THE ASTROPHYSICAL JOURNAL, 536 :266È276, 2000 June 102000. The American Astronomical Society. All rights reserved. Printed in U.S.A.(

JET-INDUCED STAR FORMATION IN CENTAURUS A1JEREMY R. MOULD,2 ALEX RIDGEWELL,2 JOHN S. GALLAGHER III,3 MICHAEL S. BESSELL,2 STEFAN KELLER,2

DANIELA CALZETTI,4 JOHN T. CLARKE,5 JOHN T. TRAUGER,6 CARL GRILLMAIR,7 GILDA E. BALLESTER,5CHRISTOPHER J. BURROWS,4 JOHN KRIST,4 DAVID CRISP,6 ROBIN EVANS,6 RICHARD GRIFFITHS,8

J. JEFF HESTER,9 JOHN G. HOESSEL,3 JON A. HOLTZMAN,10 PAUL A. SCOWEN,9KARL R. STAPELFELDT,6 RAGVENDRA SAHAI,6

ALAN WATSON,11 AND VICKI MEADOWS6Received 1999 October 19 ; accepted 2000 January 20

ABSTRACTThe inner part of the northeast middle radio lobe of the radio galaxy Centaurus A is the site of

complex interactions. This area contains a large H I cloud as well as Ðlaments of ionized gas and associ-ated blue knots, several of which exist along the northeastern edge of the radio-emitting zones. Weobserved the Ðlaments and blue knots with the Hubble Space Telescope using WFPC2, and the ionizedgas from the ground. Our sensitive, high angular resolution WFPC2 images reveal the presence of youngstars, many concentrated in what appear to be OB associations, superimposed on a background sheet ofolder stars that is typical of the Cen A halo. The ages of the OB associations are estimated to be lessthan 15 Myr from a comparison of color-magnitude diagrams with those for the Large MagellanicCloud star cluster NGC 2004, and younger stellar groups may be nearer regions of Ha emission. Wediscuss our data in the context of models for star formation stimulated by interactions between the radiojet and gas cloud.Subject headings : galaxies : individual (NGC 5128) È galaxies : jets È galaxies : stellar content È

radio continuum: galaxies

1. INTRODUCTION

Centaurus A is the closest radio galaxy. As such, it o†ersthe best opportunity to observe the e†ects of the transfer ofenergy from the active nucleus of a galaxy into the inter-galactic medium by means of the jet phenomenon (see Mor-ganti et al. 1991, 1992). We can also see in graphic detailwhat fuels the activity, in this case the intrusion into anelliptical galaxyÏs potential of a low-mass galaxy, rich in gasand dust. Secondary e†ects of this collision were docu-mented some time ago (Malin, Quinn, & Graham 1983). Ageneral review on the galaxy has been published by Israel(1999).

We previously observed a Ðeld in the southern stellarhalo of NGC 5128 (Soria et al. 1996) and found nothingunusual about the stellar population. We saw the old metal-

1 Based on observations made with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which is oper-ated by the Association of Universities for Research in Astronomy, Inc.,under NASA contract NAS 5-26555.

2 Research School of Astronomy and Astrophysics, Institute ofAdvanced Studies, Australian National University, Mount Stromlo Obser-vatory, Private Bag, Weston Creek Post Office, ACT 2611, Australia.

3 Department of Astronomy, University of Wisconsin at Madison, 475North Charter Street, Madison, WI 53706-1582.

4 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore,MD 21218.

5 Department of Atmospheric and Oceanic Sciences, University ofMichigan, 2455 Hayward, Ann Arbor, MI 48109.

6 Jet Propulsion Laboratory, 4800 Oak Grove Drive, Mail Stop 183-900, Pasadena, CA 91109.

7 IPAC, California Institute of Technology, Pasadena, CA 91125.8 Department of Physics, Carnegie-Mellon University, Wean Hall, 5000

Forbes Avenue, Pittsburgh, PA 15213.9 Department of Physics and Astronomy, Arizona State University,

Tyler Mall, Tempe, AZ 85287.10 Department of Astronomy, New Mexico State University, Box

30001, Department 4500, Las Cruces, NM 88003-8001.11 Instituto de Astronomia UNAM, 58090 Morelia, Michoacan,

Mexico.

rich population one expects to see in a luminous elliptical,set an upper limit on the size of its middle-aged stellarpopulation, and concluded that the bulk of the stars formedat least half of the age of the universe ago. That Ðeld willserve as the control sample for the current investigation intothe age of the stellar population in the Ðeld studied byGraham (1998).

In the Graham Ðeld, the extension of the nuclear jet(Clarke, Burns, & Feigelson 1986) intersects one of threeisolated H I clouds kpc from the nucleusZ15(Schiminovich et al. 1994 ; Morganti et al. 1999). This is oneof the regions near the edge of the northern radio lobe,where the presence of blue knots was noted by V. Blancoand ionized gas studied by Graham & Price (1981 ; see alsoMorganti et al. 1991 ; Graham 1998). The structure of thiszone and the excitation mechanism for the gas continue tobe debated, with suggestions including photoionization bylight beamed from the active nucleus of Cen A, shocksinduced by the relativistic plasma responsible for the radiolobe, and stellar photoionization (see discussions by Schi-minovich et al. 1994 ; Graham 1998, 1999 ; Morganti et al.1999). In the work presented here we examine stellar popu-lations within the region where the inÑuence of the activecomponent of the galaxy is likely to be important. We haveresolved clumps of young stars, conÐrm the existence ofrecently formed OB associations, and discuss their charac-teristics in terms of induced star formation models.

2. PHOTOMETRY

The images were taken on 1997 December 28 by theWFPC2 instrumentation aboard the Hubble SpaceTelescope (program 6869). The observations comprise one1100 s and two 1200 s images taken with the F814W Ðlterand three 1100 s images taken with the F555W Ðlter. TheÐeld of view is at least 15 kpc from the nucleus of NGC 5128and is identiÐed in Figure 1.

266

JET-INDUCED STAR FORMATION IN CEN A 267

FIG. 1.ÈPosition of our Ðeld in the Digitized Sky Survey. North is up ; east is to the left.

Apparent magnitudes were derived from the standardWFPC2 point-source functions developed for the KeyH0Project using the DAOPHOT/ALLFRAME ensemble ofprograms ; see Stetson (1994) for a complete description ofthis software. We also employed the Key ProjectÏs photo-

metric zero points (Hill et al. 1998). No correction was madefor charge transfer efficiency losses (Stetson 1998).

The three exposures from Ðlter F555W and the threefrom Ðlter F814W were combined to form a single image ofall six frames for each chip in the WFPC2. The WFPC2

TABLE 1

TWO BRIGHTEST STARS IN EACH CHIP

STAR POSITION MAGNITUDE

R.A. DECL.ID Chip X Y (2000) (2000) V V [I

R1 . . . . . . PC 439.1 426.1 13 26 19.9 [42 49 48.7 23.43 [0.33R2 . . . . . . PC 706.0 387.9 13 26 21.5 [42 50 03.6 23.50 [0.37R3 . . . . . . WF2 459.0 413.6 13 26 21.4 [42 48 53.2 20.09 0.29R4 . . . . . . WF2 77.8 57.9 13 26 21.2 [42 49 45.7 21.04 0.20R5 . . . . . . WF3 391.1 696.9 13 26 27.4 [42 50 15.3 22.06 [0.06R6 . . . . . . WF3 515.2 673.5 13 26 28.1 [42 50 06.5 22.07 [0.02R7 . . . . . . WF4 332.4 236.1 13 26 21.5 [42 50 23.1 21.85 0.34R8 . . . . . . WF4 54.5 67.6 13 26 21.0 [42 49 51.4 21.90 0.26

NOTE.ÈUnits of right ascension are hours, minutes, and seconds, and units of declinationare degrees, arcminutes, and arcseconds.

268 MOULD ET AL. Vol. 536

FIG. 2a

FIG. 2.È(a) WFPC2 mosaic of our Centaurus A Ðeld. Four stars are marked for comparison with Fig. 4. (b) Image observed in chip WF3. The orientationis the same as in (a).

mosaic of these medianed images is reproduced in Figure2a. A higher resolution image of the WF3 results is alsoshown in Figure 2b. These images were used to create theinitial star list. After subtracting the stars in this list fromthe images, the subtracted images were examined to identifystars obscured in the Ðrst pass by the skirts of brighter stars.The stars found in the second pass were added to the starlist. Obvious detections of background galaxies wereremoved by hand, and sharpness and uncertainty con-straints were put on the stars to produce a Ðnal star list.Including contributions from NGC 5128 and our ownGalaxy, 1075 stars were obtained from the PC frame, and20,222 were found from the three combined WF frames.

For our photometry to be on the standard system, stellarlight at large radial distances from the point source images

must be included. To calibrate these data, 20È30 programstars were used from each chip as local standards. Templategrowth curves and aperture corrections were producedfrom the program stars and applied to the rest of the photo-metry using DAOGROW and associated programs(Stetson 1990). The standard transforms between F555Wand F814W and V and I were adopted (Holtzman et al.1995). A color-magnitude diagram (CMD) plotting Vagainst V [I was produced for the stars identiÐed and isgiven in Figure 3. The main feature of this CMD, which isnot present in the previous Ðeld (Soria et al. 1996), is a mainsequence with V [ID 0 mag.

The location on the sky of the two brightest stars in eachchip are given in Table 1 along with their chip positions, Vmagnitude, and V [I color. Table 2 gives the coordinates,

No. 1, 2000 JET-INDUCED STAR FORMATION IN CEN A 269

FIG. 2b

FIG. 3.ÈColor-magnitude diagram for stars in our Centaurus A Ðeld.Circled dot : PC; circled cross : WF2; open four-point star : WF3; six-pointstar : WF4.

V magnitudes, and V [I for the remaining stars in eachchip with V \ 24 mag. We were able to identify two starspreviously observed by Graham (1999). Stars 1 and 12 fromGrahamÏs (1999) Table 3 correspond to star 3 in our Table1 and star 6 in our Table 2, respectively.

3. LAS CAMPANAS Ha IMAGES

Images of the region were obtained in Ha ] [N II] and Rat the 2.5 m telescope of Las Campanas Observatory duringthe night of 1997 April 28 (Fig. 4, showing the Ha image).The seeing varied during the course of the observationsbetween and We employed the Direct Camera and1A.1 1A.6.a 2k ] 2k CCD, together with the Harris R Ðlter and anarrowband Ðlter (FWHM\ 68 centered at 6568 TheA� ) A� .plate scale on the CCD is pixel~1, with a total Ðeld of0A.26view of for the broadband images. The useful Ðeld of8@.8view for the narrowband image is only due to the small4@.7,size of the Ðlter used. Total exposure times were 1200 s inboth R and Ha.

270 MOULD ET AL. Vol. 536

TABLE 2

REMAINING STARS DETECTED IN WFPC2 CENTAURUS A FIELD WITH V \ 24 AND [0.6\ V [ I\ 0.5

STAR POSITION MAGNITUDE STAR POSITION MAGNITUDE

ID Chip X Y V V [I ID Chip X Y V V [I

1 . . . . . . . PC 794.6 638.5 23.79 [0.02 42 . . . . . . WF2 193.0 260.7 23.98 [0.052 . . . . . . . PC 448.9 96.9 23.81 0.25 43 . . . . . . WF2 146.0 611.2 23.98 [0.083 . . . . . . . PC 698.7 370.0 23.86 0.47 44 . . . . . . WF3 163.9 733.7 22.22 0.054 . . . . . . . PC 218.8 606.5 23.94 0.44 45 . . . . . . WF3 470.2 716.3 22.28 [0.085 . . . . . . . WF2 429.6 144.6 21.75 0.07 46 . . . . . . WF3 252.9 638.6 22.61 0.276 . . . . . . . WF2 516.0 503.2 22.26 0.27 47 . . . . . . WF3 146.8 287.6 22.78 0.507 . . . . . . . WF2 388.1 385.8 22.37 0. 48 . . . . . . WF3 515.0 674.9 22.82 [0.208 . . . . . . . WF2 103.7 242.4 22.48 [0.03 49 . . . . . . WF3 555.4 466.3 22.83 [0.289 . . . . . . . WF2 392.5 359.9 22.55 [0.06 50 . . . . . . WF3 498.4 638.1 22.93 [0.1510 . . . . . . WF2 95.2 76.4 22.57 [0.01 51 . . . . . . WF3 163.1 713.8 22.95 [0.0111 . . . . . . WF2 60.4 333.7 22.60 [0.33 52 . . . . . . WF3 700.2 380.2 23.18 [0.6012 . . . . . . WF2 430.6 146.5 22.60 0.12 53 . . . . . . WF3 515.8 678.4 23.32 [0.0813 . . . . . . WF2 386.6 662.5 22.69 0.19 54 . . . . . . WF3 128.5 709.5 23.54 [0.1314 . . . . . . WF2 297.4 195.9 22.99 0.24 55 . . . . . . WF3 145.9 711.5 23.58 0.2115 . . . . . . WF2 86.2 125.7 23.05 [0.11 56 . . . . . . WF3 645.8 774.5 23.69 [0.1816 . . . . . . WF2 399.9 381.4 23.20 [0.18 57 . . . . . . WF3 490.0 715.6 23.72 [0.2817 . . . . . . WF2 569.6 294.0 23.32 0.19 58 . . . . . . WF3 76.8 99.3 23.72 [0.2118 . . . . . . WF2 432.1 148.5 23.32 0.46 59 . . . . . . WF3 465.2 649.0 23.80 [0.3319 . . . . . . WF2 374.6 366.1 23.38 [0.07 60 . . . . . . WF3 599.6 683.2 23.92 [0.3520 . . . . . . WF2 528.9 664.8 23.39 [0.28 61 . . . . . . WF3 233.7 280.4 23.99 0.2821 . . . . . . WF2 427.0 148.9 23.40 0.01 62 . . . . . . WF4 637.2 252.2 22.02 0.0722 . . . . . . WF2 554.9 468.9 23.41 0.25 63 . . . . . . WF4 746.9 470.0 22.07 [0.1623 . . . . . . WF2 157.1 681.2 23.47 0.24 64 . . . . . . WF4 87.7 377.5 22.48 0.24 . . . . . . WF2 111.8 185.7 23.48 [0.11 65 . . . . . . WF4 686.0 566.2 22.61 0.1825 . . . . . . WF2 427.7 452.3 23.50 0.23 66 . . . . . . WF4 255.6 334.7 22.68 0.1026 . . . . . . WF2 399.2 227.7 23.51 0.29 67 . . . . . . WF4 67.9 69.0 22.89 [0.0427 . . . . . . WF2 152.5 294.0 23.52 [0.08 68 . . . . . . WF4 275.3 286.1 22.97 [0.0128 . . . . . . WF2 404.2 503.8 23.55 0.25 69 . . . . . . WF4 593.1 259.7 23.23 0.0129 . . . . . . WF2 136.6 251.4 23.59 [0.57 70 . . . . . . WF4 295.4 110.8 23.35 [0.1530 . . . . . . WF2 325.0 428.9 23.61 [0.06 71 . . . . . . WF4 217.7 73.4 23.43 0.0431 . . . . . . WF2 470.0 419.4 23.66 [0.02 72 . . . . . . WF4 748.6 479.3 23.48 [0.1332 . . . . . . WF2 84.1 240.2 23.72 0.24 73 . . . . . . WF4 72.7 53.8 23.61 [0.0433 . . . . . . WF2 425.3 486.7 23.75 [0.08 74 . . . . . . WF4 198.9 101.9 23.67 [0.1234 . . . . . . WF2 52.3 300.4 23.77 0.31 75 . . . . . . WF4 262.6 153.6 23.76 [0.0135 . . . . . . WF2 120.9 144.8 23.78 [0.07 76 . . . . . . WF4 77.3 334.5 23.82 [0.4536 . . . . . . WF2 350.2 399.6 23.84 0.05 77 . . . . . . WF4 196.1 494.5 23.88 0.1437 . . . . . . WF2 274.6 286.4 23.86 [0.22 78 . . . . . . WF4 51.2 614.7 23.90 [0.5738 . . . . . . WF2 85.8 642.4 23.86 0.15 79 . . . . . . WF4 624.2 165.9 23.90 [0.0739 . . . . . . WF2 61.5 120.2 23.87 [0.14 80 . . . . . . WF4 402.5 301.5 23.92 [0.1740 . . . . . . WF2 502.5 206.0 23.91 0.09 81 . . . . . . WF4 359.3 449.2 23.95 0.3241 . . . . . . WF2 568.8 32.0 23.92 [0.13

Data reduction followed the standard procedure of biassubtraction, Ñat-Ðelding, registration, and coaddition of theimages. Both dome and twilight exposures were used toremove pixel-to-pixel variations and illumination patternsfrom the images. The two central bad columns of the chipwere linearly interpolated in each image with values fromsurrounding columns. Cosmic rays were removed from theindividual frames, using an algorithm developed by M.Dickinson (1997, private communication) for the identiÐca-tion of sharp, positive discontinuities over scales of D1pixel. This technique removed around 80%È90% of thecosmic rays. E†ects of air-mass variations were calculatedto be less than 2% and were ignored. Absolute calibrationswere obtained from observations of two spectro-photometric standards from Hamuy et al. (1994). One of thestandards was also observed during the night at di†erentazimuths to derive air-mass corrections. More details on theobserving conditions, data reduction, and calibrations arereported in Calzetti et al. (1999).

The narrowband Ðlter is large enough to include in itsbandpass the two [N II] lines in addition to Ha. However,the contribution of the strongest of the two metal lines,[N II] (6584 is weakened by the fact that while Ha isA� ),located at the Ðlter transmission peak, the [N II] line falls ina region where the transmission is the peak. Since theD12[N II] j6584 emission is typically about half the Ñux of Ha(Graham 1998), we use this Ðgure combined with the ÐlterÏstransmission efficiency to correct the Ha Ñux for the [N II]contamination in the calculation of the star formation rate(SFR) given in ° 4. The calibrated R image was used tosubtract the stellar continuum from the Ha ] [N II] image,following standard procedures. After matching the FWHMof the stars in the broad- and narrowband frames, the con-tinuum images were rescaled and subtracted from the nar-rowband images, using the Ðeld stars to obtain the scalingfactor.

The Ha ] [N II] image in Figure 4 shows that the regionhas a complex structure, with a presumably younger area (if

No. 1, 2000 JET-INDUCED STAR FORMATION IN CEN A 271

FIG. 4.ÈHa image of the Ðeld from the 2.5 m telescope of the Las Campanas Observatory. The marked stars are the same as those in Fig. 2. We also showthe approximate edge of the radio-emitting region from the Morganti et al. (1999) map and the position of the stellar associations found in the WFPC2images (see Fig. 5).

stars are responsible for the ionization) showing strong Harunning to the east and not quite parallel to the array of OBassociations seen in the present paper.

4. RECENT STAR FORMATION

We were interested in the position of the blue stars in theWFPC2 Ðeld in order to identify regions of recent star for-mation. To identify young stars, we examined stars withblue colors in the range of [ 0.6\ V [ I\ 0.5. These stars

are overlaid on the WFPC2 image montage in Figure 5. Aregion of recent star formation forms a band diagonallyacross the Ðeld. We identify three concentrations of a fewhundred parsecs in extent as stellar associations 3-6-5. Indi-vidual stellar associations do not appear to have distinctivecolor-magnitude diagrams (Fig. 6), although associations4-1-2, which lie along the band of ionized gas, have feweroptically luminous stars (supergiants) and therefore couldbe younger than their compatriots (3-6-5) with little ionizedgas.

272 MOULD ET AL. Vol. 536

FIG. 5.ÈPositions of stars with magnitudes in V \ 26.5 and V [I magnitudes between 0.5 and [1. Six associations are labelled 1È6.

Morganti et al. (1999) present a detailed study of therelationships between the northern middle radio lobe ofCen A, the H I cloud in this region, and the complex of blueknots and ionized gas that we have observed. The ionizedgas Ðlaments lie along the eastern boundary of the non-thermal radio emission, whose edge is approximately shownin Figure 4. They are more distant from the H I cloud, whichlies farther to the west, but as several authors have noted,the ionized and neutral gas have similar radial velocities(Graham 1998). This suggests that a past interactionbetween the radio jet and the H I cloud complex could beresponsible for star formation in this region (cf. Morganti etal. 1999 ; Graham 1999).

Knots of Ha emission are easily seen in correspondencewith the 4-1-2 association chain, while Ha emission is veryweak along the 3-6-5 chain, which lies farther from thenonthermal radio lobe. This indicates that star formation isstill active along 4-1-2, while it is probably subsiding along3-6-5. Absent or weak Ha implies that the last star-formingevent took place Myr ago along 3-6-5. An estimate ofZ10the Ha Ñux associated with the H II knots along 4-1-2 gives

f (Ha) B 1.3] 10~13 ergs s~1 cm~2, under the assumptionthat the [N II] emission is B50% of the Ha in this region(e.g., Graham 1998 ; see also discussion at the end of ° 3).

To get an estimate of the mean age of blue stars in thisregion of Cen A, we compared it to the LMC star clusterNGC 2004. Data for NGC 2004 are from Keller, Wood, &Bessell (1999). The age of this cluster has been calculated as107.2 yr. A CMD for NGC 2004 was formed by plotting theV -band magnitude against the V [I magnitude di†erence.The distance modulus of Cen A is 27.8, while that for theLMC is 18.5. To compare NGC 5128 to the LMC, we add9.3 to each of the observed magnitudes in NGC 2004 toshift this cluster out to the same distance modulus as ourÐeld in Cen A. Since there was no signiÐcant reddeningdi†erence between our Ðeld and NGC 2004, no colorchange is required. An overlay of the CMD for NGC 2004over our CMD is given in Figure 7, showing that the associ-ation in Cen A is of comparable age to NGC 2004.

We present some synthetic CMDs in Figure 8. It is clearfrom these Ðducial CMDs that a substantial component ofthe stellar population in the Graham Ðeld is as young as

No. 1, 2000 JET-INDUCED STAR FORMATION IN CEN A 273

FIG. 6.ÈColor-magnitude diagrams for the six associations.

NGC 2004. These synthetic CMDs were made with the helpof evolutionary calculations for Z\ 0.008 by the Universityof Padova group.

Ha in emission is also detected between the regions 4-1-2and 3-6-5, with no obvious association of blue stars under-

neath it. The Ha emission in this region has a complexstructure, with Ðlaments and arcs reminiscent of shock-excited regions, and an estimated Ñux of f (H)a B 4 ] 10~13ergs s~1 cm~2 (under the same assumption for the [N II]contribution as above), or D3 times the Ñux directly associ-

274 MOULD ET AL. Vol. 536

FIG. 7.ÈColor-magnitude diagram for stars in our Centaurus A Ðeld overlaid with the color-magnitude diagram for stars in NGC 2004. The symbols arethe same as Fig. 3, with the addition that the gray-scale solid symbols show the LMC cluster.

ated with H II knots along 4-1-2. It is likely that more thanone process contributes to the ionization of the gas, withmassive stars providing photoionization in some regionsand shocks in others. This complex situation would be con-sistent with the considerable variations in H II emission-lineintensities seen in this region (e.g., Graham & Price 1981 ;Morganti et al. 1991 ; Graham 1998).

The Ha and blue star morphology in the targeted Ðeldsuggests that star formation has been ““ propagating ÏÏ from3-6-5 to 4-1-2, possibly following changes in the direction ofpenetration of the jet into the H I cloud(s). Alternatively,star formation could have started simultaneously in the twoareas, but 4-1-2 is characterized by a larger number of denseclouds, which are thus still forming stars under the action ofthe jet. From the present data, we are unable to distinguishbetween these possibilities. Collisional ionization andnuclear ionization (favored by Graham & Price) furtherconfuse the relation between line emission and young stars.

5. THE STAR FORMATION RATE

If we assume that we are observing the results of a singleburst of star formation in the Graham Ðeld, calculating thescale of this activity is straightforward. Figure 9 shows acomparison of the main-sequence luminosity functions ofthe present Ðeld and the LMC star cluster NGC 2004,shifted in magnitude as described in the previous section.We assume that stars with [0.6\ V [I\ 0.5 are main-sequence stars and ignore those with V \ 23, which areprobably supergiants. We simplify this estimate further byassuming that the completeness of both of these luminosityfunctions is D70%. In the former case, this is a guess basedon experience with the insertion and recovery of artiÐcial

stars into Ðelds as crowded as this (e.g., Soria et al. 1996) ; inthe case of NGC 2004, this is a geometrical allowance forthe stars lost from a 1@ diameter object centered on the PC.

Assuming a mass of 2 ] 104 for NGC 2004,12 andM_taking the ratio of the number of stars with 24 \ V \ 26 in

the two panels of Figure 9, we conclude that our sample ofthe Graham Ðeld has made 1.2 ] 104 stars on a time-M

_scale of 1.6] 107 yr, the age of NGC 2004. Fuelling starformation at this rate poses no problems for an in situmodel, since approximately 6] 106 of neutral hydro-M

_gen can be found in the Ðeld by integrating the H I map ofSchiminovich et al. (1994). An estimate of the instantaneousSFR from the Ha emission along the 4-1-2 association givesa value of D1.2] 10~3 yr~1 for a 0.1È100 Salpe-M

_M

_ter initial mass function (IMF). This is approximately thesame value as found for the average SFR over 1.6 ] 107 yr.

A plausible explanation for the existence of young stars inthese numbers and at this location is that star formation inthe gas, deposited in the giant ellipticalÏs potential by thedynamical interaction, whose aftermath deÐnes todayÏsNGC 5128, was triggered by the radio jet from the activenucleus some 107 yr ago.

Our census of the recent star formation is fully consistentwith this picture. The connection with the radio jet is cir-cumstantial, but strengthened by the alignment of theassociation. A further control Ðeld in one of the other H I

clouds, say, in the southern halo of the galaxy, might

12 T. Richtler et al. (in preparation) have a measurement in progress.Provisionally, we scaled the mass of NGC 2157 (Fischer et al. 1998), takinginto account the age di†erence (Renzini & Buzzoni 1986) and luminositydi†erence (van den Bergh 1979).

No. 1, 2000 JET-INDUCED STAR FORMATION IN CEN A 275

FIG. 8.ÈV -magnitude luminosity function for stars in our Centaurus AÐeld compared to that for the NGC 2004 cluster.

sharpen this connection, as could understanding whetherany of the inner northeastern complexes of blue knots andionized gas Ðlaments have any associations with cool gasclouds.

The physical processes by which jet-cloud interactionresults in star formation are not elucidated by these results(see also Graham 1998, 1999 ; Morganti et al. 1999),although photoionizing shocks may well be relevant(Sutherland, Bicknell, & Dopita 1993). We note that theD80A short axis of the association studied here limits itsvelocity dispersion to km s~1. With the present data, it[80is unclear how the most recent jet activity relates to thelocation and morphology of observed star formation. It isunclear whether motion of the interface or di†usion hasbeen dominant in shifting and shearing what we see.Mapping the distribution of young stars over a much largerÐeld could trace the history of the interaction of the jet withhalo hydrogen clouds in this galaxy.

This research was carried out by the WFPC2 Investiga-tion DeÐnition Team for the Jet Propulsion Laboratory,and was sponsored by NASA through contract NAS7-1260.We would like to thank Cesare Chiosi for providing iso-chrones ; J. S. G. thanks the European Southern Observa-tory for partial support of a visit to their GarchingHeadquarters, where several people and especially BobFosbury provided useful comments on this project. D. C.thanks the Carnegie Observatories for granting observingtime at the 2.5 m telescope of the Las Campanas Observa-tory. A. G. R. thanks Jackie Monciewicz for assistance. Weare grateful to an anonymous referee for focusing on someof the difficulties in demonstrating jet-induced star forma-tion and to Ra†aela Morganti for helpful discussions.

FIG. 9.ÈSynthetic color-magnitude diagrams of clusters older and younger than NGC 2004.

276 MOULD ET AL.

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