Ionized gas and stars in the central kiloparsec of the type 2 Seyfert galaxy NGC 2110 - I. The data

Mon. Not. R. Astron. Soc. 352, 1180–1198 (2004) doi:10.1111/j.1365-2966.2004.08009.x

Ionized gas and stars in the central kiloparsec of the type 2 Seyfert galaxyNGC 2110 – I. The data

P. Ferruit,1� C. G. Mundell,2† N. M. Nagar,3‡ E. Emsellem,1 E. Pecontal,1

A. S. Wilson4§ and E. Schinnerer5

1CRAL – Observatoire de Lyon, 9 avenue Charles Andre, 69561 Saint-Genis-Laval cedex, France2Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD3INAF, Arcetri Observatory, Largo E. Fermi 5, Florence 50125, Italy4Astronomy Department, University of Maryland, College Park, MD 20742, USA5Jansky Postdoctoral Fellow at the National Radio Astronomy Observatory, PO Box 0, Socorro, NM 87801, USA

Accepted 2004 May 10. Received 2004 April 2; in original form 2003 May 29

ABSTRACTIn this paper, we present new results from an extensive set of Hubble Space Telescope (HST)and ground-based observations of the Seyfert galaxy NGC 2110. The HST data sets in-clude Wide Field Planetary Camera 2 (WFPC2) observations as well as the first high-spatialresolution spectroscopy ([O I], [N II], Hα and [S II] lines) of this galaxy obtained using theSpace Telescope Imaging Spectrograph (STIS). The ground-based data are three-dimensional(x, y, λ) spectrographic observations obtained using the integral field spectrograph OASIS onthe Canada–France–Hawaii Telescope, complemented by near-infrared long-slit spectroscopyobtained using NIRSPEC on the Keck–II telescope. The OASIS observations cover regionscontaining both stellar absorption lines and major optical emission lines. The NIRSPEC ob-servations cover the H and K bands.

Combined with archival HST observations, the WFPC2 data provide us with a high-spatialresolution extinction map. The OASIS data allowed bidimensional mapping of the stellar andgaseous kinematics, as well as of the spectral properties of the ionized gas. These results arecompared to those obtained in the near-infrared with NIRSPEC/Keck. Last, we used the STISdata to probe the ionized gas kinematics and properties in the inner 4 arcsec along PA = 156◦

at unprecedented spatial resolution.Our two-dimensional (2D) map of the stellar velocity field and the near-infrared stellar

velocity profile are symmetric about the nucleus, confirming the results of previous long-slitobservations. The asymmetry of the velocity field of the ionized gas is present at the samelevel for visible and near-infrared lines, indicating this is not a reddening effect. MultipleGaussian fitting of the emission-line profile allowed the contributions of the broad and anarrow components to be disentangled. The intensity peak of the [O III] narrow component islocated north of the nucleus, indicating that the bulk of the narrow [O III] emission comes fromthe jet-like structure (Mulchaey et al.) and not from the nucleus itself. We suggest that thenorthern arm is the anomalous one, contrary to what has been claimed earlier. Last, we alsoshow that the elongated region of high gas velocity dispersion located close to the nucleus anddiscovered by Gonzalez Delgado et al. is intrinsic to the narrow component.

Key words: galaxies: individual: NGC 2110 – galaxies: kinematics and dynamics – galaxies:nuclei.

�E-mail:[email protected]†Royal Society University Research Fellow.‡Kaypten Astronomical Institute, PO Box 800, 9700 AV Groningen, theNetherlands.§Adjunct astronomer at Space Telescope Science Institute, 3700 San MartinDrive, Baltimore, MD 21218, USA.

1 I N T RO D U C T I O N

NGC 2110 is a nearby, isolated S0 galaxy hosting a type 2 Seyfertnucleus. It has been extensively studied at all wavelengths and de-tailed reviews of the results obtained by various authors can be found

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Ionized gas and stars in the central kpc of NGC 2110 – I 1181

Table 1. Log of the various data sets presented in this paper.

Instrument Date Configuration ID Integration Wavelength Commentsor filter time (s) domain

OASIS/CFHT 1999/08/11 MR2 511560o 1800 6220–6990 Å OASIS run 1999IIOASIS/CFHT 1999/08/11 MR2 511562o 1800 6220–6990 Å OASIS run 1999IIOASIS/CFHT 2000/11/24 MR1 557247o 2700 4800–5550 Å OASIS run 2000IIOASIS/CFHT 2000/11/24 MR1 557249o 1800 4800–5550 Å OASIS run 2000II

NIRSPEC/Keck II 2001/11/12 H N/A 1800 14920–17846 Å 42 arcsec×0.77 arcsec slit, PA = 161.◦2NIRSPEC/Keck II 2001/11/12 K N/A 1800 19760–24128 Å 42 arcsec×0.77 arcsec slit, PA = 161.◦2

WFPC2/HST 2001/03/15 F791W u64f0101r 120 8006/1304 Å GO8610, continuumWFPC2/HST 2001/03/15 F791W u64f0102r 120 8006/1304 Å GO8610, continuumWFPC2/HST 2001/03/15 FR680P15 u64f0103f 300 6610/40 Å GO8610, narrow bandWFPC2/HST 2001/03/15 FR680P15 u64f0104f 300 6610/40 Å GO8610, narrow bandWFPC2/HST 2001/03/15 FR680P15 u64f0105f 300 6610/40 Å GO8610, narrow bandWFPC2/HST 2001/03/15 FR680P15 u64f0106f 300 6610/40 Å GO8610, narrow band

NICMOS3/HST 1998/08/27 F200N n4sb07ujq 303.85 1.985–2.005 µm GO7869, archiveNICMOS3/HST 1998/08/27 F200N n4sb07uaq 303.85 1.985–2.005 µm GO7869, archiveNICMOS3/HST 1998/08/27 F200N n4sb07uvq 303.85 1.985–2.005 µm GO7869, archiveNICMOS3/HST 1998/08/27 F200N n4sb07v4q 303.85 1.985–2.005 µm GO7869, archive

STIS/HST 2000/12/30 G750M o64f02010 1440 629–687 nm GO8610, 52x0.2 slit, PA = 155.◦65STIS/HST 2000/12/30 G750M o64f02020 1440 629–687 nm GO8610, 52x0.2 slit, PA = 155.◦65STIS/HST 2000/12/30 G750M o64f02030 1565 629–687 nm GO8610, 52x0.2 slit, PA = 155.◦65

in Gonzalez Delgado et al. (2002, GD2002 hereafter). Some of themost interesting issues have been the question of the origin of theextended, soft X-ray emission detected 4–5 arcsec north of the nu-cleus by Weaver et al. (1995; see also the discussion in Ferruit et al.(1999)) and the suggestion by Wilson & Baldwin (1985, WB1985hereafter) that the optical nucleus is offset by 1.7 arcsec from theapparent kinematic centre of the galaxy, based on gaseous velocitymeasurements. WB1985 discussed two other interpretations of thegaseous kinematics, namely effects of obscuration and the possibil-ity that the motions of the ionized gas do not reflect gravitationalbound orbital motion. Recently, GD2002 have reinvestigated thisquestion, finding that the rotation of stars along PA = 174◦ is sym-metric about the optical nucleus and that the latter is coincidentwith the kinematic centre. They conclude that non-rotational mo-tions contribute to the kinematics of the circumnuclear ionized gas.The peculiar kinematics of the nuclear ionized gas in NGC 2110originally led us to embark on an extensive program of observationsof this galaxy using the Hubble Space Telescope (HST), the ground-based three-dimensional (3D) spectrograph OASIS mounted on theCanada–France–Hawaii telescope (CFHT) and the NIRSPEC long-slit spectrograph on the Keck II telescope.

This extensive data set is presented in this paper, which is orga-nized as follows: Section 2 gives the details of the observations andtheir data reduction, Section 3 presents the procedures used for theanalysis of the data (construction of the colour map, continuum-subtracted, emission-line measurements) and Section 4 is a system-atic description of all the WFPC2, OASIS, NIRSPEC and STISresults. In Section 5, we conclude with a summary of the key find-ings of our study of NGC 2110, the detailed interpretation of whichwill be presented in a forthcoming paper.

Throughout this paper, we use a systemic heliocentric velocityof 2326 km s−1 (cz = 2335 km s−1, stellar Mg I b absorption lines,Nelson & Whittle 1995). This gives a velocity relative to the cosmic

microwave background of 2395 km s−1, as derived using the NED1

on-line velocity calculator. This corresponds to a distance of 37 Mpcand a linear scale of 179 pc arcsec−1 (H0 = 65 km s−1 Mpc−1,q0 = 0).

2 O B S E RVAT I O N S A N D DATA R E D U C T I O N

2.1 OASIS 3D spectroscopy

These 3D (x , y, λ) observations of NGC 2110 have been obtainedat the CFHT, using the integral field spectrograph OASIS. This 3Dinstrument is based on the same concept as the TIGER spectrograph(Bacon et al. 1995). A list of the OASIS NGC 2110 exposures canbe found in Table 1.

The first data set was obtained using the MR2 spectral configura-tion of OASIS, which covers the 6220–6990 Å spectral domain, witha 2.13-Å sampling. The spatial sampling was 0.41 arcsec (74 pc)per lens, yielding a total field of view of 15 arcsec × 12 arcsec(2.7 × 2.1 kpc). The (seeing limited) spatial resolution of theseobservations was roughly 0.8–1.0 arcsec (143–179 pc). The effec-tive spectral resolution of the MR2 configuration, as derived fromGaussian fitting of the unresolved [O I]λ6364 sky line, was 6.9 ±0.6 Å (full width at half maximum, FWHM hereafter). Most of the±0.6 Å uncertainty on the instrumental FWHM comes from spectralpoint-spread function (PSF) variations in our field of view.

The second data set was obtained using the MR1 spectralconfiguration of OASIS which covers the 4800–5550 Å spectral

1 The NASA/IPAC Extragalactic Data base (NED) is operated by the JetPropulsion Laboratory, California Institute of Technology, under contractwith the National Aeronautics and Space Administration.

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1182 P. Ferruit et al.

domain, with a 2.15 Å sampling. The spatial sampling was0.27 arcsec (48 pc) per lens, yielding a total field of view of 10 arcsec× 8 arcsec (1.8×1.4 kpc). The (seeing limited) spatial resolution ofthese observations was roughly 1.0–1.2 arcsec (179–215 pc). To es-timate our effective spectral resolution, we searched for Galacticand/or sky emission lines in our spectra. After spatially smoothingthe data, we were able to detect Hβ Galactic emission. Gaussian fit-ting of this spectrally unresolved line yielded a mean instrumentalFWHM of 6.3 Å. However, its weakness prevented us from obtain-ing an accurate estimate of the uncertainty of this value, which comesmainly from spectral PSF variations. We expect it to be very similarto the uncertainty measured for the MR2 configuration (±0.6 Å).In this paper, we will therefore use a value of 6.3 ± 0.6 Å for theinstrumental FWHM in the MR1 configuration.

The spectra were flux calibrated using observations of the photo-metric standard stars MR2 and MR1. In both configurations, fluxesderived from the two successive NGC 2110 exposures agree to bet-ter than 10 per cent. However, comparison with the HST imagessuggests that the absolute flux calibration of the OASIS data mayonly be accurate to only 20 per cent. No correction for atmosphericdispersion was applied. The contribution of sky emission to thespectra was only significant in the MR2 configuration, therefore,sky subtraction has only been performed for this configuration.

2.2 NIRSPEC/Keck near-infrared spectroscopy

We made use of long-slit spectroscopy of NGC 2110 obtainedwith the NIRSPEC slit spectrograph at the Keck–II telescope. The42 arcsec × 0.77 arcsec slit was oriented close to the major axis ofthe galaxy with PA = 161.◦2. Two configurations were used, pro-viding spectra covering two near-infrared spectral domains: [1.492–1.785 µm] (H band) and [1.976–2.413 µm] (K band) with 1024 pixelof 2.86 and 4.27 Å and spectral resolutions of R ∼ 1700 and 1570,respectively.

The data were obtained by nodding along the slit, leaving aneffective slit length of �15 arcsec, and providing the spectra nec-essary for sky-background subtraction. The slit was centred on thenucleus and the PA was kept fixed on the sky. The seeing duringthe observations was stable and around 0.65 arcsec. The G2V starHD 292795 was observed for atmospheric calibration. A standarddata reduction (including flat-fielding, wavelength calibration usingneon/argon lamps and atmospheric correction) was performed inIRAF following the outline given at the Keck telescope website.

2.3 HST optical imaging and spectroscopy

These HST observations of NGC 2110 have been obtained duringcycle 9 (GO 8610, PI C.G. Mundell). They consist of Wide FieldPlanetary Camera 2 (WFPC2) imaging and Space Telescope Imag-ing Spectrograph (STIS) spectroscopy. The list of the GO 8610exposures is given in Table 1.

2.3.1 WFPC2 imaging

The WFPC2 observations included wide-band continuum observa-tions using the F791W filter (corresponding roughly to an I band)and narrow-band, emission-line ([N II] plus Hα) observations us-ing the linear ramp filter FR680P15. Unlike the F606W filter usedin previous observations of NGC 2110 (snapshot survey, Malkan,Gorjian & Tam 1998), the F791W filter does not include the bright[N II], Hα and [S II] lines and is virtually emission-line free. For the

FR680P15 observations, NGC 2110 was positioned in the field ofview so that the filter bandpass was centred on the redshifted Hα

line of the galaxy. These images were processed using the stan-dard pipeline reduction with the latest calibration files, as of 2002February. The resulting F791W image was flux calibrated usingthe PHOTFLAM keyword. For the FR680P15 image, an equivalentPHOTFLAM keyword was computed with the IRAF synphot taskto allow scaling of the F791W image used for the continuum sub-traction. Due to the relatively large bandpass of the LRF filter, theresulting continuum-subtracted image (the HST [N II]+Hα image,hereafter) included flux not only from the Hα emission line but alsofrom the neighbouring [N II] doublet lines. To correct for the con-tribution of the nitrogen lines, we used SYNPHOT with a syntheticemission-line spectrum made of 11-Å FWHM Gaussian lines (as-suming a uniform [N II]λ6583/Hα ratio of 1.8). Therefore, althoughthe intensity distribution in the HST [N II]+Hα image represents amixture of the contributions of the hydrogen and nitrogen lines, thefluxes correspond to Hα only. These data, reduced using a slightlydifferent procedure, have also been presented in GD2002.

2.3.2 HST/STIS spectroscopy

The HST/STIS data consist of a set of three G750M exposures(see Table 1) along the same slit position (PA � 156◦, close to themajor axis of the galaxy). The spectral and spatial (along the slit)samplings were 1.11 Å and 0.05 arcsec, respectively. Individual ex-posures were reduced using standard pipeline reduction procedureswith the latest calibration files available at the time of the data reduc-tion. To eliminate detector defects like hot pixels, a shift of 30 pixelalong the slit had been applied from one exposure to the next. Theexposures were therefore recentred and their median computed toobtain the final spectrogram.

2.4 HST/NICMOS archival images

The NICMOS3 F200N (Quillen et al. 1999a,b) images were re-trieved from the Space Telescope/European Coordinating Facility(ST/ECF) archive at the European Southern Observatory. A ditheredimage with pixel sizes matching those of the PC was created usingthe STSDAS task CROSSDRIZZ in IRAF. It was flux calibrated usingthe PHOTFLAM keyword.

3 A NA LY S I S O F T H E I M AG I N GA N D S P E C T RO S C O P I C DATA

3.1 Colour and extinction map

F791W and dithered F200N images in magnitudes were obtainedusing the zero-point values provided in the WFPC2 and NICMOSdata handbooks. They were then used to build a colour map similarto the F606W–F200N one presented by Quillen et al. (1999b, theirfig. 4, top right), but free of contamination by line emission. Fromthis colour map, we constructed an E(B − V) image using the methoddescribed in Ferruit, Wilson & Mulchaey (1998). In defining theE(B − V) scale, we used a K0 template spectrum from the Pickleslibrary (Pickles 1998), the spectrum of which extends from 115 nmto 2.5 µm and covers the wavelength ranges of both the F791W andthe F200N filters. This procedure yielded a slope of 6.5 for the linearrelation between E(B − V) and the F791W–F200N colour, the valueof which is weakly sensitive to a change of template spectrum. Thezero point of the relation (8.7 mag) was determined from regions

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north-east, off the nucleus (corresponding to the farside of the disc)where little internal extinction is observed. Therefore, assuming auniform stellar population, the E(B − V) map traces the distributionof extinction of the starlight internal to NGC 2110 (i.e. excludingGalactic extinction). AV maps have been obtained by multiplyingthe E(B − V) map by RV = 3.1 (the standard Galactic value, Rieke& Lebofsky 1985).

3.2 Stellar kinematics and continuum subtraction

3.2.1 OASIS data

We first derived the stellar kinematics using a direct pixel fitting rou-tine. The two first velocity moments were measured by minimizingthe residuals (in a χ2 sense) between the input OASIS spectra anda convolved linear combination of stellar spectra. Spectral regionssignificantly contaminated by emission lines were systematicallydiscarded from the fit: this mainly leaves short spectral domainsaround the Mg and Fe absorption lines to constrain the fit. We thenmade use of the estimated stellar velocity and velocity dispersion tobuild an optimal template stellar continuum spectrum from a libraryof stellar and galactic spectra in both the MR1 and MR2 spectralconfigurations. These templates were then subtracted from both theOASIS MR1 and MR2 data cubes and the STIS/HST spectra, inprinciple resulting in pure emission-line data cubes.

The equivalent widths (EWs) of the Balmer lines in the templateare, however, poorly constrained due to the presence of Hβ emissionthroughout our field of view. To estimate the impact of errors in thesubtraction of the Balmer absorption lines on the inferred ionizedgas kinematics, we built a second continuum-subtracted MR2 datacube, using this time a straight line for the continuum, the level andslope of which were estimated from line-free wavelength regions.Comparison of the results of the fitting (see Section 3.3.1 for a de-scription of the fitting procedure) of the two continuum-subtracteddata cubes shows no significant differences between the inferredemission-line centroid velocities and FWHM. Differences in theintegrated fluxes are observed in regions where the EW of the emis-sion lines was small (the EW of the Hα emission line ranges from60 Å in the bright nebulosities to only 2-4 Å in low-emission-linesurface brightness areas).

3.2.2 NIRSPEC data

Stellar kinematics were also derived from both the H- and K-bandNIRSPEC spectra, using a technique similar to the one applied to theOASIS data cubes: we performed a pixel fitting using this time pub-lished standard star spectra convolved to the NIRSPEC resolutionfrom Wallace & Hinkle (1997) and Meyer et al. (1998) for the K andH bands, respectively. In the specific case of the K-band long-slitdata set, we had to add a non-stellar component (hot dust contri-bution) in the fitting procedure, represented by a simple first-orderpolynomial. This allowed us to account for the very strong dilutionof the stellar absorption CO lines around 2.3 µm at the location ofthe nucleus (see Section 4.3). The result of the fit was also used toretrieve pure emission-line spectra in both configurations.

3.3 Emission-line fitting

The basic parameters of the emission lines (intensity, centroid ve-locity and FWHM) were derived from Gaussian fitting of the linesusing the FIT/SPEC software (Rousset 1992).

3.3.1 OASIS data

In the MR2 configuration, the fit was performed using one or two‘systems’ of emission lines, each consisting of the [O I], [N II], Hα

and [S II] lines. Within each system, lines were assumed to sharethe same velocity and width (even if they originate from differ-ent species). The first fit was performed with a single system. Ex-amination of its results showed that an additional, relatively broad(∼1600 km s−1) system was needed in the nuclear regions. Thisbroad system is observed in the profile of all the lines in the MR2spectral domain (i.e. including the forbidden lines), and its presenceis therefore not the signature of a type 1 Seyfert-like, broad Hα line.A second fit was then performed, including, this time, two systems.The parameters of the broad component (centroid velocity, FWHMand line ratios) were poorly constrained and it proved necessaryto fix them during the fit (only the overall intensity level of thespectrum was left free). Their ‘nominal’ values were determinedon the high signal-to-noise nuclear spectrum. The broad compo-nent is actually spatially resolved and not confined to the nucleus(see Section 4.2). Its spectral characteristics may therefore changewithin our field of view, contrary to our assumption. Note that dueto our lower spectral resolution, we do not detect the splitting of thenarrow component into a blue and a red component; such splittingis observed by GD2002 at the (higher) spectral resolution of theirlong-slit data (see their fig. 16).

In the MR1 configuration, it was not possible to include the Hβ

and [O III] lines in the same system, as they were found to have dis-tinct velocities, especially south of the nucleus. They were thereforefitted independently. Apart from that, the fitting procedures for theMR1 and MR2 data were identical (see above). In the following, allthe FWHM given in Å are ‘as measured’, i.e. without any correc-tion for instrumental resolution, while the FWHM given in km s−1

have been corrected for instrumental resolution by subtraction of6.3 and 6.9 Å in quadrature for the MR1 and MR2 configurations,respectively.

3.3.2 NIRSPEC data

Although many emission lines can be found in our H- and K-bandspectra, in this paper we only consider the two brightest lines,namely [Fe II]λ16435.5 and H2λ21212.5. Our main goal was toobtain the velocities along the slit of the gas emitting these linesto compare them with the ionized gas velocities inferred from theOASIS data. Therefore, we used the continuum-subtracted H- andK-band spectra and fitted a single Gaussian profile to the [Fe II] andH2 lines. Velocities were corrected for barycentric motion.

3.3.3 HST/STIS data

The high-spatial resolution STIS spectroscopic data covers regionsexhibiting very different emission-line profiles, ranging from ‘sim-ple’ ones in the outer regions to extremely complex ones in thecentral 0.2 arcsec. We therefore distinguish a number of regions asfollows.

(i) The nuclear regions (labelled Nuclear regions) correspondingto regions within ± 0.2 arcsec (along the slit) of the location of thenucleus. For these regions, no satisfactory simple model describingthe line profiles was found.

(ii) Regions SA and NA (from 0.2 to 0.6 arcsec south-east andnorth-west of the nucleus, respectively), where two systems (onebroad and one narrow) were needed.

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(iii) Region NB (from 0.6 to 0.75 arcsec north-west of the nu-cleus) where three systems were needed. The first two systems werethe continuation of those observed in NA. The third one, very weakand probably spatially unresolved along the slit, was detected in fourspatial pixels only. We therefore determined its spectral character-istics from the spectrum on the central pixel and kept them fixed forthe other three spectra (similar to what has been done for the broadnuclear system in the OASIS data, see Section 3.3.1).

(iv) The outer regions (distance from the nucleus > 0.60 arcsecto the south-east and > 0.75 arcsec to the north-west, i.e. includingregions labelled NC and SB), where the line profile was accuratelydescribed by a single Gaussian profile.

It was not possible to trace the emission from ionized gas furtherthan 4 arcsec from the nucleus along the slit (signal to noise < 3 atlarger distances).

3.3.4 Error computation

An estimate of the errors on the emission-line parameters, as a func-tion of the signal to noise in the spectrum, has been computed byrepeating the fitting 500 times for each signal-to-noise level (rangingfrom 1 to 100), using artificial emission-line spectra. These artificialspectra consisted of the sum of a synthetic noise-free spectrum (withspectral characteristics typical of the observed spectra) and noise.Additional errors on the centroid velocities (OASIS data: 16 km s−1

for calibration errors; STIS data: 5 and 15 km s−1 for calibration er-rors and slit effect, respectively) and the FWHM (OASIS data: 0.6 Å;STIS data: 0.5 Å to account for slit effect) have been included in thefinal error budget.

4 R E S U LT S

4.1 Results from WFPC2 imaging

The WFPC2 continuum and emission-line images of NGC 2110have already been presented in GD2002 and are not displayed here.Integrating over a 6 arcsec×10 arcsec rectangular box centred onthe nucleus (largest side along the south–north direction) in our[N II]+Hα image, we find a total Hα flux of (5.9 ± 0.1) × 10−16

Figure 1. HST colour map of NGC 2110 constructed using the WFPC2 F791W and NICMOS F200N images of NGC 2110 (see Section 3.1). The colourscales are linear and the pixel size is 0.0455 arcsec×0.0455 arcsec. The darker regions are redder.

W m−2. Note that this error bar does not include the contribution ofchanges in the [N II]/Hα ratio with position (changes that we knowto exist, see fig. 8 in GD2002 and Section 4.2.5 in this paper). For adistance to NGC 2110 of 37 Mpc, this corresponds to an Hα lumi-nosity of 9.7 × 1033 W and yields a mass of ionized gas of �4–5 ×106 (100 cm−3/n e) solar masses (assuming a case B recombinationcoefficient for Hα of 3.03 × 10−14 cm3 s−1).

Our new colour map is shown in Fig. 1. In this map, prominentdust features are seen south-west of the major axis of the galaxy,while very little obscuration is visible north-east of it. This indicatesthat the south-west side of the disc is the near side, as here the dustin the disc is highlighted against the background continuum of thebulge. Such an orientation is also found by assuming the dust armsare trailing and noting the sense of the rotation curve (redshiftedwith respect to systemic to the south, see Fig. 2).

The strongest dust lanes 1 arcsec west and 4 arcsec south of thenucleus display F791W-F200N colours of 3.4–3.5 and 3.2–3.3 mag,respectively. This corresponds to values of AV of 1.5–1.6 and 1.3–1.4 mag, respectively (not including Galactic extinction, see Sec-tion 3.1). These values of AV are in the lower range of thosederived by (Mulchaey et al. (1994, M+1994 hereafter) from theirpre-COSTAR HST images (AV = 1.2–2.5 mag). This suggests thatthe zero point used in our relation between AV and the F791W-F200N colour is too low, i.e. that the north-east regions of the galaxyused a reference are not free of internal dust extinction. Therefore,our values of AV are probably systematic underestimates. The smallblue feature at the location of the nucleus is an artefact due to PSFmismatch between the F791W and dithered F200N images, the nu-cleus being actually quite red (M+1994; Storchi-Bergmann et al.1999; Quillen et al. 1999a,b).

Detailed examination of this colour map shows that the spiralstructure is complex. There is no evidence for a classical well-defined two-armed spiral structure, even out to large radii (see theF600W/F200N colour map of Quillen et al. 1999b, their fig. 4, or theunsharp-masked F606W image of GD2002, their fig. 18); insteadthe dust distribution is more consistent with a single spiral arm,which could be the signature of an m = 1 spiral wave mode. Com-parison between the [N II]+Hα image and the colour map shows thatthe distribution of line emission is strongly affected by extinction,especially 1 arcsec west of the nucleus where a prominent dust lane

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Figure 2. Left, OASIS map of the stellar continuum flux in the MR1 configuration (average flux over the 4760–5560 Å wavelength range) with contours ofthe WFPC2 F791W map overimposed (1, 2, 5, 10 and 20 × 10−19 W m−2 Å−1 arcsec−2). Right, OASIS stellar velocity field (v − 2326 km s−1) as derivedfrom the MR1 data, with contours of the WFPC2 F791W map overimposed (1, 2, 5, 10 and 20 × 10−19 W m−2 Å−1 arcsec−2).

is detected. This phenomenon has already been outlined for NGC2110 by Quillen et al. (1999a) and is not peculiar to this galaxy (e.g.see Quillen et al. 1999a or Ferruit, Wilson & Mulchaey 2000).

4.2 Results from the OASIS observations

4.2.1 Stellar kinematics

The map of the 4760–5560 Å stellar continuum flux reconstructedfrom the MR1 OASIS data is shown in Fig. 2. At the 1.0–1.2 arcsecspatial resolution of these data, the effects on the continuum of thedust lanes seen 1 arcsec west and 4 arcsec south of the nucleus inthe F791W image (see Fig. 2) are extremely diluted. As a result, thepresence of these dust lanes in the OASIS data is only witnessed bya weak asymmetry in the flux distribution between the north-eastand south-west sides of the galaxy.

The mean stellar velocity was measured using a direct pixel fittingtechnique as explained in Section 3.2. The signal-to-noise ratio inthe data was not high enough to derive reliable maps of higher ordermoments. The resulting stellar velocity field is displayed in the rightpanel of Fig. 2. This map indicates that the kinematic major axisis aligned with the photometric one (163◦, Alonso-Herrero et al.1998, GD2002). Its amplitude of ±160–200 km s−1 from systemicvelocity (2326 km s−1) is consistent with the values measured alongPA = 174◦ by GD2002 from their long-slit data.

4.2.2 Distribution of the ionized gas

The maps of the Hα emission, reconstructed from fitting the OASISMR2 data cube, are presented in the top left and right panels of Fig. 3for the narrow and broad systems, respectively. The morphology ofthe narrow system displays the familiar inverted S shape and exhibitsthe same asymmetry as the HST image (see fig. 5 in GD2002), thesurface brightness of the northern regions being higher than that ofthe southern ones. Faint emission is also detected 5–6 arcsec northof the nucleus. Our limited field of view toward the south preventsus from searching for a southern counterpart to this weak structure.

The distribution of the Hα emission of the broad system is muchmore compact. It is strongly peaked at the location of the nucleusand is elongated to the north, up to �3 arcsec from the nucleus. Noextension of the broad system is detected toward the south.

From analysis of the OASIS MR1 data cube, we also recon-structed maps of the Hβ and [O III] emission. The Hβ narrow-systemmap is shown in the top left panel of Fig. 4. As expected the spa-tial distribution of the Hα and Hβ emitting gas are almost identical.Extinction could have caused differences, but as we saw for the con-tinuum map (see Section 2), the effects of extinction are extremelydiluted at our spatial resolution.

The map of the [O III] emission of the narrow system is displayedin the top right panel of Fig. 4. As already known, the spatial dis-tribution of the [O III] emission is much more asymmetric than thatof the Hα emission, the southern arm being much fainter than thenorthern one in [O III]. The peak of the [O III] narrow componentis not coincident with the nucleus (defined as the peak of the con-tinuum), but is shifted 0.6 arcsec north and 0.2 arcsec west of it.The location of the peak indicates that the major contributor to thenarrow [O III] emission is therefore probably the ‘jet-like’ structure(i.e. the 0.4-arcsec long linear emission-line structure seen in theHST [N II]+Hα image, see M+1994) and not the nucleus itself.This effect is also seen on a much lower level in Hα, the peak of thenarrow Hα component being shifted by only 0.2 arcsec north of thenucleus.

Emission from the broad system can be traced further north inthe [O III] map (not shown) than in the Hα one (top right panel ofFig. 3). However, the error bars on the intensity of the broad systemat large distances from the nucleus are large so this detection shouldbe treated with caution.

4.2.3 Kinematics of the ionized gas

The Hα and [O III] velocity fields of the narrow system are shownin Figs 3 and 4. These maps are very similar to those of GD2002,despite the fact that these authors did not distinguish between thenarrow and broad systems. This is because the single Gaussian

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Figure 3. Results from two component (narrow+broad) Gaussian fitting of the OASIS MR2 data cube spectra. Top left, map of the Hα-line flux of the narrowspectral component, with contours of the 3.6-cm Very Large Array (VLA) radio map (Mundell et al., in preparation; see also Nagar et al. 1999 or Ulvestad& Wilson 1984) at 0.5 and 10 mJy beam−1 superimposed. Top right, map of the Hα-line flux of the broad component, with contours of the HST [N II]+Hα

image (see Fig. 1) at 100, 400 and 4000 × 10−19 W m−2 arcsec−2, superimposed. Bottom left, velocity field of the narrow component, with contours of thenarrow Hα-line flux map, at 5, 10, 50, 150 and 250 × 10−19 W m−2 arcsec−2, superimposed. A clipping at an Hα-peak intensity of three times the rms noisehas been applied. Bottom right, map of the FWHM of the narrow component, with contours of the narrow Hα-line flux map, at 5, 10, 50, 150 and 250 ×10−19 W m−2 arcsec−2, superimposed. A clipping at an Hα-peak intensity of five times the rms noise has been applied (corresponding to a relative accuracy of20 per cent).

profile used by these authors in their fit attaches itself naturally to thestrong narrow system, yielding centroid velocities and dispersionsrepresentative of the narrow system (but not intensities).

Beyond �2 arcsec north of the nucleus, the velocity field is actu-ally extremely flat with velocities between −100 and −150 km s−1,except on the western edge of our field of view where they are closerto systemic. This is witnessed by a 100 km s−1 difference betweenthe measured Hα centroid velocities of spectra N1 and NW (seeFig. 8 later for the location of these spectra). This structure can betraced toward the south and there is also a hint of distortions on theeastern side of the nucleus. Although these distortions have smallamplitudes and occur in regions where the signal to noise is rela-tively low, we are confident that they are real as they also appear inthe [N II] velocity field in GD2002 (right panel of their fig. 10).

We have compared the ionized gas velocities derived from Hβ,Hα and [O III] (bottom panels of Fig. 5). The Hα and Hβ centroidvelocity curves along the major axis of the galaxy agree perfectlyto within the errors. This is not the case when we compare the Hα

centroid velocities with the [O III] ones. First, south of the nucleusthe [O III] velocities are lower than the Hα ones by almost 100 km s−1

between 2 and 4 arcsec from the nucleus. Examination of the [O III]velocity curve along PA = 6◦ of GD2002 (their fig. 12) shows thatthe [O III] velocity increases sharply around 5–6 arcsec from the

nucleus (i.e. just outside of our field of view) to reach values similarto those of the Hα velocity. Both the Hα and the [O III] velocitiesthen drop by 50 to 100 km s−1 at radii >10 arcsec.

North of the nucleus (bottom right panel of Fig. 5), the Hα and[O III] velocities are identical except at radii <1.5 arcsec where the[O III] velocities remain almost constant while the Hα ones increaseto reach a value close to systemic. As an example, at the location ofthe nucleus, the [O III] velocity of the narrow system is still close toits values in the northern regions (−90 ± 20 km s−1), in agreementwith the [O III] velocity curve of WB1985 (their fig. 4) and the[O III] velocity map of Wilson, Baldwin & Ulvestad (1985, theirfig. 12). In contrast, the velocity of the Hα narrow system is closeto systemic at the nucleus (−20 ± 20 km s−1). We think that thisdifference between the Hα and [O III] velocities immediately northof the nucleus comes from the fact that the bulk of the narrow [O III]emission originates from the ‘jet-like’ structure and not from thenucleus, contrary to the situation for the Hα line (see Section 4.2.2).

GD2002 found a region of high FWHM with an elongated mor-phology in the vicinity of the nucleus (their fig. 13). This structureis also present in our FWHM maps of the narrow Hα system (bot-tom right panel of Fig. 3; see also Fig. 6). This indicates that thisbroadening is not due to the contribution of the broad system but isintrinsic to the narrow one. This region of high FWHM is centred

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Figure 4. Results from two component (narrow+broad) Gaussian fitting of the OASIS MR1 data cube spectra. Top left, map of the Hβ-line flux of the narrowspectral component, with contours of the 3.6-cm VLA radio map (Mundell et al. in preparation; see also Nagar et al. 1999 or Ulvestad & Wilson 1984) at 0.5and 10 mJy beam−1 superimposed. Top right, map of the [O III]λ5007-line flux of the narrow spectral component, with contours of the 3.6-cm VLA radio map(Mundell et al. in preparation; see also Nagar et al. 1999 or Ulvestad & Wilson 1984) at 0.5 and 10 mJy beam−1 superimposed. Bottom left, [O III] velocityfield of the narrow component, with contours of the narrow [O III]λ5007-line flux map, at 5, 25, 50, 100 and 150 × 10−19 W m−2, Å−1 arcsec−2, superimposed.A clipping at an [O III]λ5007-peak intensity of three times the rms noise has been applied (corresponding to a relative accuracy of 20 per cent). Bottomright, FWHM map of the narrow component, with contours of the narrow [O III]λ5007-line flux map at 5, 25, 50, 100 and 150 × 10−19 W m−2 Å−1 arcsec−2,superimposed. A clipping at an [O III]λ5007-peak intensity of five times the rms noise has been applied (corresponding to a relative accuracy of 20 per cent).

on the nucleus and elongated in PA � 40◦, which is some 30◦ fromthe minor axis of the galaxy. It ends roughly 2 arcsec from the nu-cleus. GD2002 give a different PA for this elongation (60–70◦, i.e.along the minor axis of the galaxy). Careful examination of the [N II]FWHM contours in their fig. 13 (right panel) shows that they areactually elongated along PA �30◦ in the inner 1–1.5 arcsec. Beyond1.5 arcsec, isocontours twist toward the minor axis of the galaxy andthis effect may account for the difference between the two papers.Along the major axis of the galaxy, this region of high FWHM isspatially unresolved in our observations. Interestingly, this regionfits almost perfectly adjacent to the radio jet ‘arms’ (see middle leftpanel of Fig. 6).

This region of high FWHM is also present in the FWHM mapof the [O III] narrow component (see bottom right panel of Figs 4and 7). It is more extended than for the Hα line and has a Z-shapedmorphology with a central elongation along PA � 50◦ instead of� 40◦ for Hα. The FWHM of the [O III] line in this region is inagreement with that of the Hα one. Overall, low surface brightnessareas east and west of the nucleus seem to have larger FWHMsthan the off-nuclear, high surface brightness ones. This trend isstronger in the [O III] FWHM map than in the Hα one and seemsto be also present in the [N II] velocity dispersion map of GD2002(right panel of their fig. 13). Higher signal-to-noise observations willbe needed to confirm this suggestion (this is especially true as low

signal to noise tends to bias dispersion measurements toward highvalues).

Comparatively, the northern and southern arms correspond to re-gions of lower FWHM (barely resolved lines in our spectra; seeFigs 8 and 9). It is worth noting that in the southern regions,the narrow [O III] lines seem to be broader than the Hα and Hβ

ones. The FWHM of the [O III] lines in spectrum S1 (see Fig. 9)is 380 ± 150 km s−1, whereas it is unresolved in Hβ. In this re-gion, similar broadening has been observed in the near-infrared[Fe II]λ1.2567 µm line (which was compared to the Pa β hydrogenline) by Knop et al. (2001).

The region at the western edge of our field of view, which wasfound to have slightly higher centroid velocities than regions to thenorth of the nucleus (Fig. 3, bottom right), exhibits intermediate Hα

linewidths (260 ± 120 km s−1 in the NW spectrum, see Fig. 8). Alarge part of this region is outside of the MR1 configuration field ofview, but it does not seem to show up in the [O III] FWHM map asclearly as it does in the Hα FWHM map.

4.2.4 Comparison between the stellar and gaseous kinematics

To compare the stellar and ionized gas velocity fields, we havereconstructed velocity curves along the major (PA = 163◦) andminor (PA = 73◦) axes of the galaxy by extracting pseudo-slits

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Figure 5. OASIS stellar and ionized gas velocity curves. Error bars displayed in these plots correspond to 1σ . Top, maps of the Hα (left) and [O III] (right)line fluxes of the narrow component, with the positions of the spectra used to build the velocity curves marked (circles, PA = 163◦, major axis; triangles, PA =73◦, minor axis). Central, plots of the stellar (triangles) and Hα (small squares) velocity fields along the photometric major (PA = 163◦, left panel) and minor(73◦, right panel) axes of the galaxy. No error bars have been estimated for the stellar velocities. Bottom, comparison of the ionized gas velocity field alongPA = 163◦, as obtained from different emission lines (Hα and Hβ, left panel; Hα and [O III], right panel).

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Figure 6. Central left, OASIS FWHM map of the narrow Hα component in the inner 5 arcsec of NGC 2110, with contours of the 3.6-cm VLA radio map(Mundell et al. in preparation; see also Nagar et al. 1999 or Ulvestad & Wilson 1984) at 0.5 and 10 mJy beam−1 superimposed. A clipping at an Hα peakintensity of five times the rms noise has been applied (corresponding to a relative accuracy of 20 per cent). Central right, OASIS map of the Hα-line flux ofthe broad component, with contours of the HST [N II]+Hα image (see Fig. 1) at 100, 400 and 4000 × 10−19 W m−2 arcsec−2, superimposed. Bottom and top,observed, continuum-subtracted OASIS MR2 spectra at four selected locations in the nuclear regions of NGC 2110 (including the nucleus).

from our 3D data cubes. The results of this exercise are displayedin Fig. 5. The top panels show the locations of the spectra usedto construct these curves for the MR2 (left panel, Hα velocities)and MR1 (right panel, [O III] and stellar velocities) configurations.In the middle panels, we compare the velocities of the stars andthe Hα emitting gas along the major and minor axes of the galaxy.Along the former one, the amplitude of the ionized gas velocitycurve south of the nucleus is higher by �100 km s−1 than the stellarone. We therefore confirm the result obtained by GD2002 along asingle slit at PA = 6◦ (their fig. 15). North of the nucleus, our stellarvelocity field is much noisier but there is a clear agreement betweenthe stellar and ionized gas velocities. Along the minor axis of thegalaxy, the stellar and ionized gas velocities are close to systemic,except west of the nucleus where the Hα centroid velocities arehigher than systemic.

4.2.5 Spectral properties of the ionized gas

The [N II]/Hα ratio of the narrow spectral component (bottom leftpanel of Fig. 8) varies by almost a factor of 2 between the southernand northern regions of the galaxy. Values as low as 0.8 are measuredin region S1, while they reach 1.2–1.4 in regions N1, N2 or N3. Thelargest [N II]/Hα ratio (1.66 ± 0.07) is observed at the location ofthe nucleus. Low surface brightness regions east and west of thenucleus exhibit values that are intermediate between those of thesouthern and northern regions. This last result must be taken withcaution as a higher [N II]/Hα ratio could be artificially created byan oversubtraction of the Hα absorption line in these regions wherethe Hα emission line has a small EW. The [O I]/Hα and [S II]/Hα

ratios (maps not shown) have very similar behavior, except that the[O I]/Hα map is more strongly peaked at the location of the nucleus

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Figure 7. Bottom right, OASIS FWHM map of the narrow [O III] component in the inner 5 arcsec of NGC 2110, with contours of the 3.6-cm VLA radio map(Mundell et al. in preparation; see also Nagar et al. 1999 or Ulvestad & Wilson 1984) at 0.5 and 10 mJy beam−1 superimposed. A clipping at an [O III]λ5007peak intensity of five times the rms noise has been applied (corresponding to a relative accuracy of 20 per cent). Other panels, observed continuum-subtractedOASIS MR1 spectra at four selected locations in the nuclear regions of NGC 2110 (same locations as in Fig. 6).

than the other ones. This may be a consequence of the high criticaldensity of the [O I] doublet.

The density sensitive [S II]λ6717/[S II]λ6731 ratio (bottom rightpanel of Fig. 8) reaches its minimum at the location of the nu-cleus with a value close to unity, yielding an electron density of�500 cm−3. For the broad component, this ratio is poorly con-strained (0.7 ± 0.7 corresponding to a nominal density value of�2000 cm−3). Away from the nucleus, the ratio increases quickly(i.e. the electron density in the [S II] emitting regions decreasesquickly) to reach values >1.4, corresponding to its low-densitylimit. There is a trend for the off-nuclear high surface brightnessregions to have slightly lower ratios (i.e. slightly higher densities)than the low surface brightness ones. Once more, higher signal-to-noise observations are needed to confirm this trend.

The [O III]/Hβ map (bottom panel of Fig. 9) shows the spectaculargradient along the inverted S-shaped structure, which was alreadydetected by Wilson et al. (1985) and can also be seen in the mapsof GD2002 (their fig. 8). From a ratio close to unity 4 arcsec southof the nucleus, it reaches almost 10 4 arcsec north of the nucleus.Our [O III]/Hβ ratios seems to be systematically smaller than thosemeasured by GD2002 (see the isocontour levels in their fig. 8), but

this is probably due to differences between our correction for theHβ absorption line and theirs. The nucleus does not really show upin this map, the nuclear [O III]/Hβ ratio being very similar to that ofthe surrounding off-nuclear regions. Note that the broad componenthas a relatively low [O III]/Hβ ratio (3.3 ± 0.7).

4.3 Results from the NIRSPEC long-slit spectra

The NIRSPEC K-band spectra of the inner 3 arcsec of NGC 2110 arepresented in Fig. 10. The nuclear spectrum is very different from theoff-nuclear spectra, exhibiting a very flat continuum and extremelydiluted absorption- and emission-line features. This is very likelydue to the additional contribution of hot dust heated by the AGN tothe nuclear spectrum (see e.g. the results of near-infrared imaging ofNGC 2110 by Alonso-Herrero et al. 1998). Note that this dilutioneffect is much weaker in the nuclear K-band spectrum of Knopet al. (2001, their fig. 9) but this is likely due to their coarser spatialresolution and sampling.

In Fig. 11 we display the position–velocity (PV) diagrams for the[Fe II]λ16435.5 (left panel) and H2λ21212.5 (right panel) emissionlines. In the inner 2 arcsec from the nucleus, the [Fe II] exhibits a

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Figure 8. Top mosaic, central, OASIS Hα emission-line map of the narrow spectral component, with isovelocity contours (from −100 to +250 km s−1

with a 50 km s−1 step) superimposed. Top mosaic, other, observed continuum-subtracted OASIS MR2 spectra at eight selected locations. Bottom left, OASISmap of the [N II]λ6583/Hα-line ratio of the narrow spectral component, with contours of the OASIS narrow Hα-line flux map, at 2, 5, 10, 100 and 300 ×10−19 W m−2 arcsec−2, superimposed. A clipping at an Hα peak intensity of nine times the rms noise has been applied (corresponding to a relative accuracy of20 per cent). Bottom right, OASIS map of the density sensitive [S II]λ6717/[S II]λ6731 line ratio of the narrow spectral component, with contours of the OASISnarrow Hα-line flux map, at 2, 5, 10, 100 and 300 × 10−19 W m−2 arcsec−2, superimposed. Darker areas on the map (lower values of the ratio) correspond todenser regions. A clipping at an Hα-peak intensity of 15 times the rms noise has been applied (corresponding to a relative accuracy of 20 per cent).

much broader and much more complex profile than the H2 line. Thisis consistent with the results of Storchi-Bergmann et al. (1999, theirfig. 4) and Knop et al. (2001, their fig. 11). This broadening of the[Fe II] line is attributed to fast shocks resulting from the interactionbetween the radio jet material and the interstellar medium of thegalaxy (Storchi-Bergmann et al. 1999; Knop et al. 2001). At largerradii, the profile of the [Fe II] line becomes narrower and simplerand the position of its centroid agrees with that of the H2 line (seealso Reunanen, Kotilainen & Prieto 2003). In the following, we willonly use the gas kinematics obtained from the fitting of the H2 lineas the single Gaussian profile we used is clearly not appropriate forthe [Fe II] line.

The stellar velocities derived from the H- and K-band NIRSPECspectra are presented in Fig. 12 (right panel). They are in goodagreement except very close to the nucleus where dilution of the ab-sorption lines made the measurement of the velocities more difficult.In the same panel, we have overplotted the stellar velocities obtainedin the visible with OASIS. Apart from discrepancies mainly due tothe different characteristics of the data sets (mostly seeing condi-tions), the velocity profiles obtained from OASIS and NIRSPEC areconsistent with each other. This indicates that reddening has littleimpact on the stellar velocity field obtained with OASIS.

In the left panel (Fig. 12), we have plotted the gas velocities in-ferred from the H2 (NIRSPEC data) and Hα (OASIS data) emission

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Figure 9. Top mosaic, central, OASIS [O III]λ5007 emission-line map of the narrow spectral component, with isovelocity contours (from −100 to +200 km s−1

with a 50 km s−1 step) superimposed. Top mosaic, other, observed continuum-subtracted OASIS MR1 spectra at seven selected locations. Bottom, OASIS mapof the [O III]λ5007/Hβ-line ratio of the narrow spectral component, with contours of the narrow [O III]λ5007-line flux map, at 5, 25, 50, 100 and 150 × 10−19 Wm−2 Å−1 arcsec−2, superimposed. A clipping at an Hβ-peak intensity of six times the rms noise has been applied (corresponding to a relative accuracy of20 per cent).

lines. Despite the fact that these two lines are probably tracing dif-ferent gaseous components, their velocities are very similar, the H2

and Hα velocities displaying the same strong kinematical asymme-try between the southern and the northern regions. This indicatesthat this asymmetry is not a reddening effect.

4.4 Results from STIS spectroscopy

4.4.1 Kinematics of the ionized gas

Fig. 13 displays the [O I], [N II]+Hα and [S II] regions of the STISG750M spectrogram. In the top panels, the cuts of the images havebeen adjusted to show the fainter outer regions, while the bottompanels are a zoom on the inner 1-arcsec (179-pc) region. In thetop spectrograms, the line emission can be traced up to 4 arcsecfrom the nucleus. The distinction between the ‘simple’ outer re-gions and the ‘complex’ inner ones (see Section 3.3.3) is obvious in

these images. Comparison between the southern and northern partsof the outer regions shows the difference in velocities observed inthe OASIS data (see Section 4.2). Many of velocity substructuresare also seen, especially north of the nucleus. The narrow compo-nent of the lines can be traced down to radii of 0.1–0.2 arcsec, butin the inner 0.5 arcsec south of the nucleus and 1 arcsec north ofthe nucleus, it is superimposed on an additional broader compo-nent. In the inner 0.2 arcsec, the line profiles become much morecomplex.

The results of fitting single or multiple Gaussian profiles to theemission lines in the spectrograms are shown in Figs 14 and 15(centroid velocities and velocity dispersions) and Figs 16 and 17(line ratios). The profiles of the [N II]+Hα blend of lines of the inner0.2-arcsec spectra are shown in Fig. 18. The definition of the regionnames (SA, Nuclear regions, NA, NB) used in these diagrams, aswell as the actual number of Gaussian profiles used for the fittingin these different regions can be found in Section 3.3.3

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Figure 10. NIRSPEC K-band spectra extracted at different radii alongPA = 161.◦2. Spectra correspond to distances to the central luminosity peak(from top to bottom) of −3.095, −2.087, −1.079, −0.071, 0.937, 1.945and 2.953 arcsec. Positive offsets are to the south-east. The spectra werearbitrarily (vertically) shifted for clarity. Their flux is per unit of wavelength.

Fig. 14 presents the Hα centroid velocity and velocity dispersionas a function of the distance along the slit for the outer regions.The lower spatial resolution, stellar and Hα velocities obtainedfrom the OASIS data have been overplotted in the middle panel.South of the nucleus, OASIS and STIS Hα velocities are in excel-lent agreement. A clear drop in velocity is detected 1 arcsec southof the nucleus in the STIS data as the slit crosses one of the southernemission-line clumps. Smaller amplitude oscillations of the velocityfield are present at radii > 1.5 arcsec south of the nucleus.

North of the nucleus, the STIS Hα velocity field is less regularand displays strong wiggles, with velocities ranging from −100 to

Figure 11. Left, PV diagrams along PA = 161.◦2 extracted from the NIRSPEC data sets and showing the [Fe II]λ16435.5 (left panel) and H2λ21212.5 (rightpanel) emission lines. In these plots, the south-east direction is to the left. The broad component associated with the nuclear activity is clearly seen in the [Fe II]line. The gas velocity curve measured from the near-infrared H2 emission line is overplotted on both panels.

−200 km s−1. Note that the error bars on the STIS velocities includean estimate of the contribution of the slit effect (our 0.2-arcsec slitwidth is significantly larger than the spatial resolution), so thesewiggles cannot be caused by gradients in the intensity distributionwithin the slit. The ‘redder’ velocities are observed where the slitcrosses the northern part of the inverted S-shaped structure. Boththe OASIS velocity field and the one of GD2002 are much smootherand do not show any signature of the blue wiggles. This is due tothe fact that, at the low spatial resolution of these ground-basedobservations (typically five to ten times lower than that of the STISobservations), the dominant contributor to the OASIS spectra is thebright and extended inverted S-shaped structure. The contribution ofthe dimmer, and probably more compact, blue regions is thereforesmall. This explains why they do not appear in the (luminosity-weighted) centroid velocities inferred from the OASIS and IFS data.

In previous papers (WB1985; Wilson et al. 1985; GD2002), it hasalways been assumed that the asymmetry of the ionized gas velocityfield was due to anomalously large positive velocities south of thenucleus. In particular, GD2002 assumed that the stellar velocitiesrepresented circularly rotating gas disc velocities and therefore in-terpreted the mismatch between the ionized gas and stellar velocitiessouth of the nucleus as evidence for anomalously redshifted ionizedgas velocities. We emphasize the fact that the amplitude of stellarvelocity curves is expected to be smaller than that of a circularlyrotating disc. This result, the structure of the northern velocity curvein the high spatial resolution STIS data, as well as the anomalousemission-line ratios strongly suggest, instead, that it may be thenorthern arm that displays anomalously small negative velocities(i.e. only � −100 km s−1). The plot of the FWHM of the Hα lineas a function of the distance from the nucleus in the extranuclearregions (right panel of Fig. 14) displays strong oscillations. North ofthe nucleus, the arm of the inverted S-shaped structure correspondsto one of the minima in FWHM. The largest FWHM values (stillin the extranuclear regions) are observed 1.5 arcsec south of the nu-cleus, in a region with no apparent associated feature in the HST[N II]+Hα image. This peak in the FWHM curve is followed by aquick decrease toward FWHM values in the 3–4 Å range.

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Figure 12. Kinematics extracted from the NIRSPEC data sets. Left, centroid velocities of the near-infrared H2 emission line (triangles) as compared to theoptical Hα line (filled circles). The stellar velocities obtained from the CO bandhead in the K band (crosses) are overplotted for comparison. Right, stellarkinematics extracted from the CO bandhead (at 2.3 µm, crosses), from the NIRSPEC H-band spectra (filled circles) and from the OASIS MR1 data cube (emptysquares) all along PA = 161.◦2.

Figure 13. Top, to the left, WFPC2 [N II]+Hα narrow-band image of the central 4 arcsec×8 arcsec region of NGC 2110 (see Fig. 1) with the position of the0.2-arcsec wide slit overimposed. The image has been rotated so that the direction of the slit (PA = 156◦) is vertical, with north-west to the top. From left toright, zooms on interesting regions of the STIS spectrogram corresponding to the [O I], [N II]+Hα and [S II] lines. Bottom, same as above but for the inner2 arcsec only.

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Figure 14. Hα centroid velocity and velocity dispersion in the outer regions inferred from the STIS data. The error bars displayed in these plots correspond to1σ . Left, WFPC2 [N II]+Hα narrow-band image of the central 4 arcsec×8 arcsec region of NGC 2110 (see Fig. 1) with the position of the 0.2-arcsec wide STISslit and contours of the 3.6-cm VLA radio map (Mundell et al. in preparation; see also Nagar et al. 1999 or Ulvestad & Wilson 1984) at 0.5 and 10 mJy beam−1

superimposed. The image has been rotated so that the direction of the slit (PA = 156◦) is vertical, with north-west at the top. Central, plot of the centroidvelocity of the Hα line as a function of the position along the STIS slit (central regions excluded), as derived from single Gaussian fitting of the lines (seeSection 3.3.3). The stellar (squares) and Hα (triangles) velocities along the same PA inferred from the OASIS data are also plotted. Right, plot of the FWHMof the Hα line as a function of the position along the STIS slit, as derived from single Gaussian fitting of the line (see Section 3.3.3).

Figure 15. Hα centroid velocity and velocity dispersion in the inner 2 arcsec of NGC 2110 inferred from the STIS data. Error bars diplayed in these plotscorrespond to 1σ . Left, WFPC2 [N II]+Hα narrow-band image of the central 2 arcsec×2 arcsec region of NGC 2110 (see Fig. 1) with the position of the0.2-arcsec wide STIS slit and contours of the 3.6 cm VLA radio map (Mundell et al. in preparation; see also Nagar et al. 1999 or Ulvestad & Wilson 1984)at 0.5 and 10 mJy beam−1 superimposed. The image has been rotated so that the direction of the STIS slit (PA = 156◦) is vertical, with north-west at the top.Central, plot of the centroid velocity of the Hα line as a function of the position along the STIS slit (inner 0.2 arcsec excluded), as derived from single ormultiple Gaussian fitting of the lines (see Section 3.3.3). Right, plot of the FWHM of the Hα line as a function of the position along the STIS slit, as derivedfrom single or multiple Gaussian fitting of the line (see Section 3.3.3).

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Figure 16. Line ratios in the outer regions inferred from the STIS data. Error bars displayed in these plots correspond to 1σ . Left, WFPC2 [N II]+Hα

narrow-band image of the central 4 arcsec×8 arcsec region of NGC 2110 (see Fig. 1) with the position of the 0.2-arcsec wide STIS slit and contours of the3.6-cm VLA radio map (Mundell et al. in preparation; see also Nagar et al. 1999 or Ulvestad & Wilson 1984) at 0.5 and 10 mJy beam−1 superimposed. Theimage has been rotated so that the direction of the STIS slit (PA = 156◦) is vertical, with north-west at the top. Top, central, plot of the [N II]/Hα ratio as afunction of the position along the STIS slit (central regions excluded), as derived from single Gaussian fitting of the STIS data. Values inferred from the OASISdata along the same PA are displayed as triangles. Top right, same as the top, central panel, but for the [O I]/Hα ratio. Bottom central, same as the top centralpanel, but for the [S II]/Hα ratio. Bottom right, same as the top central panel, but for the [S II]λ6717/[S II]λ6731 ratio.

The centroid velocities and velocity dispersion curves for the in-ner (radii <1 arcsec) regions are displayed in Fig. 15. There is agood continuity between the velocities and FWHM measured inthe outer regions and those obtained for the narrow system in re-gions SA, NB and NA. This indicates that we are tracing the samegaseous component. North of the nucleus, an additional broad sys-tem is detected at radii <0.6–0.7 arcsec (regions NA and NB). Thismatches well the location and extent of the ‘jet-like’ structure andsuggests that this broad system is associated with this structure.In particular, region NA corresponds exactly to the area where theslit follows the brightest regions of the ‘jet-like’ structure (see theleft panel of Fig. 15). Assuming that the STIS instrumental resolu-tion is 2.3 Å (STIS instrument handbook for an extended object andthe 52 × 0.2 slit), the FWHM of the broad component in the NAregion is typically �600 km s−1, compared to roughly 150 km s−1

for the narrow component. The centroid velocities of the narrow andbroad components are very similar in region NB and in the north-ern parts of NA, but start to differ as we get closer to the nucleus

(the broad component becomes bluer than the narrow one). Lastly,an extremely blue component (centroid velocity < −400 km s−1) isdetected in region NB. Its weakness prevents us from deriving anaccurate measurement of its spectral characteristics.

South of the nucleus and still within 1 arcsec of it, the velocityfield of the narrow component is very flat, with centroid velocitiesaround +100 km s−1 (Fig. 15). A blue, broad component is also de-tected with velocities and FWHM similar to those of its northerncounterpart, and may represent an extension of the northern coun-terpart across the nucleus. Error bars on the spectral characteristicsof this southern broad component are relatively large due to its lowsignal-to-noise ratio.

As we get close to the nucleus, the [N II] and Hα lines becomemore and more blended (see Fig. 18). In the inner 0.2 arcsec, wewere not able to obtain a stable fit of the emission-line profiles. Arelatively accurate fitting was achieved using three to four systemsof lines, but their spectral properties were not stable at all from onespectrum to the next so this modelling seems useless and is probably

C© 2004 RAS, MNRAS 352, 1180–1198


Figure 17. Line ratios in the inner 2 arcsec of NGC 2110 inferred from the STIS data. Error bars displayed in these plots correspond to 1σ . Left, WFPC2[N II]+Hα narrow-band image of the central 2 arcsec×2 arcsec of NGC 2110 (see Fig. 1) with the position of the 0.2-arcsec wide STIS slit and contours ofthe 3.6-cm VLA radio map (Mundell et al. in preparation; see also Nagar et al. 1999 or Ulvestad & Wilson 1984) at 0.5 and 10 mJy beam−1 superimposed.The image has been rotated so that the direction of the STIS slit (PA = 156◦) is vertical, with north-west at the top. Top central, plot of the [N II]/Hα ratio as afunction of the position along the STIS slit (inner 0.2 arcsec excluded), as derived from single or multiple Gaussian fitting of the STIS data. Top right, same asthe top central panel, but for the [O I]/Hα ratio. Bottom central, same as the top central panel, but for the [S II]/Hα ratio. Bottom right, same as the top centralpanel, but for the [S II]λ6717/[S II]λ6731 ratio.

non-physical. It is also likely that Gaussian profiles are not a goodapproximation to the actual line profiles in these regions (especiallyfor the broad components).

4.4.2 Spectral properties of the ionized gas

The dependence of the lines ratios on radius in the outer regions isshown in Fig. 16. The [N II]/Hα gradient between the southern andnorthern regions of the galaxy (see Fig. 8 in this paper or fig. 8 inGD2002) is clearly detected in the STIS data. North of the nucleus,there is a strong increase in [N II]/Hα as the slit crosses the northernarm of the inverted S-shaped structure. The ratio is close to unity1.5 arcsec north of the nucleus and reaches values of 1.6–1.8 between2 and 2.5 arcsec from the nucleus. As was the case for the wigglesin the velocity field, this gradient is not detected in the lower spatialresolution 3D data (this paper and fig. 8 in GD2002). The OASIS[N II]/Hα ratio north of the nucleus actually reflects the mean ratioover the northern arm. No clear gradient is seen in the [O I]/Hα and[S II]/Hα ratios as the slit crosses the northern arm. There is a hintthat the density sensitive [S II] ratio is lower in this region, but theerror bars are large. This would confirm the results of the OASISobservations (see Fig. 8).

The same curves have been constructed for the inner regions(radii < 1 arcsec) of the galaxy (see Fig. 17). Contrary to whathappened for the velocity field, the [N II]/Hα ratio of the narrowcomponent changes quickly as we cross region NB, to reach values

close to 1.7 in region NA. In the southern regions, the [N II]/Hα

ratio remains relatively low, with values around 1.2–1.3. The broadcomponents display [N II]/Hα ratios very similar to those of thenarrow component. In contrast, the [O I]/Hα and [S II]/Hα ratiosare different for the broad and narrow components.

Overall, the density sensitive [S II] ratio of the narrow componentincreases with radius, indicating that the central regions are denserthen the outer ones (as expected). For signal-to-noise reasons, the[S II] ratio of the broad component could only be estimated in regionNA. It is very low (between 0.5 and 0.7) and indicates electronicdensities >2000 cm−3.

5 C O N C L U S I O N

In this paper, we have presented the results of an extensive set of ob-servations of the type 2 Seyfert galaxy NGC 2110. In the following,we summarize the most important results, the detailed interpretationof which will be presented in a forthcoming paper.

(i) The post-COSTAR HST [N II]+Hα map reveals the detailsof the ‘jet-like’ and inverted S-shaped structures first observed byM+1994. Inside a 6 arcsec×10 arcsec region, we infer a total massof ionized gas of 4–5 × 106 solar masses. The [N II]+Hα morphol-ogy is strongly affected by the extinction features observed in thenew F791W-F200N colour map.

C© 2004 RAS, MNRAS 352, 1180–1198


Figure 18. Profile of the [N II]+Hα blend of lines in the inner 0.2 arcsecof NGC 2110 (STIS data). The position written adjacent to each spectrumcorresponds to the distance from the nucleus along the STIS slit. Units arearbitrary and the spectra have been normalized and shifted vertically to avoidany overlap.

(ii) Examination of various colour maps (this paper, Quillenet al. 1999a,b) shows that the spiral structure is complex (see alsoGD2002). We find no evidence for a classical, well-defined two-armed spiral; instead the overall dust distribution is more consistentwith a tightly wrapped single-armed spiral.

(iii) We have obtained the first 2D map of the stellar velocity fieldin NGC 2110. It is symmetric about the nucleus, both in the opticaland in the near-infrared, with an amplitude of ±160–200 km s−1. Wetherefore confirm and extend the results of the long-slit observationsof GD2002. We also found that the kinematic major axis of thegalaxy is very close to the photometric one.

(iv) In the inner 1 arcsec, the bulk of the emission of the [O III]narrow system is not associated with the nucleus but with the ‘jet-like’ structure. This is not the case for the Hα narrow system.

(v) The asymmetry in the velocity field of ionized gas discov-ered by WB1985 and Wilson et al. (1985) and recently mapped byGD2002 is clearly detected in our observations. Strong differencesbetween the stellar and ionized gas kinematics are observed south ofthe nucleus. This asymmetry is also present at the same level in thevelocity field of near-infrared lines, indicating that it is not a redden-ing effect. We suggest that contrary to what was assumed in previouspapers, the northern arm is, in part, responsible for the asymmetryof the velocity field. The difference between the stellar and ionizedgas kinematics south of the nucleus is in fact not surprising giventhat the stellar velocities are expected to be systematically smallerthan those of a dynamically colder (gaseous) system. This pictureis consistent with the Seyfert-type emission-line ratios detected onthe northern side of the nucleus.

(vi) We detect the region of high FWHM reported by GD2002 inour Hα and [O III] narrow system FWHM maps. This indicates thatthis broadening is not due to a stronger contribution of the broadcomponent but is intrinsic to the narrow one.

(vii) The gradient of excitation between the southern and northernregions of NGC 2110 (WB1985; Wilson et al. 1985; GD2002) isclearly detected in the [O III]/Hβ and [N II]/Hα ratio maps.

AC K N OW L E D G M E N T S

This paper is based on observations collected at the Canada–France–Hawaii Telescope, which is operated by CNRS of France, NRC ofCanada and the University of Hawaii, and on observations with theNASA/ESA HST obtained at the Space Telescope Science Institute,which is operated by the Association of Universities for Researchin Astronomy, Inc., under NASA contract No. NAS 5-26555. PFacknowledges support by the Programme National Galaxies, underfunding of the programme ‘Le budget energetique des nebulositesetendues dans les galaxies de Seyfert’. This research was supportedin part by NASA through HST grant No. GO8610.

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This paper has been typeset from a TEX/LATEX file prepared by the author.

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Ionized gas and stars in the central kiloparsec of the type 2 Seyfert galaxy NGC 2110 - I. The data

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Transcript of Ionized gas and stars in the central kiloparsec of the type 2 Seyfert galaxy NGC 2110 - I. The data