THE ASTROPHYSICAL JOURNAL, 481 :735È740, 1997 June 11997. The American Astronomical Society. All rights reserved. Printed in U.S.A.(

AN ULTRAVIOLET AND NEAR-INFRARED VIEW OF NGC 4214 : A STARBURSTINGCORE EMBEDDED IN A LOW SURFACE BRIGHTNESS DISK

MICHAEL N. WILLIAM W. DENISE A. WENDY L.FANELLI,1,2,3 WALLER,1,3 SMITH,1,4 FREEDMAN,5BARRY SUSAN G. ROBERT W. MORTON S.MADORE,6 NEFF,1 OÏCONNELL,7 ROBERTS,8

RALPH ANDREW M. AND THEODORE P.BOHLIN,9 SMITH,1 STECHER10Received 1996 July 5 ; accepted 1996 December 31

ABSTRACTDuring the Astro-2 Spacelab mission in 1995 March, the Ultraviolet Imaging Telescope (UIT)

obtained far-UV (j \ 1500 imagery of the nearby Sm/Im galaxy NGC 4214. The UIT images have aA� )spatial resolution of D3A and a limiting surface brightness, mag arcsec~2, permitting detailedk1500 [ 25investigation of the intensity and spatial distribution of the young, high-mass stellar component. Thesedata provide the Ðrst far-UV imagery covering the full spatial extent of NGC 4214. Comparison with acorresponding I-band image reveals the presence of a starbursting core embedded in an extensive lowsurface brightness disk.

In the far-UV (FUV), NGC 4214 is resolved into several components : a luminous, central knot ; aninner region kpc) with D15 resolved sources embedded in bright, di†use emission ; and a popu-(r [ 2.5lation of fainter knots extending to the edge of the optically deÐned disk (r B 5 kpc). The FUV light,which traces recent massive star formation, is observed to be more centrally concentrated than theI-band light, which traces the global stellar population. The FUV radial light proÐle is remarkably wellrepresented by an R1@4 law, providing evidence that the centrally concentrated massive star formation inNGC 4214 is the result of an interaction, possibly a tidal encounter, with a dwarf companion(s).

The brightest FUV source produces D8% of the global FUV luminosity. This unresolved source, cor-responding to the Wolf-Rayet knot described by Sargent & Filippenko, is located at the center of theFUV light distribution, giving NGC 4214 an active galactic nucleusÈlike morphology. Another strongsource is present in the I band, located 19A west, 10A north of the central starburst knot, with no FUVcounterpart. The I-band source may be the previously unrecognized nucleus of NGC 4214 or an evolvedstar cluster with an age greater than D200 Myr. The global star formation rate derived from the totalFUV Ñux is consistent with rates derived using data at other wavelengths and lends support to the sce-nario of roughly constant star formation during the last few hundred million years at a level signiÐcantlyenhanced relative to the lifetime averaged star formation rate. The hybrid disk/starburst-irregular mor-phology evident in NGC 4214 emphasizes the danger of classifying galaxies based on their high surfacebrightness components at any particular wavelength.Subject headings : galaxies : irregular È galaxies : individual (NGC 4214) È infrared : galaxies È

ultraviolet : galaxies

1. INTRODUCTION

Ultraviolet imaging enables detailed investigation ofextreme Population I material in disk galaxies by““ Ðltering ÏÏ out the light of the underlying cool stellar popu-lation that dominates the luminosity at optical/near-infrared (NIR) wavelengths. In late-type galaxies, the globalextinction is often found to be low, mag (e.g.,A

V\ 1 Hunter

1 Laboratory for Astronomy and Solar Physics, Code 681, NASA/Goddard Space Flight Center, Greenbelt, MD 20771.

2 fanelli=kuylym.gsfc.nasa.gov.3 Hughes STX Corporation, 4400 Forbes Boulevard, Lanham, MD

20706.4 National Research Council Postdoctoral Fellow.5 Carnegie Institute of Washington, 813 Santa Barbara Street, Pasa-

dena, CA 91101.6 IPAC, California Institute of Technology, Pasadena, CA 91125.7 Astronomy Department, University of Virginia, Charlottesville, VA

22903.8 National Radio Astronomy Observatory, Edgemont Road, Charlot-

tesville, VA 22903.9 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore,

MD 21218.10 Laboratory for Astronomy and Solar Physics, Code 680, NASA/

Goddard Space Flight Center, Greenbelt, MD 20771.

& Gallagher permitting direct imaging of young,1986),massive stellar populations with good signal-to-noise ratiosin acceptable exposure times.

Of particular interest is the connection between thepresent-day far-ultraviolet (FUV) appearance of galaxiesand the morphology of systems at high redshift. For zB 2,the FUV is redshifted to D4500 Visible-band images ofA� .high-redshift systems taken with 8È10 m class telescopes, ororbiting telescopes, sample the redshifted FUV portion of agalaxyÏs spectral energy distribution. Therefore, an under-standing of the FUV properties of local systems is essentialfor exploring galaxy evolution at high redshift.

NGC 4214 is a nearby (DD 4.2 Mpc), bright (BT

\ 10.24mag), blue (B[V \ 0.46) late-type galaxy exhibiting manyof the tracers for vigorous, recent high-mass star formation.As one of a few ““ prototype ÏÏ irregular galaxies, NGC 4214has been studied in terms of its multiwavelength morphol-ogy gas kine-(Allsopp 1979 ; Hunter 1982 ; Schild 1984),matics Geller, & Huchra and star(Hartmann, 1986),formation history et al. Hunter, &(Huchra 1983 ; Gallagher,Tutukov et al. & Filippenko1984 ; Thronson 1988 ; Sargent

hereafter et al. Optically, NGC1991, SF91; Leitherer 1996).4214 shows a blue, high surface brightness inner region,

735

736 FANELLI ET AL. Vol. 481

vaguely S-shaped, with nascent spiral segments extendingfrom this central region at position angles of D160¡ andD340¡. Ðrst noted a fainter, more symmetricSchild (1984)disk using imagery in the V and I bands. This disk exhibitsan exponential light proÐle for 35A \ r \ 125A, and its cen-troid is o†set from the brightest continuum knot. Thecentral regions of NGC 4214 possess several bright knots ;their photometric properties are consistent with clusters ofOB stars et al. Optical spectra(Hunter 1982 ; Huchra 1983).presented by & Filippenko indicate thatSargent (1991)several knots are unusual, exhibiting emission lines charac-teristic of Wolf-Rayet stars. The brightest knot (their knot1) also shows broad Ha emission with a FWZI of D2900km s~1, which is attributed to the WR stars.

The star formation history of NGC 4214 has been studiedby et al. using infrared and CO observ-Thronson (1988)ations and by et al. with IUE spectroscopyHuchra (1983)and optical/NIR photometry. Huchra et al. favor a model inwhich D5% of the mass has participated in a recent burst ofstar formation superposed on an older stellar populationwith a relatively constant star formation rate (SFR).

et al. Ðnd that the total SFR is B0.5È1Thronson (1988) M_yr~1, a rate which has been maintained within a factor of 4

for the last billion years. The central SFR, averaged over aHubble time, however, appears to be much lower than thecurrently observed rate.

Available satellite UV imagery (j \ 3000 of galaxies isA� )still limited. The Ultraviolet Imaging Telescope (UIT), partof the Shuttle-based Astro Observatory, was designed toprovide wide-Ðeld, photometric imagery of a variety of cel-estial targets with good spatial resolution. In this paper wepresent the Ðrst full-Ðeld FUV imagery of NGC 4214. TheUIT data complement a Hubble Space Telescope (HST )Faint Object Camera (FOC) image (j D 2200 discussedA� )by et al. of the central 22A ] 22A. In weLeitherer (1996) ° 2discuss the observations. In the morphology of massive° 3star formation as observed in the UIT images is describedand compared with the global morphology visible atoptical/NIR wavelengths. Some implications of these dataare discussed in ° 4.

2. OBSERVATIONS

NGC 4214 was observed by the UIT during the Astro-2mission on 1995 March 13. The UIT obtained exposures of106 and 1060 s in the broad-band B1 Ðlter (jeff B 1520 A� ,*j B 350 The UIT images were recorded on IIaO ÐlmA� ).with a D40@ Ðeld of view (FOV) and a spatial resolution ofD3A. Flight Ðlm was digitized with a PDS microdensi-tometer, producing an image scale of 1.14A pixel~1. Conver-sion of linearized intensity units to Ñux was done using IUEspectrophotometry of stars observed with UIT. A detaileddiscussion of the telescope, detectors, and standard pro-cessing of UIT data is in et al. No sky back-Stecher (1992).ground was detected in the long exposure, mag(k1500\ 26arcsec~2). The lack of a measurable background is not sur-prising, given the darkness of the FUV sky et al.(Waller

The principal photometric uncer-1995 ; OÏConnell 1987).tainty is produced by low-level nonuniformities attributedto the development and digitization of the Ðlm. Magnitudesare expressed in the monochromatic system, logmj\ [2.5

where is the incident Ñux density in ergs s~1Fj [ 21.1, Fjcm~2 A� ~1.Direct CCD images in the Johnson B and Cousins I

bands were obtained with the Palomar 5 m telescope in

1994 February, as part of the Freedman et al. Astro-2 GuestInvestigator program. Images were obtained in good seeingbut nonphotometric conditions with a FOV of 9.7@. AsNGC 4214 almost Ðlls this Ðeld, background measures areproblematic. For this paper we utilize the I-band image.Although its absolute intensity level is unknown, a radiallight proÐle can be derived from these data for comparisonto the FUV.

3. MULTIWAVELENGTH MORPHOLOGY

3.1. Far UltravioletIn (Plate 27) the FUV image of NGC 4214 isFigure 1

presented. The FUV emission is patchy and centrally con-centrated with no obvious global symmetry. High surfacebrightness knots are visible, embedded in di†use emission at

(2.5 kpc) with three distinct knots observed in ther [ 2@center. The brightest source, which we refer to as the ““ UVnucleus ÏÏ has an unusually intense peak surface brightnessD15.9 mag arcsec~2 and corresponds to the Wolf-Rayetknot described by & Filippenko andSargent (1991)

et al. as ““ knot 1.ÏÏ A second UV knot isLeitherer (1996)present, separated by D11A at P.A.D 113¡ with k1500 D 1.2mag fainter. Approximately 16A south of the secondary UVpeak is a J-shaped structure marginally resolved into fourknots. The central region is surrounded by strong di†useemission, ellipsoidal in extent, with a major axisP.A.D 153¡ ^ 5¡. This di†use emission declines to a surfacebrightness level mag arcsec~2 at a radial dis-k1500 [ 25tance of D2@. At larger radii D20 distinct sources arevisible ; the most distant is located 5.1@ from the UV nucleus,

corresponding to a projected distance of 6.2(D1.2 ] R25),kpc.FUV light is produced by stars with stars ofM Z 2 M

_;

lower mass are too cool to emit signiÐcant light at thesewavelengths et al. Early B stars are the domi-(Fanelli 1992).nant luminosity source at 1500 except in very youngA� ,starbursts where the shorter lived O stars will also beimportant. The relative contribution of O and B stellartypes depends on the duration and initial mass function ofthe burst. The knots seen in are OB star clusters,Figure 1the youngest clusters providing the ionizing sources for thebright Ha emission described by andHunter (1982) SF91.

The origin of the spatially di†use UV light is more prob-lematic. Di†use light can be produced by scattering of star-light by dust grains, nebular continuum and line emission,and a distributed, spatially unresolved population of hotstars. A UV-bright, di†use stellar component can arise fromB stars formed during the last few hundred million yearsand then spatially dispersed during their lifetimes. Theintensity of di†use stellar UV-light depends the recent starformation history and on several poorly known physicalparameters, e.g., the initial mass function of OB stars, thefraction of OB stars formed in bound clusters versus thoseformed in unbound associations, and the fraction of clustersthat remain bound (and identiÐable as discrete sources)during their UV-luminous phase.

Emission lines are not expected to be important in theFUV band. Although the UV imagery cannot distinguishbetween dust-scattered light and direct radiation from unre-solved stars, the optical colors, metallicity, molecularcontent et al. and low extinction et(Becker 1995), (Leithereral. indicate only modest amounts of dust. We believe1996)that starlight is the primary source of the di†use emission,

0 50 100 150 200 25026

24

22

20

18

16

Radius (arcsecs)

µ

(

mag

s ar

csec

-1)

FU

V

Far-UV

I-band

1.0 1.5 2.0 2.5 3.0 3.5 4.0

26

24

22

20

18

16

Radius (arcsec)0.25 0.25

µ

(m

ags

arcs

ec -1

)F

UV

No. 2, 1997 ULTRAVIOLET AND NEAR-IR VIEW OF NGC 4214 737

produced by massive stars with ages Myr, (M [ 4[200spectral types earlier than DB9).M

_,

The FUV data provide a snapshot of the spatial distribu-tion of high-mass stars. We note three major attributes :(1) the FUV light is centrally concentrated, (2) it is wellresolved into distinct knots, and (3) the brightest observedUV source is found to be at the center of the global FUVlight distribution.

3.2. I Band(Plate 28) shows the CCD image obtained in theFigure 2

I-band. In the nuclear region, the starburst knots areprominent, indicating that the young population dominatesthe light distribution even at near-IR wavelengths. But theglobal morphology is strikingly di†erent ; the I-band lightshows much greater symmetry than that observed in theFUV. A smooth, azimuthally symmetric disk, Ðrst describedby is visible with no FUV counterpart. Addi-Schild (1984),tional UV-bright knots are also visible in the I band, buttheir contrast against the di†use (unclustered) light isgreatly reduced compared with the contrast at FUV wave-lengths. The bar-shaped region at may not be ar [ 30Astructure of the old population but simply an illusioncreated by the chain of bright knots, also suggested bySchild. However, the I-band contours are distinctly twistedat intermediate radii, indicating some global perturbationin the stellar disk. At large radii the isophotes become morecircular.

If the extended, axisymmetric distribution of I-band lightis produced by stars, a sufficiently deep image, in theabsence of background light, should detect the FUV emis-sion from these stars. At locations not coincident with UV-bright knots, the stellar population is expected to becomposed of older stars, which produce minimal FUV light.The (FUV[ V ) colors for an F0 star (main-sequence life-time D2 Gyr) is approximately ]5, for a G2 dwarf, approx-imately ]9. reports a V -band surfaceSchild (1984)brightness of 23.0 mag arcsec~2 at a radius of D90A (I-bandsurface photometry is not available). Because the expectedFUV surface brightness from an old population would be

several magnitudes fainter than the FUV detection limitmag arcsec~2), this emission is not detected in(k1500 Z 25.5

the UIT image.shows additional sources not present in theFigure 2

FUV image. A bright source, denoted here 4214-I1, is seen19A west, 10A north of the central knot, a second source islocated D16A northwest of 4214-I1. Foreground galacticÐeld stars are present at this wavelength range, but webelieve the chance of two objects projected onto the near-nuclear region of NGC 4214 and aligned with the UV knotsto be low. 4214-I1 is marginally extended with a measuredFWHM approximately twice that of Ðeld stars in this frame.Both sources are visible in the red *jD 70(j0D 6477, A� )continuum image presented by and are further dis-SF91cussed below.

3.3. Comparison of the Radial and Azimuthal L ightThe radial dependence of the light distribution is mea-

sured by integrating the observed intensity within circularannuli of increasing radius, centered on the UV nucleus.The choice of circular apertures was dictated by the opticalmorphology ; the disk is observed with an approximatelyface-on orientation. Contaminating foreground stars havebeen masked out of the I-band image. No foregroundobjects are evident in the FUV data. The azimuthally aver-aged radial light proÐles are displayed in BecauseFigure 3a.of the absence of an absolute calibration, the I-band proÐlehas been arbitrarily normalized to the FUV at r \ 30A.Uncertainties in the subtracted I-band sky background areshown as lines bracketing the measured proÐle.

The FUV-light radial-light distribution is not wellapproximated by an exponential decline. A steep innerregion is seen in the FUV, due to both the UV nucleus andthe surrounding starburst regions, which is not present inthe I band. The I-band proÐle displays less structure anddeclines exponentially to r D 100A (2 kpc). At larger radiithere appears to be excess light over that expected byextrapolating an exponential proÐle from the inner region.Since NGC 4214 almost Ðlls the CCD Ðeld of view, thisexcess light may arise from uncertainty in the sky back-

FIG. 3a FIG. 3b

FIG. 3.ÈRadial surface brightness proÐle of NGC 4214 in the FUV and the I band derived using circular apertures centered on the FUV nucleus. Theerror envelope (solid and dashed lines) reÑects the uncertainties in the incident Ñux and the sky backgrounds. The I-band image is not absolutely calibrated,therefore the I-band radial proÐle has been arbitrarily normalized to the FUV proÐle at a radius of 30A. (a) Surface brightness vs. radius in arcseconds. TheFUV light displays a steeper radial proÐle than the I-band light, implying that massive stars are more concentrated than the underlying disk population.(b) Surface brightness vs. radius~1@4 in arcseconds. The FUV light is well represented by an R~1@4 law. The straight line illustrates the expected behavior inthese units for an R1@4 proÐle and is not a Ðt to the data.

11 12 13 14 15 16 170

2

4

6

8

N(m

)

FUV Magnitude

738 FANELLI ET AL. Vol. 481

ground but may also reÑect a real divergence from an expo-nential decline. Additional observations are planned toresolve this issue.

In the FUV radial light proÐle is displayed as aFigure 3bfunction of radius~1@4. We Ðnd that the FUV light isremarkably well-represented by an R1@4 law, especially atr \ 40A(D0.8 kpc). An R1@4 light proÐle is characteristic ofdynamically hot systems, such as elliptical galaxies or spiralbulges, which are posited to be old, relaxed stellar systems.However, in NGC 4214, the R1@4 proÐle is produced byyoung, Population I stars. Lacking kinematical informa-tion, we cannot address the dynamical state of the youngstellar component.

et al. suggest an interaction/merger sce-Hartmann (1986)nario to explain the velocity gradients observed in theionized gas. Mergers between disk galaxies can produceoptical r1@4 proÐles (e.g., et al.Schweizer 1996 ; Bergvall

However, NGC 4214 does not display the strongly1989).disturbed morphology typical of major mergers ; in particu-lar, the I-band disk shows smooth outer contours. Morelikely, NGC 4214 is interacting with the nearby dwarfirregular companion NGC 4190, as suggested by Allsopp

The H I distribution in NGC 4214 is extended(1979).toward NGC 4190 which are separated by(Allsopp 1979),29.5@, corresponding to D36 kpc on the sky.

We suggest that the FUV light distribution providesadditional evidence that the centrally concentrated massivestar formation in NGC 4214 is the result of a tidal encoun-ter with a dwarf companion. We note that the radial FUVlight proÐle of the peculiar Sbc galaxy NGC 3310, whichshows more robust evidence for a recent merger/interaction,is also described by an R1@4 law et al.(Smith 1996).

There are currently few extant FUV light proÐles of gal-axies for comparison. et al. present multi-Cornett (1994)band surface photometry for the face-on ScI galaxy NGC628, whose FUV radial proÐle shows an approximatelyexponential decline. They Ðnd that the disk continuum scalelengths decrease with increasing wavelength, such that theratio of FUV to R-band scale lengths is D1.5. In NGC 4214the sense of the radial light proÐle is opposite ; the FUV lightis more centrally concentrated than the red light. The con-trast between the FUV and I-band morphology is furtherillustrated in (Plate 29), where the I-band intensityFigure 4contours are overlain on the FUV image. At the currentepoch, massive star formation is centrally concentrated,although aggregates of OB stars are detected at all radii.

3.4. T he UV Knot PopulationWe have measured the integrated FUV intensity of 30

sources to estimate their OB star content. The distributionof measured FUV magnitudes is presented in noFigure 5 ;intrinsic extinction corrections have been applied. For com-parison, a single early-O dwarf with andM

V\[5.5

(FUV[ V ) \ [4.7 et al. would have an(Fanelli 1992)FUV magnitude\ 17.9 for the assumed distance. Therange of observed magnitudes is The11.6\mFUV\ 16.3.faintest knot included in is equivalent to D5 O5 VFigure 5stars. These data illustrate the potential of UV imagery toisolate and measure small concentrations of young stars inotherwise complex stellar populations.

The UV nucleus is the brightest FUV source observed inNGC 4214. The integrated intensity of this source is

ergs cm~2 s~1 ~1, equivalent to D340F1500D 7.8] 10~14early-O dwarfs. et al. Ðnd that this knotLeitherer (1996)

FIG. 5.ÈThe distribution of observed FUV magnitudes for young starclusters. No correction for internal extinction has been applied ; Galacticextinction is negligible. At the adopted distance of 4.2 Mpc, a single O5dwarf would have an (unextincted) FUV magnitude of D18, whereas a 3Myr old cluster with M \ 104 (mass range 100È1 power-lawM

_M

_,

IMF with an exponent \ [1.35, would have mag.Z\Z_) mFUVD 13.7

contains roughly twice the number of O stars found in 30Doradus in a region of comparable size and produces lessthan D0.5% of the B-band luminosity. At FUV wave-lengths it produces D8% of the total observed intensity. Ifthe extinction applicable to this source at 1500 is similarA�to that adopted by Leitherer et al., the equivalent number ofmassive stars would be approximately doubled. We notethat the brightest starburst knot is centrally located withrespect to the FUV light distribution. As the disk isobserved to be only slightly inclined to the plane of the sky,the UV nucleus is physically located near the morphologi-cal center of the galaxy.

3.5. Global PropertiesThe total FUV Ñux is D9.6] 10~13 ergs cm~2 s~1 ~1,

corresponding to mag. Using integrated mag-mFUV\ 8.95nitudes from the 3RC Vaucouleurs et al.(de 1991),(FUV[ B) \ [1.29 and (FUV[ V ) \ [0.83. Galacticextinction is assumed to be negligible along this line ofsight, although we note that E(B[V ) \ 0.02 will produceD0.15 mag of FUV extinction for the standard Galacticreddening relation ; FUV to optical colors are at leastuncertain to this degree. These values are e†ected by anunknown amount of intrinsic extinction, but in general theyshould be indicative of irregular galaxies with a modestlevel of massive star formation. They can be directly com-pared to the colors of high-redshift systems observed withHST and the new generation of 8È10 m class telescopes.

4. DISCUSSION

To aid interpretation of the FUV luminosity of NGC4214, we construct simple starburst models for two limiting

No. 2, 1997 ULTRAVIOLET AND NEAR-IR VIEW OF NGC 4214 739

cases : a 3 Myr burst with an initial mass function (IMF)slope \ [1.35 and a mass range Z\ 0.4100 [ M

_[ 2,

and a continuous star formation (CSF) model with theZ_

,same parameters. The model uses current evolutionarytracks et al. et al. and(Schaller 1992 ; Schaerer 1993) Kurucz

model atmospheres to create synthetic spectra at(1992)each model time step.

In the CSF model the intrinsic FUV luminosity reachesan asymptotic value after D200 Myr for this set of modelparameters. The observed total FUV luminosity implies astar formation rate of 0.05 yr~1. If half the intrinsic UVM

_luminosity is absorbed, the star formation rate woulddouble. We emphasize that this rate applies only to starsmore massive than 2 FUV data provide no constraintsM

_.

on the formation history of lower mass stars, as these starscontribute negligible Ñux at 1500 Extending the modelA� .IMF to with the Salpeter slope wouldMlower\ 0.1 M

_imply a total formation rate D0.2 yr~1. These rates areM_dependent on both the assumed slope of the IMF at low

masses and the assumed extinction. Given the uncertainties,the star formation rate estimated from the FUV data isconsistent with those derived by Hunter, &Gallagher,Tutukov and supports the scenario of roughly con-(1984)stant star formation over the most recent few hundredmillion years.

NGC 4214 contains sufficient neutral gas to maintain thisrate for a Hubble time, MH ID 1.4] 109 M

_(Huchtmeier

& Seiradakis However, the H I distribution is sub-1985).stantially extended & Seira-(Allsopp 1979 ; Huchtmeierdakis while the FUV data indicate that recent1985),massive star formation is strongly centrally concentrated.About 35% of the total FUV light arises from the inner 30A,corresponding to a radius of D600 pc. As noted in ° 3.3,perturbations from nearby dwarf companions have beensuggested by et al. as the source for theHartmann (1986)disturbed velocity Ðeld seen in the ionized interstellarmedium and by to explain the complexAllsopp (1979)structure observed in the H I distribution. Both authorsprovide evidence for a kinematically disturbed disk. GasinÑow caused by the same interaction is a plausible mecha-nism for fueling the current centrally enhanced star forma-tion episode. Molecular gas has been detected in thenear-nuclear region and mapped by et al.Becker (1995).Higher resolution H I and CO observations are needed toprobe more fully the central gas kinematics.

To illustrate the nuclear knot structure, we present acomparison of the central region in (Plate 30). TheFigure 6striking aspect of is the bright knots visible in the IFigure 6band that are absent in the FUV. The I-band source north-west of the UV nucleus may be a compact nucleus withinNGC 4214. noted that NGC 4214 did notSchild (1984)appear to have a nucleus and that the nuclear knots haveoptical colors of H II regions. We are unable to determine acolor for this source with the present data set, but theabsence of any enhancement in the FUV light at its location

indicates that this source does not contain substantialnumbers of OB stars. An archival HST WFPC2 image(F606W) shows an enhancement of faint sources surround-ing 4214-I1 compared to adjacent regions, lending strongsupport to its intrinsic nature.

If not a true ““ nucleus,ÏÏ 4214-I1 may be an evolvedversion of the star cluster identiÐed as the UV nucleus. Forthe single generation cluster model described above (100[

the FUV luminosity declines by 2.4, 4.7, andM/M_

[ 2),5.9 magnitudes for t \ 20, 100, and 200 Myr, respectively.For t D 200 Myr, the turno† mass is D4 spectral typeM

_,

B8. By this age, an optically prominent star cluster wouldproduce little FUV Ñux and would be difficult to detectagainst the di†use UV light visible in NGC 4214.

If massive star formation has been approximately contin-uous, then clusters similar to that powering the UV nucleusshould also form continuously, unless massive star forma-tion and superÈstar cluster formation are decoupled pro-cesses. Such clusters are FUV luminous beyond theirionizing stage and should easily be detected in nearbysystems with space-based UV imagers up to ages D100È200Myr. 4214-I1 may be such a cluster, sufficiently evolved thatits FUV Ñux has faded several magnitudes, yet sufficientlymassive to be easily visible against the galactic backgroundat longer wavelengths. Physically it may be a compactnucleus, or an evolved superÈstar cluster with an agegreater than 200 Myr. These data illustrate the potential formapping the spatial pattern of massive cluster formationover the last Gyr, when good spatial resolution and UVsensitivity are available.

Although NGC 4214 is classiÐed as a Magellanic““ irregular,ÏÏ the dominant feature of its I-band morphologyis an azimuthally symmetric disk with a uniform radialdecline in surface brightness. Massive star formation is con-centrated in the inner galaxy, where it dominates the lightdistribution even in the NIR. Comparison of the FUV andI-band morphology emphasizes the long-recognized dangerof classifying systems based on their high surface brightnesscomponents at a particular wavelength. Most of thepublished images of NGC 4214 & Tammann(Sandage

& Bedke highlight the central, blue1974 ; Sandage 1988)star forming region. We suggest that similar selection e†ectscomplicate interpretation of galaxies observed at high red-shift with HST . These images may not reÑect the true globalstructure.

This paper is based on observations with the UltravioletImaging Telescope Ñown on the Shuttle-based Astro-2Spacelab mission. We thank the many people who contrib-uted to the success of the Astro-2 mission, especially ourpayload specialist, Ron Parise, and the crew of STS 67. Wealso thank the referee for several constructive commentsthat measurably improved this work. UIT research wassupported by the Spacelab Office under project 440-51.

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FIG. 1.ÈFar-UV (j D 1500 image of NGC 4214 obtained with the Ultraviolet Imaging Telescope from Shuttle Endeavour on 1995 March 13. ThisA� )image is a combination of 1060 and 106 s exposures, taken through the UIT B1 Ðlter. For a constant energy distribution, for this Ðlter. Intensityjeff \ 1520 A�is shown as surface brightness in units of magnitudes arcsec~2 in the range An angular size of 1@ corresponds to a physical scale of D1.2 kpc16 \ kFUV\ 24.at the adopted distance of 4.2 Mpc. The box outline deÐnes the Ðeld of view of the I-band CCD image displayed in Fig. 2.

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FIG. 2.ÈI-band image of NGC 4214 (j D 8000 obtained with the Palomar 5 m telescope on 1994 February 13 and 14. The orientation is the same asA� )The image is not calibrated and is displayed with a logarithmic stretch to enhance the appearance of the faint, azimuthally symmetric disk in which theFig. 1.

star-forming core is embedded.

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FIG. 4.ÈI-band intensity contours overlain on the FUV image. The images were aligned by assigning the peak Ñux of the UV nucleus to the same pixel.The azimuthal symmetry of the I-band light contrasts sharply with the elongated and centrally concentrated FUV light, yet FUV emission extends to theedge of the optically deÐned disk.

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FIG. 6.ÈCentral region of NGC 4214 in the FUV (upper panel) and the I band (lower panel). The extracted regions are 175A ] 87A in size, corresponding to3.6] 1.8 kpc at the adopted distance. The ““UV nucleus ÏÏ is centered in each panel. Note the two bright sources northwest of the UV nucleus in the I-bandimage that lack FUV counterparts. The brighter of these two sources may be the previously unrecognized nucleus of NGC 4214.

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