arXiv:astro-ph/0311323v1 13 Nov 2003 h edl eetbeotcleiso rvdn nexcel an providing emission optical detectable readily the H rotating regularly 1 as fsa omto nteds fe h omto fthe of formation high the a after with disk be- the component. presumably in II luminosity, formation population I unit population per by of dominated GCs cause galaxies fewer Disk contain GCs. 4 eac which least 1994) at Irwin have & Gilmore (Ibata, ex Sagitarius (Hod Fornax and The spheroidals 1961) dwarf likely. massive formation most the their the are make ceptions because to probably mass GCs, insufficient contain have not do sphero Dwarf generally bulges. galaxies prominent with e ( galaxies in spiral Dominant as central as well and such (dE), dominant elliptical is dwarf population (E), liptical stellar old n the particularly where are able They (GCs). clusters globular contain to yial r leadntcniee ieyhssfrpopul for They hosts stars. likely into considered systems. mass not GC baryonic and their blue of are much typically form to yet have rpittpstuigL using typeset Preprint 1 2 4 3 6 5 7 11 8 12 9 10 A . P eateto hsc n srnm,TeJhsHpisUniv Hopkins Johns The Astronomy, and Physics of Department C/ikOsraoy nvriyo aiona at Cru Santa California, of University Observatory, UCO/Lick ..Kc bevtr,6-10MmlhaHy,Kmea HI Kamuela, Hwy., Mamalahoa 65-1120 Observatory, 20771. Keck MD W.M. Greenbelt, Center, Flight Space Goddard NASA atoSine&Tcnlg,PO o 7,Ma,C 80542-06 CO Mead, 670, Box P.O. Technology, & 21218. Science MD Bartko Baltimore, Drive, Martin San 3700 STScI, aa nttt fPyis h erwUiest,Jerusal University, Hebrew The Physics, of Institute Racah eateto srnm n srpyis h Pennsylvani The Astrophysics, and Astronomy of Department eatet eAtooí srfsc,PnicaUniver Pontificia Astrofísica, y Astronomía de Departmento 7 ednOsraoy otu 53 30R edn Netherla Leiden, RA 2300 9513, Postbus Observatory, Leiden uoenSuhr bevtr,Karl-Schwarzschild-Stra Observatory, Southern European twr bevtr,Uiest fAioa usn Z857 AZ Tucson, Arizona, of University Observatory, Steward L J l aaiswt asv l tla ouain r thoug are populations stellar old massive with galaxies All G 95i neteegsrc aayhaving rich gas extreme an is 2915 NGC M ETTERS ⊙ / L lses l fwihsatt eov nosas have stars, into resolve to domina matter start dark which gas-rich of the all of clusters, images Surveys for Camera prl,o nrr icmtne eansseddi their in headings: suspended Subject remain formed circumstances to has rare on population in go or stellar could spirals, old systems the such history, of interaction much subsequent when key a epoch resembles the 2915 cont por NGC - cluster that small suggests normalized This a mass clusters. only and galaxy luminosity since the However, that likely galaxies. The spiral clusters. to globular compared Galactic metal-poor of typical dex, eko h uioiyfnto fMlyWygoua cluste globular Way Milky of function luminosity the of peak , B ACCEPTED , ehv icvrdtregoua lsesbyn h Holmb the beyond clusters globular three discovered have We ICVR FGOUA LSESI H RT-PRLNC2915 NGC PROTO-SPIRAL THE IN CLUSTERS GLOBULAR OF DISCOVERY ⊙ L (Meurer .B F. B A ESSER T ENÍTEZ E tl mltajv 11/12/01 v. emulateapj style X G ARTKO C ERHARDT ROSS I 1. 10 iketnst vr5tmsbeyond times 5 over to extends disk ..M A.R. , 1 tal. et INTRODUCTION .C M. , 6 1 ..B R.J. , ..F P.D. , aais trcutr aais niiul(G 95 — 2915) (NGC individual galaxies: — clusters star galaxies: halos .M R. 96 eefe CF6.Its MCBF96). hereafter 1996, LAMPIN ARTEL OUWENS ELDMAN EURER 3 1 .M F. , ..M G.K. , 1 1 1 ..B J.P. , ..B T.J. , ..G D.A. , IRRHCLGLX EVOLUTION GALAXY HIERARCHICAL ENANTEAU s ,D878Grhn,Germany. Garching, D-85748 2, sse m sal91904. Israel em, ILEY M ,C 95064. CA z, LAKESLEE ia Cat sidad ROADHURST tt nvriy 2 ae a,Uiest ak A16802 PA Park, University Lab, Davey 525 University, State a 21. ..W R.L. OLIMOWSKI nds. HI M 11 riy 40NrhCalsSre,Blioe D228 meu 21218; MD Baltimore, Street, Charles North 3400 ersity, / 70. .P M. , 96743 L HI D,as cD), 1 B ..T H.D. , lc eCie ail 0,Snig 2 Chile. 22, Santiago 306, Casilla Chile, de ølica / ´ otice- p etr,accepted Letters, ApJ L ratio HITE lent idal 1 ABSTRACT ous B .S M. , ge OSTMAN ht l- = h y - 7 1,5 ..B R.A. , 1 .G C. , M RAN .Z W. & , 1 V IRIANNI 606 pcfi rqec fcutr sa iiu normal, minimum a at is clusters of frequency specific eeomn obcm ytm ieNC2915. NGC like systems become to development ainn19,hratrMC4,isH its MMC94), Mackie, Meurer, hereafter (BCD; 1994, dwarf Carignan compact & blue mo a optical of that its is in while phology Furthermore, ratios mass-to-light (MCBF96). known galaxy coincident highest single the a not of distribution; one has mass 2915 NGC the for tracer dynamical 66 ( F606W bandi re olo o tla etn orefrthe for source heating stellar a for look to order Sur- in for obtained Camera Ford Advanced (ACS; Telescope veys Space Hubble in found optical. the in apparent not are which arms spiral egradius berg esn fteiae a oeuigthe using done was images the of cessing xoueo 2480 of exposure mgscvrpoetdrdio 45 of radii projected cover images edcnee t09 at centered field a ik htisewl edsusdi eaaeatce(Meu article separate a in discussed al. be et will issue That disk. Hc 99.W sdteprogram the used We 1999). (Hack 5 4 ul-plre litclglxe,eov nonormal into evolve galaxies, elliptical larger build-up n shge,lk htse nelpia aaisand galaxies elliptical in seen that like higher, is ent RONWALL .R P. , ..K R.A. , s hi oossgetamtliiy[Fe metallicity a suggest colors Their rs. = ROWN nti etr erpr h icvr ftrelmnu GCs luminous three of discovery the report we Letter, this In C ieFedCmr WC mgswr bandof obtained were images (WFC) Camera Field Wide ACS hs nteeryheacia sebyo galaxies of assembly hierarchical early the in phase − ino h ytmhsbe uvydi smore is it surveyed been has system the of tion HENG e lecmatdafglx G 95 The 2915. NGC galaxy dwarf compact blue ted 1 r aisi ubeSaeTlsoeAdvanced Telescope Space Hubble in radius erg u iteo h tla ik eedn nthe on Depending disk. stellar the of little but , 8 03 npeaain eefe Meu03). hereafter preparation, in 2003; ..F H.C. , OSATI . o–. a,sgicnl rgtrta the than brighter significantly mag, –9.8 to 9 5 V ..B C.J. , IMBLE 1 606 8 .I L. , 12 R ,adF1W( F814W and ), Ho ..S W.B. , ORD 3 NFANTE 114 = ..H G.F. , URROWS 1 s tal. et ..I G.D. , 2600 , 2. aais vlto galaxies: — evolution galaxies: h PARKS AAADANALYSIS AND DATA ′′ 25 eotie ,2 mgsfratotal a for images 4 2, 2, obtained We . 9 02 mgso G 95ta were that 2915 NGC of images 2002) ARTIG 5 ..K J.E. , m s ..C E.S. , LLINGWORTH 5220 , MLCTOSFOR IMPLICATIONS : 36 5 ..T Z.I. , I 814 . s 5 8 –76 48, ..A D.R. , . RIST ,rsetvl.Tebscpro- basic The respectively. ), s HENG ′′ [email protected] ntefitr 45 ( F475W filters the in o257 to SVETANOV 5 M.P. , ◦ 3 Apsis 2 RDILA N.J.G. , 35 N. , ′′ / ′ hra h Holm- the whereas , H] I 52 CALACS ikceryshows clearly disk 1 (Blakeslee 1 , ≈ . ′′ , J00.The (J2000). 4 − 1 . 9 pipeline g tal. et 475 ally H rer r- ), I 2 Meurer et al.

2002) to align and combine the images encorporatinggeometric 4.1. Cluster formation efficiency correction and rejection of cosmic rays and hot pixels. Because of the blue core and gas rich nature of NGC2915 Here we present photometry in the natural system of the fil- we had not expected to find GCs in our images. However, ters, with zeropoints selected so that Vega would have a magni- in retrospect this discovery should not have been surprising. tude of 0.0 in all bands. In order to compare our observations GCs have previously been discovered around morphologically with previous work, we convert the previous work to this sys- similar systems; Östlin, Bergvall & Rönnback (1998) find a tem, as needed, using the calibrations of Sirianni et al (2003, considerable population of at least 65 old GCs about the Blue in preparation). The most important correction is to the V 606 Compact (albeit not dwarf) galaxy ESO 338-IG04. In addition, photometry, since the F606W filter straddles the wavelength of NGC2915 is not a dwarf system in terms of mass, and optical traditional V and R filters. imaging shows that it to be dominated by an older stellar popu- 3. RESULTS lation for R > 0.5 Kpc (MMC94). These facts suggest that old GCs might have been expected. Table 1 presents adopted global properties for NGC2915. Are the number of clusters found anomalous? To address this The foreground extinction, E(B −V), is from the Schlegel, we estimate the total number of clusters in the system. Unfor- Finkbeiner & Davis (1998) extinction maps. It is significantly tunately, our images sample only a small fraction of the galaxy. − ′ larger than E(B V)=0.15 ± 0.05 estimated by MMC94, but If the GCs are distributed spherically out to RHI = 10 (where consistent with the position of the field star Red Giant Branch the H I distribution ends; MCBF96), then we have only sur- (RGB; Meu03). Extinction corrected photometry employing veyed 3.5% of the available area. Here we consider two cases the Cardelli, Clayton & Mathis (1989) extinction curve is de- for the possible distribution of old clusters. Case (1) assumes noted with a “0” subscript. The distance, D was derived from that we are lucky and have managed to observe all the clus- the field star RGB tip (Meu03). It is consistent with but im- ters in NGC 2915. While this is unlikely, it provides a strict proves on previous estimates D =5.3 ± 1.3 Mpc (MMC94) and lower limit to the cluster formation efficiency. The more likely D =3.8 ± 0.5 Mpc (Karachentsev et al. 2003). The remaining case (2) is that the globular cluster system is like that of more quantities in Table 1 were derived from MMC94 and MCBF96 luminous systems - having a spherically symmetric power-law after correcting to the new E(B −V) and D. radial distribution in number per unit area - N(R) ∝ Rα (Harris As shown in Fig. 1, the three sources are clearly GCs whose 1991). We assumes this extends between R =0.5 Kpc and RHI, brightest stars are resolved. Table 2 compiles the properties of where the minimum R insures that the predictions are finite and the clusters. The photometric quantities were measured using was chosen to correspond to the size of the central star form- ′′ a circular aperture having a radius of r =3 , with the local sky ing population (MMC94). For −2.5 <α< −0.5 we estimate ′′ ′′ subtracted using an annulus having radii of 5 and 7.5 . The that the total number of clusters in the system is 7 to 12 times cluster size r1/2 is the circular aperture radius encompassing higher than found on our images. For case (2) we adopt a cor- half the V606 light as measured from curves of growth. rection factor of 9. Hence there are at least 3 GCs in the system Compared to Galactic GCs, these clusters are large and lu- (case 1), with a more likely number being ∼ 27 (case 2). minous, but not abnormally so. Only 16% of the clusters in the In principle, we should correct the total GC estimate for the 13 Harris (1996) database have V606 luminosities brighter than finite luminosity sampling of the images. However, SExtrac- G3; only three clusters are more luminous than G1. The clus- tor (Bertin & Arnouts 1996) catalogs of our images show that ters’ r1/2 ranges from about 5 to 9 pc, placing them in the upper we can detect extended objects down to V606 ≈ 27 which cor- quartile of Galactic GCs which have r1/2 ranging from 0.3 to responds to an MV606 ∼ −2 for sources in NGC2915. This is 24.7 pc (Harris 1996). The clusters are noticeably elongated well below the peak of the GC luminosity function MV ∼ −7.5 with ellipticity ǫ ≡ 1 − b/a similar to the canonical flattened (Secker 1992; revised to agree with the Hipparcos RR Lyrae Galactic GCs M22 and ω Cen (ǫ =0.14, 0.17, respectively; Har- zero point, e.g. Carretta et al. 2000). Hence we apply no lumi- ris 1996). The combination of high luminosity and appreciable nosity sampling correction. The fact that we only have found flattening is also seen in the cluster M31-G1 (Pritchet & van GCs much brighter than this peak is somewhat puzzling. den Bergh 1984; Meylan et al. 2001). We will consider three measures of cluster formation effi- − − 0.4(MV +15) The (g475 V606)0 and (V606 I814)0 colors of the clusters are ciency. First, the specific frequency, SN = NT 10 , is compared to GCs (Harris 1996) in Fig. 2. Their the V luminosity normalized cluster content (Harris & van den colors are virtually identical implying similar metallicities, as- Bergh 1981). SN is typically around 1 for spiral galaxies and suming they are old and nearly coeval. We derive their metallic- increases towards earlier galaxy types, with E galaxies having ity by fitting the metallicty - color relationship from the Harris SN ∼ 4. In the center of galaxy clusters SN ranges from 2.5 database after converting the colors to (V606 − I814)0. We em- to 12.5 (Blakeslee, Tonry, & Metzger 1997; Blakeslee 1999). ployed an unweighted least squares fit with an iterative 2.5σ We consider two measures of the mass normalized contribu- − + − 9 rejection resulting in [Fe/H] = 5.37 5.36(V606 I814)0 with a tion: ηGC the number of clusters per 10 M⊙ in dynamical dispersion of 0.29 dex. The metallicity for the three clusters mass (Blakeslee et al. 1997), and ǫcl, the fractional baryonic is then [Fe/H] = −1.9 ± 0.4 dex, consistent with low metallic- (gas and stars) mass in GCs (McLaughlin 1999). As done ity Galactic GCs. Our stellar population analysis, in progress by McLaughlin (1999) we assume each cluster has an average 5 (Meu03), indicates that the stars at the outskirts of the clusters mass hM i =2.4 × 10 M⊙ when calculating ǫ . Blakeslee − cl cl have very similar I814,0 versus (V606 I814)0 color-magnitude di- et al. (1997) find an average ηGC =0.7 with a scatter of 30%, agrams, dominatedby a narrowand blue RGB. This is also con- while McLaughlin (1999) computes an average ǫcl = 0.0026 sistent with low [Fe/H], if the clusters are old. with a 20% uncertainty. Table 3 tabulates SN, ηGC and ǫcl for NGC2915 under the 4. DISCUSSION above two cases. Relevant quantities used for these calculations 13 http://physwww.physics.mcmaster.ca/∼harris/mwgc.dat Globular clusters in NGC2915 3 are given in Table 1. Table 3 also lists literature values and un- et al. 1999). Indeed, models of present day evolution of galaxy certainties of the efficiencies for “normal” systems. The uncer- clusters invoke mildly truncated in field galaxies tainty of our estimates are large: for a Poissonian distribution accreting onto the cluster as the cause of the Butcher-Oemleref- yielding a count of 3, the 95% confidence limits on the cluster fect (Balogh, Navarro & Morris 2000; Kodama & Bower 2001). formation efficiencies are 0.3 and 2.3 times the estimated value. It is likely that in their early evolution clusters were assembled We find that at a minimum (case 1) SN is close to normal for a from building blocks similar to NGC2915 which had their ISM while while ηGC and ǫcl are low compared to nor- stripped from them by these mechanisms. The stripped gas was mal globular cluster systems. In the more likely case (2), SN is then virialized to become the cluster X-ray halo. In this sce- very high compared to normal gas rich systems. ηGC and ǫcl are nario galaxy clusters have high SN because they formed out of also somewhat high compared to literature values, but in rea- subclumps which had already efficiently formed star clusters sonable agreement considering the Poissonian uncertainty of but which never had the chance to form disks. our measurements. While we can not rule out the possibility In a less hostile environment, a building block similar to that NGC2915 is like a normal spiral galaxy in terms of SN, NGC2915couldgo onto forma normalspiralgalaxy. If the H I it is more likely that it has a high luminosity normalized clus- disk of NGC2915 were to be perturbed enough to efficiently ter content, whereas the mass normalized content is closer to form stars in a fairly quiescent fashion this would result in an 8 normal. additional 7×10 M⊙ of stars forming with no additional GCs. Assuming a M/LV ∼ 1M⊙/LV,⊙ for the additional stars then 4.2. A missing link in galaxy evolution? the system would evolve towards SN ∼ 2, a fairly normal value It is interesting to interpret these results within the framework for spiral galaxies, while ηGC and ǫcl remain fixed at their nor- of hierarchical evolution through comparison with other sys- mal values. The SN in NGC2915 is then anomalously high because the formation of its disk has not proceeded, presum- tems. Blakeslee (1999) has discussed how the SN of the young Milky Way must have been fairly high, similar to the values for ably due to a lack of external perturbations (MCBF96). If so, cluster E galaxies, after the formation of the Galactic halo but then we may expect other galaxies with high MHI/LB ratios before stellar disk formation. The increase in luminosity from and extended H I disks to also have a significant GC popula- later star formation in the disk would cause a decrease in the tion, especially if they have a strong old population. While this scenario seems compelling, we caution that it is Galactic SN to its present low value, while leaving the number of GCs per unit mass unchanged. In NGC2915 we have an ex- possible that NGC 2915 only superficially resembles the build- ample of a present-day galaxy with an old stellar component, ing block we describe. We have not proven that the number of including GCs, but a spiral disk that is still mainly in the form clusters is anomalously high. Furthermore, we can not yet rule of gas. out the possibility that the clusters are of higher metallicity and In this sense, NGC2915, like the centers of rich galaxy clus- younger than Galactic GCs. This is shown in figure 2 where we overplot Bruzual & Charlot (2003) population models on the ters, has an elevated value of SN compared to typical spirals be- cause of a lower efficiency for converting gas into stars. Clus- two color diagram illustrating the strong age-metallicity degen- ters typically have 2–5 times as much mass in gas as in stars eracy for the filters used here. If the clusters are not old, they (Arnaud et al. 1992), similar to what we see in NGC2915. The may represent the remnants of a starburst occurring as recently difference, however, is that the gas in clusters now resides in as a few Gyr ago. We are undertaking additional observations the hot intracluster medium, while the gas in NGC2915 still re- (imaging and spectroscopic) to get a more accurate census of tains the potential to be converted into stars. To a large extent, star clusters in the NGC2915 system and determine their na- ture. The results of these studies should determine whether the fate of the gaseous disk (and the future evolution of SN in NGC2915) must depend on the surrounding environment. NGC2915 is just a gas rich galaxy with a peculiar star forma- Simulations show that when a disk galaxy enters the envi- tion history, or whether it truly presents us with a rare local ronment of a rich galaxy cluster, much of the gas in its disk and view of how galaxies looked in the epoch of cluster assembly. halo will be removed by the combination of tidal and ram pres- sure stripping (Abadi, Moore & Bower 1999; Bekki, Couch & ACS was developed under NASA contract NAS 5-32864, Shioya 2002; Bekki et al. 2001; Gnedin 2003). For example, if and this research has been supported by NASA grant NAG5- a system similar to the Galaxy were to fall into a rich cluster it 7697. We thank the technical and administrative support staff would have its gas disk truncated down to a radius of a few kpc of the ACS science team for their committed work on the ACS within a few tens of Myr by ram pressure strippingalone (Abadi project.

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TABLE 1 NGC2915 PROPERTIES

Quantity Value Units Description E(B −V) 0.28 ± 0.04 mag foregroundextinction D 4.1 ± 0.3 Mpc Distance 8 Mg 7.4 × 10 M⊙ ISM mass 8 M⋆ 3.2 × 10 M⊙ Mass in stars 10 MT 2.1 × 10 M⊙ Total dynamical mass MV –16.42 mag Absolutemag V band 8 LB 3.4 × 10 LB,⊙ B band luminosity MT /LB 62 solar Mass to light ratio

TABLE 2 CLUSTER PROPERTIES

a a − − ID RA Dec R MV 606 (g475 V606)0 (V606 I814)0 r1/2 ǫ (J2000) (J2000) (kpc) (pc)

G1 092556.273 -763514.53 3.0 -9.82 0.62 0.66 8.8 0.11 G2 092527.445 -763631.55 3.3 -9.04 0.57 0.65 4.6 0.16 G3 092541.954 -763633.70 2.4 -8.92 0.59 0.64 5.9 0.13

aThe absolute accuracy of the positions ∼ 0.′′2 in each coordinate is set by the astromet- ric calibration which employed 86 stars in the HST Guide Star Catalog version 2 (http://www- gsss.stsci.edu/gsc/gsc2/GSC2home.htm). The relative accuracy of the positions is < 0.′′05 in each coordinate.

TABLE 3 GLOBULAR CLUSTER SYSTEM PROPERTIES

Quantity Case(1) Case(2) Literature

SN 0.81 7.3 0–12.5 ηGC 0.14 1.3 0.7 ± 0.2 ǫcl 0.00067 0.0061 0.0026 ± 0.0005 6 Meurer et al.

FIG. 1.— Three color images of the three globular clusters; from left to right: G1, G2, G3. The images are shown on the same logarithmic stretch, with F814W, F606W, and F475W representing red, green and blue respectively. Each image is 8′′ on a side and has the same orientation. The large arrow of the compass in the middle panel points to north, the small arrow points to east.

FIG. 2.— Color-color plots of the NGC 2915 globular clusters (pink filled diamonds) compared to Galactic globular clusters (crosses; from Harris 1996), and “simple stellar population” models from Bruzual & Charlot (2003; colored connected dots). All data have been corrected for Galactic extinction. The red line segment shows the E(B −V) = 0.1 reddening vector (Cardelli et al. 1999). Population models having ages of 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1.0, 2.0, 5.0, 10, 15, and 20 Gyr are plotted in four constant metallicity model sequences having [Fe/H] =, –2.25 (black); –1.65 (purple); -0.33 (green); and 0.09 (orange). All models employ the standard stellar evolution and Initial Mass Function prescriptions from Bruzual & Charlot (2003).