Clues to Nuclear Star Cluster Formation from Edge-On Spirals

Home , Central massive object, NGC 4144, NGC 4183, NGC 4206

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Cambrid St., Garden 60 Fellow, Postdoctoral CfA a Currently 5 fte7 aeo aaiso yeSdadlater and Scd type of galaxies face-on 77 the of 75% rgtncersa lseshv enosre in observed been have clusters star nuclear Bright and wells potential gravitational deep their to Due cetdfrpbiaini h srnmclJournal L Astronomical using the typeset Preprint in publication for Accepted ujc headings: Subject holes. black supermassive and dwarfs, 9p,tpclo ausfudfrohrncercutr.W lop also We super a host clusters. also an nuclear may propose 4206) other NGC we for (in results clusters found observational nuclear values the of of another typical pc, 19 oetecneto ewe u bet n asv lblrclus globular massive and compo objects disk-like our the between tranforming connection for st the mechanisms between note period possible the mome discuss estimate angular We and lose structure. then spheroidal and older disks, an nuclear compact in episodically form G 24nces edtriealwrlmto h yaia aso 2 of mass dynamical the on limit lower a determine we nucleus, 4244 NGC w rmr tla ouain.Epotn msinlnsta pert appear that lines emission Exploiting populations. stellar more or two M ouain i pcrsoyadmlibn htmty hsnuc either This of ages photometry. with multi-band populations and single-stellar by spectroscopy via populations .-. antdsburta h peodcmoet,suggestin components, ages spheroid have the than bluer magnitudes 0.3-0.6 S/C.Teecutr aemgiue ( magnitudes have clusters These HST/ACS. rfre ntoo he ae.Tencercutrdssrnsh disks/rings and cluster spheroid nuclear a The of combination cases. a three by of fit two well in are preferred clusters flattened the r infiatyfltee n hweiec o utpe coincident within multiple, to aligned for are evidence clusters Ho three show these galaxies. of and dE elongations and flattened spirals significantly late-type face-on, are of studies previous in efid9ncercutrcniae nasml f1 deo,late-t edge-on, 14 of sample a in candidates cluster nuclear 9 find We ⊙ oee,sgicnl etrfist h pcrsoyadphot and spectroscopy the to fits better significantly However, . LE ONCERSA LSE OMTO RMEG-NSPIRAL EDGE-ON FROM FORMATION CLUSTER STAR NUCLEAR TO CLUES < 1. y.I G 24 h ers ftencercutr,w furth we clusters, nuclear the of nearest the 4244, NGC In Gyr. 1 A INTRODUCTION T E tl mltajv 6/22/04 v. emulateapj style X aaisnce aaissa lses–glxe:prl–galaxies:for – galaxies:spiral – clusters galaxies:star – galaxies:nuclei nvriyo ahntn srnm et,Bx318,Sea 351580, Box Dept., Astronomy Washington, of University nvriyo ahntn srnm et,Bx318,Sea 351580, Box Dept., Astronomy Washington, of University nvriyo ahntn srnm et,Bx318,Sea 351580, Box Dept., Astronomy Washington, of University nvriyo ahntn srnm et,Bx318,Sea 351580, Box Dept., Astronomy Washington, of University e:niiul(C55,NC44,NC4206) NGC 4244, NGC 5052, (IC ies:individual cetdfrpbiaini h srnmclJournal Astronomical the in publication for Accepted nsitu in uineJ Dalcanton J. Julianne itrP Debattista P. Victor alW Hodge W. Paul nlC Seth C. Anil ABSTRACT omto ehns o ula lsesi hc stars which in clusters nuclear for mechanism formation M ge, I − ∼ ∼ 0 y or Myr 100 1 n ie ( sizes and 11) lses ihtpclhl-ih ai f35p.How- pc. globu- 3.5 than with brighter of clusters, significantly lar are radii clusters half-light globular these Way nuclear ever, typical late-type Milky that with face-on, to found of clusters, similar dimensions (2004) sample have stud- al. their clusters Böker be et From galaxies, can and spiral detail. prominent in very are ied clusters star clear 2006). Côté al. 2005; et al. al. nuclear et et possess (Grant frequently Mastropietro clusters also originate 2006), star (e.g. al. may well, et galaxies which al. Beasley 2005; et as spiral presence galaxies, (Carollo low-mass elliptical spirals the phenomena from Dwarf nuclear by type other complicated ap- earlier 2002). and is clusters in bulges study Similar of frequent their be although (2002). to al. Bökerpear et by studied and ∼ nlt-yesia aaiswt iteo oble nu- bulge, no or little with galaxies spiral late-type In ∗ 10 − ◦ 6(¨kre l 02.Terhg uioiisare luminosities high Their 2002). al. (Böker et 16 fteglxe’mjrae.Structurally, axes. major galaxies’ the of ‡ † ∼ rfraineioe obe to episodes formation ar y,adwt asso 2-5 of masses with and Gyr, 1 httesasi hs components these in stars the that g ee,treo h ula clusters nuclear the of three wever, r mtyaeotie ycombining by obtained are ometry v 66-84 ( F606W-F814W ave eff es(e.g. ters asv lc oe ae nour on Based hole. black massive I eettnaieeiec that evidence tentative resent et noshris ealso We spheroids. into nents tmo etvrial oform to vertically heat or ntum ercutri qal elfit well equally is cluster lear te A98195 WA ttle, te A98195 WA ttle, te A98195 WA ttle, te A98195 WA ttle, ∼ iko ig ihtedisk the with ring, or disk a tutrlcmoet.The components. structural admgiue agn between ranging magnitudes band p aaisosre with observed galaxies ype c iia otoefound those to similar pc) 3 originate o .5 ω rcntantestellar the constrain er − +1 e) ultra-compact Cen), 1 . . 7 2 × ain–galax- – mation 10 ∼ ∼ 1 6 M ′′ -)colors V-I) ∼ ⊙ rmthe from S . Gyr 0.5 within × 10 6 − 8 2 Seth, Dalcanton, Hodge, and Debattista at least in part due to their large masses; Walcher et al. clusters form in situ from gas channeled into the center (2005) have used high-resolution spectra to derive typical of galaxies (Milosavljević2004). In this paper we present 6 masses ∼3×10 M⊙ for nine galaxies in the Böker et al. observational evidence that favors the in situ formation (2002) sample, similar in mass to the most massive Milky scenario. In particular, we find nuclear clusters that are Way cluster, ω Cen (Meylan & Mayor 1986). However, elongated along the major axis of the galaxies and have the brightness of nuclear clusters also results in part from two coincident components. These clusters appear to the presence of young stellar populations. Photometry consist of a younger flattened disk or ring and an older and spectra of a number of clusters suggest that nu- spheroid. Based on the evidence we present, we suggest clear clusters in both spiral and dwarf ellipticals have a detailed formation mechanism in which nuclear clus- populations much younger than globular clusters (e.g. ters form in repeated star formation events from very Ho et al. 1995; Lotz et al. 2004). In late-type galaxies, compact nuclear gas disks. most clusters appear to have stars with ages <100 Myr In §2 we describe the imaging and spectroscopic ob- (Walcher et al. 2006). Furthermore, recent spectral stud- servations used in the paper. §3 focuses on an analysis ies have shown that nuclear clusters are made up of com- of the morphology and luminosities of the clusters as re- posite stellar populations, with most having substan- vealed by HST observations. The stellar populations of tial old (&1 Gyr) stellar component (Long et al. 2002; the multiple-component clusters are examined in §4. In Sarzi et al. 2005; Walcher et al. 2006; Rossa et al. 2006). §5 we derive a dynamical mass for the NGC 4244 nuclear Not only are nuclear clusters present in a large frac- cluster, and then in §6 present tentative evidence that the tion of present day spirals and dwarf ellipticals, they also NGC 4206 nucleus may harbor both a star cluster and have been invoked as the progenitors of massive globu- a supermassive black hole. We present a detailed forma- lar clusters and ultra-compact dwarfs. In a hierarchical tion mechanism and discuss it in the broader context in merging scenario, many galaxies will be tidally stripped §7, and then summarize our findings in §8. during mergers leaving behind only their dense nuclei 2. OBSERVATIONS (e.g. Bekki & Chiba 2004). Nuclear cluster properties 2.1. match the half-light radii and exceptional luminosity of Identification of Nuclear Clusters massive globulars and UCDs (Mackey & van den Bergh Most of the data presented in this paper were taken 2005; Phillipps et al. 2001; Jones et al. 2006). Further- with the Hubble Space Telescope’s (HST) Advanced more, the multiple stellar populations seen in ω Cen and Camera for Surveys (ACS) as part of a Cycle 13 snap- G1 (e.g. Bedin et al. 2004; Meylan et al. 2001), and the shot program. Sixteen galaxies were observed in both evidence for possible self-enrichment among the bright- the F606W and F814W filters, with exposure times of est globular clusters in ellipticals (Harris et al. 2006) ∼700 seconds. Details of these observations and their strengthen the association between these bright compact reduction are presented in Seth et al. (2005) (Paper I). objects. We identified nuclear clusters (NCs) by careful exam- Finally nuclear clusters appear to be intimately con- ination of the central regions of our galaxies. The exact nected to the formation of supermassive black holes locations of the galaxy centers were determined using (SMBHs) at the centers of galaxies. For both fits to 2MASS data presented in Paper I. Nuclear clus- spiral and elliptical galaxies, a clear correlation is ters were distinguished based on their proximity to the seen between nuclear cluster mass and bulge mass photocenter of the galaxy, their brightness/prominence, (Côtéet al. 2006; Wehner & Harris 2006; Ferrarese et al. and their extent (i.e. if they appeared resolved). At the 2006; Rossa et al. 2006). This correlation appears to fall distance of most of our galaxies, NCs such as those ob- along the well-known M• −σ relation between black hole served by Böker et al. (2002) should be at least partially mass and bulge or galaxy mass (e.g. Ferrarese & Merritt resolved. 2000; Ferrarese 2002). Both SMBHs and nuclear clusters Table 1 lists the galaxies in our sample and whether appear to contain the same fraction of the total galaxy each possesses a NC (shown in the fifth column). In mass, supporting the idea that they are two related two of the sixteen galaxies in the sample, the central re- types of central massive object (Wehner & Harris 2006). gion was not covered by the ACS images (denoted with Whether a central massive object is a nuclear cluster or a “N/A” in Table 1). Of the remaining 14 galaxies, a SMBH appears to depend on mass; early-type galax- seven have prominent and/or extended sources located 10 ies more massive than ∼10 M⊙ appear to have SMBHs, within ∼2 arcseconds of the photometric center of the while lower mass systems commonly host nuclear clusters galaxy. Another two have possible detections of NCs, (Ferrarese et al. 2006). These findings strongly suggest where the candidate clusters were not very prominent a similar formation mechanism for nuclear clusters and (denoted with a “Maybe”). The remaining five galaxies SMBHs. Increasing evidence also points to massive star have no obvious candidate NCs. Of these five definite clusters as sites of intermediate-mass black hole (IMBH) non-detections, three (IRAS 06070-6147, IRAS 07568- formation (e.g. Miller & Colbert 2004; Gebhardt et al. 4942, & NGC 4631) are in galaxies where strong dust 2005; Patruno et al. 2006). These IMBHs may also fol- lanes could have easily obscured any extant cluster. The low the M• − σ relation (Gebhardt et al. 2002). other two of the non-detections (NGC 55 and IC 2233) In this context, a detailed understanding of how nu- are in low mass galaxies without strong dust lanes, sug- clear clusters formed may reveal the general mechanism gesting that these galaxies truly lack NCs. driving the formation of central massive objects of all In summary, we have detected candidate NCs in 50% to types. Two basic scenarios have been suggested (e.g. 65% of our sample. Eliminating the galaxies with strong Böker et al. 2004): (1) nuclear clusters form from multi- dust lanes, the fraction of galaxies with detected NCs ple globular clusters accreted via dynamical friction (e.g. may be as high as 82%. These detection fractions are Tremaine et al. 1975; Lotz et al. 2001), and (2) nuclear consistent with other studies, both for late-type galax- Nuclear Star Clusters in Edge-on Spirals 3

Fig. 1.— Images in the F814W filter of the nine candidate nuclear clusters. Each image is 2′′ on a side, and the bar in the bottom right corner indicates a length of 10 pc. Images have been rotated so that the x-axis lies along the position angle of the major axis of the galaxy disk. ies, where Böker et al. (2002) finds ∼75% of systems have spectral coverage from 3700A˚ to 5500A˚ on the blue side clusters, and for early-type galaxies observed as part of and 5600A˚ to 7200A˚ on the red side. A 1.5′′ slit was the ACS Virgo Cluster Survey (Côtéet al. 2006). How- used. Two exposures of 15 and 30 minutes were ob- ever, our sample is less well-suited than these studies for tained centered on the nuclear cluster and oriented along constraining the frequency of nuclear clusters because the the major axis of NGC 4244 with a position angle of - edge-on perspective can result in more chance superpo- 43◦. We note that the elongation of the nuclear cluster sitions and greater obscuration from dust. is aligned to within ∼10◦ of the galaxy’s major axis (see Fig. 1). Seeing at the time of observation was ∼1.0′′, 2.2. Apache Point Observatory 3.5-m Spectrum and thus the cluster was essentially unresolved. We reduced these spectra using a PyRAF pipeline designed for To better constrain the stellar populations of the DIS data reduction (written by Kevin Covey) which uti- NGC 4244 nuclear cluster (the nearest in our sample), lizes standard IRAF onedspec and twodspec routines. we obtained long-slit spectra on May 19, 2005 with The NGC 4244 cluster observations were wavelength cal- the Double Imaging Spectrograph (DIS) on the Apache ibrated using lamp spectra, and a further correction was Point Observatory (APO) 3.5m telescope. The DIS spec- applied to remove small shifts visible in the sky lines. trograph obtains both blue and red spectra simulta- The velocities were calibrated to the local standard of neously. We used the “medium-resolution” blue grat- rest using rvcorrect and dopcor. Observations of ing (1.22A/pixel)˚ and the “high-resolution” red grating Feige 66 at nearly identical airmass were obtained for (0.84A/pixel).˚ The resulting images had good quality 4 Seth, Dalcanton, Hodge, and Debattista

TABLE 1 Galaxy Sample and Nuclear Cluster Properties

Galaxy Type MB Vmax Possess Multiple MF 814W mF 606W − mF 814W m-M [km/sec] NC Components IC 2233 SBcd -16.60 79 No N/A 29.96b IC 5052 SBcd -17.23 79 Yes Yes -10.77 1.08 28.90a IRAS 06070-6147 Sc -20.50 133 No;Dust N/A 31.22d IRAS 07568-4942 SBc -17.70 142 No;Dust N/A 30.06c IRAS 09312-3248 Sc -17.90 98 Maybe No -9.24 1.36 30.61d NGC 55 SBm -17.05 67 No N/A 26.63a NGC 891 Sb -18.78 214 N/A N/A 29.61e NGC 3501 Sc -17.37 136 Yes No -10.58 1.62 31.04d NGC 4144 SBc -17.20 67 Maybe No -8.33 0.85 29.36a NGC 4183 Sc -18.41 103 Yes No -11.15 0.93 31.35f NGC 4206 Sbc -18.46 124 Yes Yes -14.17 0.93 31.29g NGC 4244 Sc -17.08 93 Yes Yes <-12.65i 0.77 28.20a NGC 4517 Sc -18.18 137 Yes Maybe -11.34 1.77 29.30c NGC 4565 Sb -20.47 227 N/A N/A 31.05h NGC 4631 SBcd -19.75 131 No;Dust N/A 29.42a NGC 5023 Sc -16.29 77 Yes Maybe -9.79 1.26 29.10a

Note. — Types and velocities from HYPERLEDA/LEDA (Paturel et al. 1995, 2003). (a) Seth et al. (2005) (b) Bottinelli et al. (1985), (c) Bottinelli et al. (1988), (d) assuming H0=70 km/sec, (e) Tonry et al. (2001), (f) Tully & Pierce (2000), (g) Gavazzi et al. (1999), (h) Jensen et al. (2003) (i) The derived magnitudes on the NGC 4244 cluster are lower limits due to the central pixels being saturated in both bands.

flux calibration shortly before the observations; however our sample have magnitudes and sizes similar to those the night was not photometric and thus the absolute flux observed in other galaxies. scale is not fully constrained. The two observations of the NGC 4244 cluster differ in flux scale by under 10%, 3.1. Model fitting and our absolute flux is probably uncertain by a simi- The dimensions and scales of the clusters were deter- lar amount. Combined, the two spectra give a S/N of mined via 2-D fitting to analytical functions (e.g. King ∼35 per pixel. The cluster spectrum was extracted from profiles). Because the objects are very compact, it was within a 2′′ diameter aperture, and regions used to de- necessary to include the effects of the PSF. We used two fine the background subtraction were within the confines similar methods to fit the nuclear clusters. First, we used of the galaxy, 20′′-30′′ (∼1/4th the scale length of the the ishape program (Larsen 1999), which was designed galaxy) along the major axis in both directions. Any to fit semi-resolved clusters. Unfortunately, ishape only contamination from the underlying galaxy component works with elliptically symmetric objects, which some of should be very small, as the source was well above the our objects clearly are not. Therefore, we wrote our own background at all wavelengths. We analyze this data in code in IDL, using the existing MPFIT2DFUN package to §4 and §5. perform Levenberg-Marquardt least squares fits. 3. NUCLEAR CLUSTER MORPHOLOGIES AND In both ishape and our program, analytical functions LUMINOSITIES are convolved with the PSF before fitting to the data. The most basic result of this paper is that nuclear We performed this convolution using a position-variable clusters are not all simple, single-component objects, PSF derived from our data (see Paper I for details), and as is evident from inspection of Figure 1, which shows over-sampled by a factor of 10 to minimize the effects the F814W images of all nine nuclear cluster candi- of aliasing and binning. A separate PSF was generated dates. Of these candidates three (IC 5052, NGC 4206, for each cluster based on its position on the chip. We & NGC 4244) look like miniature S0 galaxies, possess- conducted tests to insure that our program give similar ing both an elongated disk or ring component and a results to the ishape program. spheroidal component. These components are both com- We used ishape (1) to determine what types of func- pact with physical dimensions of tens of parsecs or less. tions best fit the data, and (2) to assess if each clus- We will show in §3.1.3 that these components appear to ter is indeed resolved. We fit Gaussian, King, Moffat, physically overlap. We therefore refer to these clusters and Hubble profiles to each cluster and found that King as multi-component clusters throughout the rest of the and Moffat profiles provided the best overall fits in terms paper. Two other galaxies, NGC 4517 & NGC 5023 also of the reduced χ2, as also found by Böker et al. (2004). show tentative evidence for similar structures. In this Both profiles provided equally good fits to our clusters, section we focus on modeling the morphology of the clus- and we chose to continue our fitting with elliptical King ters, in particular those that have multiple components. profiles of the form: We will show: (1) the nuclear cluster elongations are closely aligned with the disk of the host galaxy, (2) models with both a spheroidal and disk or ring components 1 1 fit the multi-component clusters significantly better than Σ(z)=Σ0 − (1)  z 2 rtidal 2  single-component models, and (3) the nuclear clusters in 1 + ( rcore ) 1 + ( rcore ) q q  Nuclear Star Clusters in Edge-on Spirals 5

z 2 with z = x2 + q s where Σ(z) is the projected surface brightness, x and z are the coordinates along the major and minor axes, and q is the axial ratio (minor/major). We have fixed the concentration (c ≡ rtidal/rcore) to be 15 for our fits, based on the fit concentrations of our brightest clusters and similar to that found for Galactic analog ω Cen (Trager et al. 1995). We report only half-light/effective radii as this quantity is well determined even for relatively low signal-to-noise data (Carlson & Holtzman 2001), and does not depend strongly on the chosen model (see Fig. 2, Böker et al. 2004). Given sufficient signal-to-noise, semi-extended sources can be detected with intrinsic widths as small as 10% of the PSF (Larsen 1999). The FWHM of our PSF is ∼0.9 pixels, meaning objects with widths smaller than a pixel should be easily resolvable. The signal-to-noise of our fits ranged from ∼50-1500. For four of the galaxies (IC 5052, Fig. 2.— Elliptical King profile fit results: the axial ratio q is NGC 4206, NGC 4244, & NGC 5023), the FWHM of the plotted against the position angle of the major axis of the nuclear fitted ishape King profiles is greater than one pixel, and cluster relative to the galaxy disk (∆PA). The connected data the objects are clearly partially resolved. The χ2 values points connect the fits for each galaxy in the two filters. Open 2 circles indicate the three galaxies (IRAS09312-3248, NGC 3501, of these fits is 2-20 times better than the χ of a point- NGC 4183) for which good fits were not achieved, due to the NC source fit (see last column, Table 2). The remaining fits compactness and complexity of the surrounding emission. This have FWHM >0.2 pixels, and have χ2 values 1-3× the figure shows that most of the clusters we have detected were 2 significantly flattened, and that the flattening aligns closely with best-fitting point-source χ . Of these, NGC 4144 and the plane of the galaxy. NGC 4517 are still clearly resolved, while IRAS 09312- 3248, NGC 3501, & NGC 4183 (all of which are at large distances) are only marginally resolved. suggesting that the single component King pro- 3.1.1. Elliptical King Model Fits file is not a good fit to the data. We note that Table 2 shows the results of our program’s fits to single- NGC 4517, which has a disk-like appearance in component elliptical King profiles with c = 15. These fits its outer isophotes, also has a high χ2 value. In show: NGC 4244 the central eight pixels were saturated and therefore excluded from the fit. 1. All the well resolved clusters are flattened, as shown in Figure 2. The median axis ratio (q ≡ b/a) 3.1.2. is 0.81, with q ∼ 0.4 for NGC 4206 and NGC 4244, Elliptical King + Disk Models the two systems with the most prominent disk (see To account for the flattened residuals remaining af- §3.1.2). ter fits to the elliptical king profiles in the IC 5052, NGC 4206, and NGC 4244 nuclear clusters, we have fit 2. The position angle of the three multi-component an additional morphological component to these clusters. nuclear clusters (IC 5052, NGC 4206, & NGC 4244) In this section we examine fits to disk models, in the next are all aligned within 10 degrees of the galaxy disk section we consider fits to ring models and compare the position angle. This can be seen plainly in Fig- disk and ring fits. ures 1 and 2. The NGC 4517 and NGC 5023 clus- As a disk model we adopted the surface brightness pro- ters also are somewhat elongated along the plane file of a transparent, self-gravitating, edge-on exponential of the galaxy. disk (van der Kruit & Searle 1981):

3. The effective radius (also known as the half-light x x 2 z radius) ranges between 1 and 20 pc, with most of Σ(x, z)=Σ0 K1 sech (2) hx hx z0 the clusters having effective radii between 1-4 pc. Figure 3 demonstrates that the measured effective where x and z are the major and minor axis coordinates, radii are similar to those observed for other ob- Σ0 is the central surface brightness, K1 is the modified jects including late-type nuclear clusters (filled cir- Bessel function, hx is the disk’s exponential scale length cles Böker et al. 2004) and galactic globular clus- and z0 is the scale height of the disk. We note that we ters (open triangles Harris 1996). tried other vertical profile fits (e.g. simple exponential) which were too concentrated near the midplane to pro- 4. After subtraction of the King profile fit, the resid- vide good fits to the data. uals of IC 5052, NGC 4206, and NGC 4244 clearly Figure 4 shows that the inclusion of a disk compo- show evidence for a flat disk- or ring-like component clearly improves the fit relative to the single com- nent (see the middle column of Fig. 4). The re- ponent elliptical King profile. Quantitatively, the χ2 of duced χ2 of these three galaxies is also very high, the fits shown in Table 3 is significantly reduced relative 6 Seth, Dalcanton, Hodge, and Debattista

TABLE 2 Elliptical King Profile Fits

a 2 2 Galaxy Pix. Scale Filter reff q ∆PA Reduced χPS /χ [pc/pixel] [pc] (b/a)[◦] χ2 [ishape] IC 5052 1.46 F606W 3.64 0.76 -3.8 23.8 5.78 IC 5052 1.46 F814W 2.93 0.80 -6.6 18.1 7.00 IRAS 09312-3248 3.21 F606W 2.46 0.68 43.1 2.8 1.50 IRAS 09312-3248 3.21 F814W 2.24 0.86 39.8 3.5 1.42 NGC 3501 3.91 F606W 6.82 1.00 89.9 8.2 1.41 NGC 3501 3.91 F814W 3.58 0.61 19.2 10.1 1.38 NGC 4144 1.81 F606W 1.46 0.85 83.4 3.2 2.69 NGC 4144 1.81 F814W 1.99 0.81 74.8 4.6 2.30 NGC 4183 4.51 F606W 2.76 1.00 -40.8 9.6 1.20 NGC 4183 4.51 F814W 3.02 0.78 68.8 6.4 1.12 NGC 4206 4.39 F606W 16.30 0.37 5.6 54.7 2.39 NGC 4206 4.39 F814W 15.81 0.40 5.3 42.4 2.75 NGC 4244 1.06 F606W 2.92 0.46 -10.0 94.1 20.73 NGC 4244 1.06 F814W 3.35 0.49 -9.9 82.3 19.63 NGC 4517 1.76 F606W 1.71 0.85 -22.9 46.0 1.20 NGC 4517 1.76 F814W 1.33 0.85 -16.1 64.2 1.23 NGC 5023 1.60 F606W 12.54 0.81 16.2 13.4 4.27 NGC 5023 1.60 F814W 10.22 0.83 18.6 11.1 6.89

Note. — (a) ∆PA is the position angle relative to the galaxy disk.

Fig. 3.— The fitted effective radius vs. the absolute aperture magnitude in the F814W filter. Large filled and open circles as in Fig. 2. Small symbols denote the following: Filled circles – late-type nuclear clusters from Böker et al. (2004). Open circles – dE,N nuclei from De Propris et al. (2005) (transformed to MI using Padova models which suggests stellar populations older than 1 Gyr have F814W-F850LP colors of 0.25±0.1 mags). Filled triangles – massive globular clusters in NGC 5128 observed by Martini & Ho (2004). Open triangles – Galactic globular clusters from the Harris (1996) catalog (note that ω Cen is the brightest of these). Filled Stars – Fornax UCDs from Drinkwater et al. (2003), assuming V-I of 1.1±0.1 as shown in Mieske et al. (2002). Open Stars – faint fuzzy M31 clusters from Huxor et al. (2005). Open Squares – young massive clusters (YMCs) from Bastian et al. (2006). Note that Padova models suggest MF 814W ∼ MI to within a few hundreths of a magnitude. Nuclear Star Clusters in Edge-on Spirals 7

if rin

2 z 2 2 Σ(x, z)= 2ρ0 sech rout − x z0 q else : Σ(x, z)= 0

This model is brightest for rin

TABLE 3 Elliptical King + Disk Profile Fits

Galaxy Pix. Scale Filter Disk Ell. King ∆PA Reduced 2 [pc/pixel] µ0 hx z0 µ0 reff q [◦] χ [mag/✷′′] [pc] [pc] [mag/✷′′] [pc] IC 5052 1.46 F606W 17.27 3.54 1.47 16.15 3.97 1.00 -10.8 20.3 IC 5052 1.46 F814W 16.28 3.48 1.19 14.74 3.55 1.00 -12.0 15.7 NGC 4206 4.39 F606W 17.44 31.00 15.76 12.87 4.94 0.45 5.1 26.8 NGC 4206 4.39 F814W 16.61 31.36 17.42 12.00 5.32 0.44 5.0 16.7 NGC 4244 1.06 F606W 13.64 2.51 1.64 16.29 10.79 0.78 -10.1 15.1 NGC 4244 1.06 F814W 13.18 2.73 1.42 13.84 5.70 0.73 -9.6 16.3

TABLE 4 Elliptical King + Ring Profile Fits

Galaxy Filter Ring Ell. King ∆PA Reduced 2 rout rin z0 Lring/ µ0 reff q [◦] χ ′′ [pc] [pc] [pc] Ltot [mag/✷ ] [pc] IC 5052 F606W 11.83 6.83 1.44 0.18 15.64 3.24 1.00 -11.0 16.9 IC 5052 F814W 12.13 6.91 1.61 0.14 14.36 3.05 1.00 -11.5 13.2 NGC 4206 F606W 84.82 0.44 18.56 0.53 13.57 7.69 0.48 5.3 32.5 NGC 4206 F814W 83.78 0.44 20.69 0.53 12.59 7.70 0.47 5.2 21.5 NGC 4244 F606W 10.42 0.11 0.92 0.18 13.19 4.07 0.58 -10.1 26.7 NGC 4244 F814W 10.75 0.14 0.91 0.16 12.51 4.25 0.59 -9.7 23.5 where the data is well-fit, but differ substantially when to-light ratios. Over time, as the young stellar popula- the fits are poor. tions age the disks will become less prominent. Figure 3 plots the F814W aperture magnitudes mea- The observed color of the clusters (see Table 1) can sured for all nine nuclear candidates against their fitted constrain the total dust column to the center of the host half-light radius from the elliptical King profiles. We galaxies. We estimated rough lower and upper limits also plot magnitudes and sizes of other compact stel- on the total reddening by comparing the colors of the lar systems, assuming that MI ∼ MF 814W since the NCs to that of an old and red, or young and blue stellar F814W filter is very similar to the Johnson I filter, with population. For most of the galaxies, the F606W-F814W single-stellar populations expected to be ∼0.03 magni- NC colors of ∼1 suggest that the reddening lies between tudes brighter in F814W than in I (Girardi 2006). E(B-V) of 0 and 1. However, for NGC 3501 & NGC 4517 Examination of Figure 3 shows that the magnitudes the reddening must be greater than 0.7 mags. and sizes of the objects in our sample are similar to the 4. NCs found by Böker et al. (2002, 2004) (filled circles) for NUCLEAR CLUSTER STELLAR POPULATIONS their sample of face-on, late-type galaxies. Böker et al. In this section we demonstrate that the multiple struc- (2004) measure sizes for only the brighter clusters in their tural components seen in some NCs in §3 also have dis- sample, but find NCs with measured magnitudes as faint tinct stellar populations. In particular, the disk com- as MI = −8.6. Because of their galaxies’ face-on ori- ponents are made of younger stars than the spheroidal entations, clusters with disk and spheroid components components. We first examine color maps of the clusters such as we have observed would have gone unnoticed in and argue that the disks most likely have populations their study. The morphology of the M33 nuclear cluster younger than 1 Gyr. We then examine the bright nearby is flattened (ǫ = 0.17) along an axis parallel to the ma- NC in NGC 4244 in detail, using multi-band photometry jor axis of the galaxy (Lauer et al. 1998; Matthews et al. and spectroscopy (see Fig. 6) to show that the spectra is 1999), as would be expected for a multi-component clus- best fit by two or more epochs of star formation. ter observed at the inclination of M33’s disk. The stellar Two recent papers have presented very thorough stud- populations in the spectra of both M33’s NC (Long et al. ies of the stellar populations of nuclear clusters in both 2002) and some of the objects in the Böker et al. (2002) early and late type spiral galaxies (Walcher et al. 2006; sample (Walcher et al. 2006), are also similar to what we Rossa et al. 2006). Both use fitting of spectra to convinc- see in §4.2 for NGC 4244’s NC. It is therefore clear that ingly show that nuclear clusters contain multiple gen- the nuclear clusters observed here are similar to those erations of stars. For a sample of nine bright nuclear found previously. Furthermore, the multi-component clusters in late-type galaxies, Walcher et al. (2006) has morphology may be a common feature of nuclear clus- shown that all of the clusters observed had stellar popu- ters. lations younger than 100 Myr. The typical mass of the 5 We also note that the two most luminous clusters youngest stars was a few×10 M⊙. However, in most are those with the most prominent disk components of the clusters, the bulk of the stellar mass had an age (NGC 4206 & NGC 4244). This correlation is not sur- of &2.5 Gyr. Our findings on the stellar populations in prising, as the disks seem to be composed of recently our multi-component clusters, presented below, are fully formed stars (see §4) and would therefore have low mass- consistent with these papers. Nuclear Star Clusters in Edge-on Spirals 9

Fig. 5.— Maps of the F606W-F814W [VEGAMag] color for each cluster. Contours showing the F606W brightness are overlaid in black. The clusters with obvious disk components (IC 5052, NGC 4206, & NGC 4244) all show bluer disks and redder spheroids. The color bars on the right side indicate values for the F606W-F814W color. Colors have been corrected for foreground reddening. The central eight pixels of the NGC 4244 NC were saturated and thus don’t give accurate colors. The circled region in NGC 4244 shows the suspected HII region used to obtain a dynamical mass estimate in §5.

4.1. Color Maps lyzed by Walcher et al. (2006) showed evidence for stellar Figure 5 shows F606W-F814W color maps for the three populations with ages <100 Myr nuclear cluster candidates identified as having multiple The suggested interpretation of the color difference as components. In each cluster, an obvious and significant a difference in stellar population may be complicated by color difference exists between the disk and spheroidal a number of factors. First, even if two separate compo- components. The disks are 0.3-0.6 magnitudes bluer in nents exist, both likely contribute to the observed color F606W-F814W than the reddest parts of the spheroids. at most locations. This overlap suggests that the actual The observed color difference between the disk and spread in age is larger than implied by the color differ- spheroid components is most simply interpreted as a dif- ences. Second, the colors could be affected by the pres- ference in stellar population. Despite the unknown red- ence of dust structures on the same scale as the clusters dening, we can place upper limits on the age of the disk or by emission line contribution. Based on the symme- component. Using the Padova single stellar population try of the nuclear clusters (e.g. the disk is roughly the models (Girardi 2006), for a 10 Gyr old stellar popula- same color on both sides of the nucleus), and the spectral tion with metallicities between [Fe/H] of -1.7 and -0.4, results from §4.2 we believe that the color difference is the F606W-F814W color ranges from 0.73 to 0.91, with unlikely to be due solely to dust or emission lines and redder colors for more metal-rich populations. The dif- instead really does reflect a difference in stellar popula- ference in color between a 10 Gyr and 1 Gyr population tion. is .0.25 magnitudes regardless of metallicity, suggesting 4.2. Stellar Populations in NGC 4244 that disks cannot be composed solely of intermediate age (>1 Gyr) stars. At ages younger than 1 Gyr, however, In the previous section we showed that the color off- the color quickly becomes bluer. Thus the > 0.3 mag set between the disk and spheroid components of the color difference is evidence that the disks have stellar nuclear cluster indicates that the disk has a systemati- populations younger than 1 Gyr. Put another way, we cally younger stellar population. However because of the find that the disk components all must have intrinsic col- uncertain reddening, the ACS colors alone cannot accu- ors bluer than F606W-F814W=0.65, which corresponds rately constrain the ages of the stellar populations. In to single-stellar populations younger than 1 Gyr. Our this section we use our APO spectrum and photometry analysis of the spectrum of NGC 4244’s NC (§4.2) con- of the NC in NGC 4244 from the Sloan Digital Sky Sur- firms that stars with ages .100 Myr are present. Fur- vey (SDSS) and 2MASS survey to constrain the cluster’s thermore, each of the nine nuclear cluster spectra ana- age. Before describing the details of this analysis we first describe the photometry. 10 Seth, Dalcanton, Hodge, and Debattista

Fig. 6.— A portion of the APO/DIS spectrum of the NGC 4244 NC with best-fitting models overplotted. Only the portion of the spectrum with the most significant spectral features are shown. Top Panel – single-age best fits at 70 Myr and 0.8 Gyr - the ages at which there are minima in the joint-χ2 distribution (see Fig. 7). The most age-sensitive portion of the spectrum lies around the Ca H line at ∼3940A,˚ although numerous other features also cannot be fit well with a single-age population. Middle Panel – the best two-age fit, including both stars with ages of 0.1 Gyr (dark gray line) and 1.0 Gyr (light gray line). This fit is a significant improvement relative to either single-age fit. Bottom Panel – Residuals for all three fits.

The nuclear cluster in NGC 4244 is prominent, and saturated in the ACS data (taken with gain=1 e−), we stands well above the emission from the galaxy disk. were unable to derive accurate photometry from the HST Because of this, it was detected as a point source in images. both the SDSS and 2MASS surveys. This data com- 4.2.1. Single-Age Fits pliments the spectroscopic data by giving a large wavelength baseline to constrain the mass, reddening, and In principle, the photometry and spectrum can be ages of the NC (de Grijs et al. 2003). In both surveys, fit with arbitrary stellar populations, but in practice the source is essentially unresolved, and photometry is this leads to far too many degeneracies. To simplify determined from PSF-fitting. In the SDSS survey, the the problem we started by assuming the NGC 4244 DR4 object ID is 58773909806344622, with PSF mag- cluster contains stars of a single age and then deter- nitudes of 17.81, 16.62, 16.18, 15.96, and 15.73 in the mine a best-fitting extinction, mass and χ2 as a func- u′, g′, r′, i′, and z′ filters. In 2MASS, the object ID is tion of age from both the photometric and spectroscopic 12172945+3748264, with magnitudes of 14.36, 13.75, and data separately. For the photometric data, we used the 13.55 in J, H, and K bands. The errors on the SDSS single-stellar population models from the Padova group magnitudes are ∼0.02 mags, while the 2MASS observa- (Girardi et al. 2004) using the Marigo (2001) treatment tions have errors of ∼0.05 mags. The difference in res- of the AGB. These models have been produced for both olution and photometric methods between the two sur- the SDSS and 2MASS photometric systems and assume veys means there may be some relative systematic error a Kroupa (2001) initial mass function (IMF) between 0.1 in the magnitudes. Therefore, any conclusions based on and 100 M⊙. The extinction was calculated in Av assum- the photometry alone should be considered with caution. ing Rv =3.1 using coefficients from Girardi et al. (2004) We note that because the central pixels of the NC were and Cardelli et al. (1989) for the Sloan and 2MASS data respectively. For the spectroscopic fitting, single stellar Nuclear Star Clusters in Edge-on Spirals 11 population spectral templates from Bruzual & Charlot (2003) were used. These templates assume a Chabrier (2003) IMF from 0.1 to 100 M⊙. They have 1A˚ resolution, and were redshifted and interpolated to the wavelengths of our observed spectrum. The reddening was applied to the spectrum using the Cardelli et al. (1989) prescription. We fit the feature rich blue side of the DIS spectrum extending from 3650A˚ to 5500A˚ which contains Balmer, He I, Mg, and Ca H & K lines (see Fig. 6). The signal-to-noise of this portion of the spectrum ranged from 20 to 60. For both the photometry and spectroscopy, we determined the best-fitting mass and reddening at a given age using the IDL CURVEFIT routine. All fits were done assuming a distance modulus of 28.20 (D=4.4 Mpc, Seth et al. 2005), and a metallicity of [Fe/H]=-0.4, similar to the expected current gas phase metallicity of NGC 4244 (Garnett 2002). This is the same metallicity derived from composite age fits for a majority of the clusters found by Walcher et al. (2006). Fur- thermore, Rossa et al. (2006) and Walcher et al. (2006) have shown that the inferred age distribution does not depend strongly on the metallicity. Figure 7 shows the results of our fits for single stellar populations with ages between 107 and 1010 years for both the spectroscopy and photometry. The top panel indicates the χ2 of the best-fitting model at each age. Fig. 7.— Photometric and spectroscopic fits of the mass and reddening of the NGC 4244 NC using single-stellar population mod- The spectrum is best fit by populations younger than 2 9 6 els. Top panel – the reduced χ values of the best-fitting mass and .10 years, stellar masses of 2-3×10 M⊙ and decreasing reddening at each age. Middle panel – The best-fitting reddening, 2 extinction with increasing age. The χ has a minimum bottom panel – the best fitting mass. The dashed vertical lines in- at ages of ∼0.8-1.0×109 years (just before the onset of dicate the two minima in the joint χ2 distribution at 70 Myr and the RGB), and then increases rapidly towards older ages. 0.8 Gyr. This results from the template spectrum becoming redder than the observed spectrum. A shallower χ2 minima is also seen in the spectra at ∼50-100 Myr. The fits to the photometric data have two comparable tral and photometric data, or (2) by fitting the spec- χ2 minima at ∼108 and ∼109 years, with the younger trum to a constant star formation rate (SFR) model age being somewhat preferred. The minima at these two from Bruzual & Charlot (2003). These approaches both ages is more pronounced in the photometric data than give significantly better fits to the data than single-stellar in the spectroscopic data, perhaps as a result of the pho- population models, reducing χ2 values by 50% or more. tometric data’s longer wavelength coverage. Combining However, it is also clear that there is no unique solution; the photometric and spectroscopic results, the minima in the data can be fit equally well by many different combina- the joint χ2 are at 70 Myr and 0.8 Gyr. The masses from tions of stellar populations and reddening values. Note 6 the photometric data at these minima are 3-5×10 M⊙, that we assumed a single extinction (AV ) value for all somewhat more massive than the spectral data. This dif- components. ference may result from light lost outside the 1.5′′ wide Of the nine NC spectra presented in Walcher et al. slit, uncertainty in the absolute calibration of the spec- (2006), our spectrum of the NGC 4244 NC most closely troscopic data, and/or differences in the assumed IMF. resembles their spectrum of the NGC 7793 NC. From Spectral fits from the two joint-χ2 minima are shown in their fits to composite stellar populations, they find that Figure 6. The younger age appears to fit the bluer part of the NGC 7793 cluster has an average stellar age of the spectrum better, while the older age more accurately ∼2 Gyr, but has young stars (.100 Myr) that make up fits many of the lines and the redder continuum. The 10% of the mass. This is qualitatively similar to what Ca H line is particularly age sensitive, being much too we find below for the NGC 4244 NC. shallow in the younger age and too deep in the older age. Two Age Models: The simplest stellar population 4.2.2. model consistent with the color maps is a two-age stellar Multi-Age Fits population: a young population associated with the disk, The color difference between the disk and spheroid and an older population associated with the spheroid. components discussed in §4.1 and the recent results of Taking the two minima seen in the single-stellar popula- Rossa et al. (2006) and Walcher et al. (2006) strongly tion fits at ages of 70 Myr and 0.8 Gyr as a starting point, suggest that NCs contain stellar populations of multi- we explored what combination of two populations gives ple ages rather than the single-age populations assumed the best fit to both the spectroscopic and photometric above. We have tried to constrain the ages and masses data. For a young component with ages between 50-100 of the multiple stellar populations in NGC 4244’s NC Myr and an intermediate-age component with ages be- in two ways: (1) by fitting a weighted average of two tween 0.6 and 1.0 Gyr, the best fits to the spectroscopy 5 or three stellar populations of different ages to the spec- have AV ∼ 0.5, masses of 0.5-3×10 M⊙ for the young 12 Seth, Dalcanton, Hodge, and Debattista

6 stellar population, total masses of 2-4×10 M⊙, and re- Constant SFR model: Using the Bruzual & Charlot duced χ2 values of ∼6. However, for the photometry, (2003) codes, we generated a model spectrum assuming a the best fits favor an AV > 1 and masses dominated by constant SFR from 12 Gyr to the present. The best-fit of the young component. These photometric fits clearly do this model to the data gives a current mass of the cluster 6 not reproduce the shape or lines in the spectra, thus we of 6.2×10 M⊙, and a reddening of AV =0.72. The re- 2 2 restricted ourselves to lower AV solutions. When AV is duced χ of the fit is 5.7, within 10% of the reduced χ of restricted to be less than one, the photometric fits are the best fits obtained for the two- and three-age models. in relatively good agreement with the spectroscopic fits. The spectral shape of the constant SFR model appears to However, the implied stellar masses for both the young be somewhat redder than the data, suggesting either the and intermediate-age components are up to two times need for relatively more young stars than in the constant larger than the spectroscopic best-fit models, similar to SFR model, or for a more realistic metallicity evolution, what was seen in the single-age fits. Overall, we find that in which the older stars are bluer and more metal-poor. ages of 0.1 and 1.0 Gyr do the best job of matching both The former point is supported by Walcher et al. (2006), sets of data. The spectroscopic best fit at these ages is who have shown that in their sample of clusters, the star 5 shown in Figure 6, and has AV = 0.46, 1.9×10 M⊙ of formation rate in the past 100 Myr is an order of mag- 6 0.1 Gyr old stars and 3.1×10 M⊙ of 1 Gyr old stars. nitude greater than the star formation rate between 0.3 Based on the color map of the nuclear cluster, we ex- and 20 Gyr. However, this enhancement of recent star pect the younger stellar component in the two-age fits formation may result from a bias in their sample towards be associated with the disk. We can test the consis- brighter nuclear clusters. tency of this idea by comparing our spectral fits to the luminosities of the disk component decomposed from the 4.2.3. Summary of Stellar Populations in the HST data in §3. Assuming that the disk is dominated NGC 4244 NC by ∼70 Myr old stars, the expected F606W-F814W color We draw the following conclusions from the spectral from the Padova models is 0.25 (Girardi 2006). The ob- and photometric fits of the NGC 4244 NC: served disk component color is 0.4, requiring an extinction AV = 0.47 mags. Using this extinction and mass- 1. Multiple stellar populations definitely provide im- to-light ratios from the Padova models, we derive a mass proved fits over single-stellar populations for both 5 for the disk component of 1.2×10 M⊙. Both the mass the spectroscopy and photometry of the NGC 4244 and extinction derived from the luminosities of the disk NC. However, many different combinations of component in the HST data agree with the young stellar masses and ages fit the data well. mass and extinction determined in the two-age spectral fits. This agreement strongly suggests a link between 2. The best-fitting single stellar populations have ages the blue disk component and the young stellar compo- of either ∼70 Myr or ∼0.8 Gyr. nent that appears to be required to fit the blue spectral 3. The total current mass of the cluster appears to be features. 7 between 0.2 and 1×10 M ⊙. Three Age Models: We also explored the sensitivity of our fits to the inclusion of a third, old (10 Gyr) stellar 4. If we assume only two ages are present, the population. We find that a large mass of old stars is youngest (. 100 Myr) stellar component appears 5 consistent with both the photometric and spectroscopic to have ∼10 M⊙ of stars. This mass is consistent data. Best-fits to both data sets have low AV (<0.4), with the luminosity of the disk component decom- 7 total masses of ∼10 M⊙ with 90% of that mass in the posed using the morphological fits in §3. 5 old component, and ∼10 M⊙ in the young component, similar to the two-age fits. The inclusion of the older 5. For all multi-age fits, the mass of stars younger component significantly improves the photometric fit to than 100 Myr is a small fraction (< 5%) of the the data (reduced χ2 =1.7), suggesting older stars may total mass of the cluster. indeed be present. However, the spectral fit is insensitive 6. Stars older than 1 Gyr may be present in large to this old component; the best-fit spectrum is nearly 7 numbers (∼10 M⊙). Inclusion of an old stellar identical to the two-age fits, despite 90% of the mass being contained in the old component. population particularly improves our fits to the The effects of disk contamination on the photome- SDSS and 2MASS photometric data. try and spectra should be minimal given the promi- 7. A constant SFR fits the data as well as assuming nence of the NGC 4244 NC. Futhermore, by compar- two or three discrete ages. ing HST/STIS and ground-based spectra, Walcher et al. (2006) show that the stellar population immediately sur- 5. A DYNAMICAL MASS ESTIMATE FOR THE NGC 4244 rounding the nuclear cluster in late-type galaxies seems CLUSTER to have a population similar to the cluster itself. Both Emission lines, including Hα, [NII], [SII], and [OII] Rossa et al. (2006) and Walcher et al. (2006) find some are seen in our spectrum of the NGC 4244 NC. For the evidence for an underlying old stellar component in many Hα line, the emission component is redshifted relative to of their nuclear cluster spectra. We note that all the the underlying stellar absorption lines (see Fig. 8). We masses quoted here refer to the initial mass in stars re- utilize this offset between the emission and absorption quired to fit the current spectral data. Due to stellar components to obtain a dynamical mass estimate for the evolution, the current mass of a cluster dominated by NGC 4244 cluster. old stars would be ∼2× lower than the initial mass, given Examination of the color map for NGC 4244 in Fig- reasonable initial mass functions. ure 5 shows a very blue compact source located along Nuclear Star Clusters in Edge-on Spirals 13

the galaxy disk. Figure 8 shows the fitted emission and absorption lines, as well as the best fit of absorption + emission for the Hα line after fixing the velocities of the absorption and emission to the values derived from the NaI, [SII], and [NII] lines. This Hα line fit does a good job of replicat- ing the observed line shape thereby suggesting that the velocities we have derived are correct. Assuming the projected radius of the HII region is close to the true radius, and that it is bound to the NGC 4244 NC nuclear cluster, the ∆v = 24.1 ± 7.0 km/sec offset between emission and absorption implies a nuclear clus- +1.7 6 ter mass of 2.5−1.2 × 10 M⊙ within a radius of 19 pc. If the object is indeed rotating around the cluster, but is in front of or behind the cluster (thus making its projected distance larger), then the mass we derived is a lower limit on the mass of the cluster. This rough estimate of the dynamical mass of NGC 4244’s NC is consistent with the stellar masses determined for the one- and two-age fits in §4.2. It is also Fig. 8.— Spectral lines used to derive the relative offset between emission and absorption. Dark lines show the actual spectrum, consistent with the constant SFR or three-age model fits 6 while lighter lines show the best fitting gaussian or double-gaussian (M∼6×10 M⊙) if the HII region is seen in projection model. The number in the title of each panel gives the fitted ve- and lies ∼25 pc from the center of the nuclear cluster. locity and error bar. The spectra has a S/N∼35/pixel. The estimated mass is also similar to the typical mass determined by Walcher et al. (2005) for late-type galaxy nuclei. They derived masses for nine nuclear clusters the NC plane on the left (NE) side, 0.88′′(18.6 pc pro- from the Böker et al. (2002) sample by combining size jected) from the NC center. This compact blue object and velocity dispersion measurements. Their total range 5 7 has an F606W-F814W color of ∼0.3, which is ∼0.4-0.6 of masses is 8×10 M⊙ to 6×10 M⊙, with seven of the 7 mags bluer than the disk and spheroid components of nine clusters having masses between 0.1 and 1×10 M⊙. the cluster. Assuming moderate reddening and blending We note that they have determined these masses as- with the outer isophotes of the cluster, the color of the suming spherical symmetry; if these clusters have disk blue knot is consistent with this object being an HII re- components similar to the ones we have observed here, gion. HII regions in our filter combination appear very the masses derived by Walcher et al. (2005) may be un- blue because there are strong emission lines located in derestimates, since they do not take into account the the F606W filter and not in the F814W filter. Measure- rotational support that would be expected in a disk. ments of HII regions in NGC 55 (the nearest galaxy in Given that their clusters contain young stellar popula- our sample) suggests HII regions have typical colors of tions (Walcher et al. 2006), this effect could be signifi- F606W-F814W=-0.2 with a ∼0.2 mag scatter. cant. This likely HII region falls within the slit used for our 6. nuclear cluster spectrum. Therefore, we identified this NGC 4206: POSSIBLE INDICATION OF AN AGN COMPONENT object as the likely source of the emission lines in our spectra. We verified this by extracting spectra from the In this section, we present evidence that the nuclear left (NE) and right (SW) sides of the cluster. The left cluster in NGC 4206 may contain an AGN component. side spectrum shows significantly enhanced emission lines This evidence is very tentative and requires confirmation relative to the right side. However, because the seeing with follow-up observations. was ∼1′′, we were unable to spatially separate the emis- An SDSS spectrum (MJD 53149, Plate 1612, Fiber 603) exists for the central regions of NGC 4206. Due sion component from the nuclear cluster. We now pro- ′′ ceed under the assumption that the emission lines in our to the 3 diameter (∼250 pc) aperture, this spectrum is spectrum are associated with this compact blue object. dominated by the light from the central regions of the To determine the relative velocity of the emission and galaxy and not by the nuclear cluster. In addition to absorption components we fit the [SII] doublet and [NII] a dominant old stellar spectrum, narrow Hα, [NII], and line to determine the emission line velocity, and fit the [SII] emission are clearly seen. No broad emission lines NaI doublet to determine the absorption line velocity. are seen. Examining the HST color map, the center of We obtained a weighted mean velocity of 270.5±5.4 the nuclear cluster is the bluest area anywhere within the km/sec for the emission lines and 246.3±4.3 km/sec for SDSS spectral aperture. Since emission line sources are the NaI absorption line assuming air wavelengths for very blue in our filter combination, the central portion the lines from the Atomic Line List1. This absorp- of the nuclear cluster is the likely source of the emission tion velocity matches the systemic velocity of NGC 4244 lines. The emission lines are not offset in velocity from (244.4±0.3 km/sec, Olling 1996) to within the error bars, the absorption spectrum (as was seen in NGC 4244) fur- as would be expected for the nuclear cluster. We note ther supporting their nuclear origin. that this HII region is counter-rotating with respect to The ionizing source of the observed emission lines could be either star formation or AGN activity at the center 1 http://www.pa.uky.edu/˜peter/atomic/ of the nuclear cluster. These two ionizing sources can be 14 Seth, Dalcanton, Hodge, and Debattista distinguished using the line ratio of [NII] and Hα. Be- Walcher et al. (2006) that an initial seed star cluster is cause the Hα line is contaminated by stellar absorption, required for subsequent nuclear star cluster formation. we model the underlying stellar continuum with a single The details of an in situ formation mechanism can be stellar population ∼3 Gyr in age with AV ∼ 0.5. After constrained by our observations. To explain presence of subtracting this spectrum, the line ratio of the remain- redder/older spheroids in the multi-component clusters ing emission lines is log([NII]/Hα)∼−0.3. Based on the and the absence of disk components in a majority of our recent analysis of SDSS spectra by (Obric et al. 2006), nuclear clusters, we propose a model in which nuclear this line ratio suggests that the source is a “mixed” star- cluster disks form episodically and are transformed into a burst/AGN. NGC 4206 may therefore host a small AGN rounder distribution over time due to loss of angular mo- within the core of the nuclear star cluster. If confirmed mentum or dynamical heating. There are two important by follow-up observations, this would make it only the timescales in this model: (1) the period of time between second late-type spiral known to have both a SMBH and star formation episodes, and (2) the time it takes stars in massive nuclear star cluster, the other being NGC 4395, a disk morphology to end up in a spheroidal distribution. an Sd galaxy with MB ∼−17.5 hosting a nuclear cluster We examine these two timescales in detail below. with MI ∼ −10.1 (Matthews et al. 1999), and a black 4 5 7.1.1. Period Between Star Formation Episodes hole with a mass of ∼10 − 10 M⊙ (Filippenko & Ho 2003). In earlier type spirals, other examples exist of A number of authors have previously suggested that the co-existence of nuclear star clusters and AGN (e.g. star formation in the nuclear regions of late-type spirals Thatte et al. 1997; Davies et al. 2006). We plan to test may be episodic. The physical motivation for episodic for the presence of AGN components in all of our multi- star formation is that a massive star formation event component nuclear cluster candidates with near-IR inte- will feedback on the local environment preventing star gral field unit spectroscopy using adaptive optics. formation for the next ∼108 years (Milosavljević2004). 7. Although this feedback may destroy the gas disk and DISCUSSION shut off star formation, it does not disrupt the disk of We now discuss how our data constrains the formation recently formed stars, whose lifetime we discuss below in mechanisms of nuclear clusters. We suggest a forma- 7.1.2. tion mechanism in which stars in nuclear clusters form Assuming that the present epoch is not a special time episodically in disks and then over time lose angular mo- in the life of the nuclear clusters, one can estimate the mentum or are heated vertically, ending up in a more period on which star formation recurs. Observation- spheroidal distribution. After discussing this mechanism ally, Davidge & Courteau (2002) argue for a period of in some detail, we examine the relation of nuclear clus- ∼0.3 Gyr based on the detection frequency of Paα emis- ters to other massive star clusters and supermassive black sion in galaxy nuclei. Schinnerer et al. (2003) also deter- holes. mine a recurrence period of a few hundred Myr to 1 Gyr based on the gas accretion rates observed in IC 342’s nu- 7.1. Formation Mechanisms cleus. More recently, from analysis of the youngest stellar Our observations of multi-component clusters place components in the spectrum, Walcher et al. (2006) has significant constraints on nuclear cluster formation mech- found the typical time between star formation bursts to 5 anisms. The presence of young NC disks (<1 Gyr) be ∼70 Myr with a typical mass of roughly 1.6×10 M⊙. aligned with the disks of the host galaxies strongly sug- Our observations of the NGC 4244 nuclear cluster are gests that they are formed in situ from gas accreted consistent with these previous estimates. Based on the 5 into the nuclear regions. We have shown in §3.2 that ∼10 M⊙ of stars in the young component (§4.2), ap- these disks may be a common feature of nuclear clus- proximately 25 episodes of disk formation must have oc- 6 ters. Such disks cannot be formed via the accretion of curred to build up the 2.5×10 M⊙ of the cluster (§5). If globular clusters by dynamical friction. While globu- this occurs over a Hubble time, then the period between lar cluster accretion very likely occurs, this mechanism bursts is ∼0.5 Gyr. has been shown to be relatively inefficient in late-type 7.1.2. galaxies (Milosavljević2004). We propose here a scenario Timescale of Disk Transformation in which the dominant formation mechanism for nuclear Given that nuclear cluster disks are observed in <50% clusters is via in situ formation in disks. Although the of nuclear clusters, and that these disks all have stel- mechanism will need to be tested with detailed simula- lar populations younger than a Gyr, a mechanism must tions, it is consistent not just with our data, but with exist that destroys or disrupts this disk on a timescale both the evidence for young and composite stellar popu- shorter than ∼1 Gyr. We suggest that angular mo- lations in many nuclear clusters (e.g Walcher et al. 2006), mentum loss or dynamical heating causes the disks and the direct detection of a molecular gas disk coinci- to be transformed over time, thus creating the more dent with the nuclear cluster in IC 342 (Schinnerer et al. spheroidal, older components seen in our nuclear clus- 2003). ters. Although mechanisms of angular momentum loss On a large scale (∼1 kpc), both dissipative merging have not been studied previously in nuclear clusters, of molecular clouds and magnetorotational instabilities these objects are dynamically similar to globular clus- have been shown to be capable of supplying the nu- ters, for which the theory of rotation and rotational cleus of a late-type galaxy with sufficient gas to form evolution has been well-developed (Agekian 1958; King nuclear clusters (Milosavljević2004; Bekki et al. 2006). 1961; Shapiro & Marchant 1976; Fall & Frenk 1985; Our proposed in situ formation mechanism is concerned Davoust & Prugniel 1990; Longaretti & Lagoute 1997). with what occurs once the gas reaches the center. Our As they age, star clusters flattened due to rotation model is also fully consistent with the suggestion by will become increasingly round due to the preferential Nuclear Star Clusters in Edge-on Spirals 15 evaporation of high angular momentum stars (Agekian in rings is consistent with the proposed formation model. 1958). We can compare the timescale of this angu- The ring in IC 5052 is internal to the cluster itself, so lar momentum loss to the expected lifetimes of our after loss of angular momentum or dynamical heating, disks. The timescale over which the ellipticity of a rotat- its stars would still contribute to the stellar distribution ing stellar cluster decreases is &40× the median relax- of the spheroid. The radius of the ring depends on the ation time, τrh (Shapiro & Marchant 1976; Fall & Frenk current mass of the star cluster in both mechanisms for 1985). For globular clusters in the Milky Way, τrh is ring formation described above. This correlation might typically ∼108 years. However, this timescale is longer provide a natural explanation for the clear correlation for more massive clusters like ω Cen, with τrh =3.5 Gyr of nuclear cluster size and luminosity observed in dwarf (Shapiro & Marchant 1976). For the NGC 4244 NC we elliptical galaxies by Côtéet al. (2006). A similar expla- have calculated a median relaxation time for the whole nation could work for formation in disks if the size of the cluster of ∼2 Gyr (Eq. 8-72 Binney & Tremaine 1987). disks depends on the mass of the nuclear cluster. De- For just the disk component, assuming it has a mass tailed modelling of disk/ring formation and dissipation 5 of ∼10 M⊙ and a half mass radius of ∼3 pc, we get would be required to test the feasibility of this idea. × 8 τrh =4 10 years. These relaxation times suggest that 7.1.4. the timescale for evaporation of angular momentum in Other Formation Scenarios NGC 4244 is in the range of 16-80 Gyr. This timescale Our proposed scenario naturally explains the forma- is significantly longer than the 100s of Myrs over which tion of multi-component clusters. However other scenar- we expect the disk to be transformed into a spheroidal ios may also be consistent with our observations. While component. our observations seem to require some in situ formation It therefore appears that angular momentum loss due to explain the presence of young stellar disks, they do to processes internal to the clusters are unlikely to trans- not rule out the possibility that the multi-component form the disk components into spheroids on sufficiently nuclear clusters were formed in part from the merging of short timescales. Instead, we must look to external pro- globular clusters (e.g. Tremaine et al. 1975; Lotz et al. cesses that can affect the the dynamical evolution of the 2001). In this “hybrid” scenario, the young disk com- nuclear cluster. We have identified a few alternative ponents may be a late-time veneer on an already mas- mechanisms for transforming the stellar disk component sive cluster formed from merged globular clusters. How- into a spheroid. These include: (1) small angular off- ever, we note that Milosavljević(2004) has calculated sets of the nuclear disk from the plane of the galaxy, and that the timescale for dynamical friction in late-type spi- (2) slight positional offsets of the cluster from the center rals is & 3×109 years even for relatively massive clusters 6 of the galaxy potential, (3) torques on the stellar disk (10 M⊙) located within the central kpc of the galaxy. from successive gas accretion. A forthcoming paper will This quantity increases for less massive and more distant discuss this problem in more detail. clusters. He concludes that this timescale is thus too long to be the dominant mechanism for nuclear cluster forma- 7.1.3. Rings tion, and shows that magnetorotational instabilities can We have been assuming thus far that stars are formed transport sufficient quantities of gas into the galaxy cen- initially in disks, as suggested by our observations of ter to account for the observed NCs in late-type spirals. NGC 4244 and NGC 4206. However, in §3.1.3, we showed 7.2. Relationship to Other Massive Star Clusters that one of the three multi-component NCs, in IC 5052, was best fit by a spheroid + ring model with inner and Our observations have implications beyond just the outer ring radii of 6.8 and 11.8 pc. We discuss here how formation of nuclear star clusters in late-type galaxies. such a ring could form and examine rings in context of We briefly review here the possible connections between our proposed nuclear cluster formation model. late-type spiral nuclei and dwarf elliptical nuclei, and the Based on an elongated near-infrared component in evidence that some massive globular clusters and ultra- IC 342, Böker et al. (1997) suggested that the observed compact dwarfs are stripped nuclear clusters. molecular ring (with radius2 of ∼65 pc) results from an 7.2.1. The relation between spiral and dE nuclei inner Lindblad resonances (ILR) with a stellar bar. If a similar mechanism is at work in forming the ring in The recent study by Côtéet al. (2006) shows that the IC 5052, it would require the star cluster itself to have a nuclei of dE and late-type spiral galaxies have compara- barred disk morphology. From HST imaging of nuclear ble sizes, luminosities, and frequencies of detection (see clusters in face-on spirals, there is no evidence for bars on also Fig. 3). They argue that this similarity suggests a these scales (Böker et al. 2004). A ring could also result nuclear cluster formation mechanism that does not de- from tidal stripping of an infalling molecular cloud or pend strongly on galaxy properties. However, the forma- star cluster. Assuming a pre-existing cluster with mass tion scenario suggested by our observations for nuclear 6 5 clusters in late-type galaxies cannot take place in the gas- of 3 × 10 M⊙ and a 10 M⊙ molecular cloud, the molecular cloud would be tidally stripped between radii of 8 free dE galaxies. We therefore interpret the similarity and 20 pc for molecular cloud number densities ranging of the nuclei in these galaxies as support for dE galaxies between 5 × 104 and 5 × 103 cm−3. Thus, tidal stripping evolving from low-mass spiral and irregular galaxies (e.g. of a molecular cloud is a feasible mechanism for creating Davies & Phillipps 1988). In this scenario, a dE,N galaxy a ring such as the one in IC 5052. would result from a nucleated late-type spiral galaxy af- Episodic star formation occuring at least occasionally ter depletion of gas through star formation or galaxy har- rassment (Moore et al. 1996). If so, then the ages of the 2 The radius of their ring was recalculated assuming the Cepheid nuclear clusters in dE galaxies can constrain the epoch distance to IC 342 of 3.3 Mpc derived by Saha et al. (2002). when these systems were stripped. 16 Seth, Dalcanton, Hodge, and Debattista

7.2.2. Massive star clusters as stripped galaxy nuclei reason, Côtéet al. (2006), Ferrarese et al. (2006) and Wehner & Harris (2006) suggest that nuclear clusters There is direct evidence that some massive globular and SMBHs be thought of as a single-class of Central clusters are associated with stripped galaxies. The galac- Massive Objects. They suggest a transition at ∼107 tic globular cluster M54 appears to be the nucleus of M⊙ between nuclear clusters and SMBHs. However, the the Sag dSph (Layden & Sarajedini 2000), and six of detection of numerous lower mass SMBHs at the cen- the most massive globular clusters in NGC 5128 appear ters of early and late-type dwarf galaxies (Barth et al. to be surrounded by extratidal light (Harris et al. 2002; 2005; Greene et al. 2005), and the co-existence of nuclear Martini & Ho 2004). star clusters and SMBHs in NGC 4395 (Filippenko & Ho Stripped nuclear clusters have also been invoked to 2003) and perhaps in NGC 4206 (§6), suggest that this explain the multiple stellar populations observed in the transition may not be abrupt. most massive nearby globular clusters (e.g Gnedin et al. The formation of black holes in the centers of nu- 2002). These observations include the large metal- clear clusters may be related to the intermediate mass licity spread (Smith 2004) and double main-sequence black holes (IMBHs) found in the centers of massive non- (Bedin et al. 2004) in ω Cen, and the spread in red gi- 4 nuclear star clusters. A 1.7×10 M⊙ black hole has been ant branch colors for G1 in Andromeda (Meylan et al. reported by Gebhardt et al. (2005) in the globular clus- 2001). More recently, Harris et al. (2006) have observed ter G1 in Andromeda (which is also a possible stripped a trend towards redder colors with increasing luminosity 2 4 nuclei), based on stellar dynamics. A 10 -10 M⊙ black for the brightest blue globular clusters in massive ellip- hole has also been identified in one of M82’s young mas- tical galaxies. This trend is interpreted as evidence for sive clusters, based on X-ray observations (Patruno et al. self-enrichment in these clusters pointing to an extended 2006). These black holes are thought to form either by star formation history more typical of nuclear clusters early merging of massive young stars within the clus- than globular clusters. A similar redward trend is seen ter, or by dynamical interaction in the core of the clus- for dE nuclei by Côtéet al. (2006). Self-enrichment is ters over long time periods (Miller & Colbert 2004, and naturally explained in an in situ formation mechanism, references therein). If IMBH formation is common in but is not easily explained if these clusters are formed by dense massive clusters, then nuclear clusters should fre- merging less massive clusters via dynamical friction. quently host black holes as well. Subsequent mergers of The ultracompact dwarf galaxies recently found in many IMBHs could then form supermassive black holes the Fornax (Phillipps et al. 2001; Drinkwater et al. 2003) (SMBHs) (Miller & Colbert 2004). As the mass of the and Virgo (Jones et al. 2006) clusters may also be re- SMBH increases, these mergers will likely destroy the nu- lated to nuclear star clusters. One possible mechanism clear clusters (see §5.2.2 of Côtéet al. 2006), distributing for their formation is “galaxy threshing”, in which nu- their stars into the bulge. cleated galaxies lose their envelopes leaving behind only their nuclei as UCDs (Bekki et al. 2001). In the MI vs. 8. SUMMARY log(reff ) diagram in Figure 3, NGC 4206’s NC has sufficient size and luminosity, even after modest age fading, to We have examined the nuclear regions of fourteen be a UCD progenitor. The galaxy threshing scenario has nearby galaxies using HST/ACS images in the F606W and F814W filters. We find nuclear cluster candidates also been shown to be feasible by Côtéet al. (2006) for ′′ dwarf elliptical nuclei, although De Propris et al. (2005) within 2 of the photocenter of nine of these galaxies. We suggests that dE,N are in general more compact than have also obtained multi-band photometry and a spec- UCDs. One of the galaxy nuclei they study does have trum of the NGC 4244 nuclear cluster, the nearest object similar properties to the UCDs, as well as possessing spi- in our sample. Analyzing these nuclear clusters, we find ral arms which indicate the galaxy may be an anemic the following: spiral. 1. From the edge-on perspective, three of the nine cluster candidates (IC 5052, NGC 4206, 7.3. Connection to Black Holes NGC 4244) have morphologies suggesting they As residents of the same neighborhoods, it is natural have both disk and spheroid components. These to suspect that there may be some connection between components are very compact with scale lengths nuclear clusters and supermassive black holes (SMBHs). and half-light radii of 2-30 pc. Star clusters with The presence of SMBHs in the nuclei of >30 galax- multiple visible components have not previously ies have been confirmed through dynamical measure- been seen. ments of their masses (reviewed by Ferrarese & Ford 2005). The masses of SMBHs are known to correlate 2. The three multi-component nuclear clusters have ∼ ◦ with the mass and/or velocity dispersion of the bulge elongations oriented to within 10 of the major- axis of the galaxy. component (the M• − σ relation, Ferrarese & Merritt 2000; Gebhardt et al. 2000). Recent work on nu- 3. The magnitudes and sizes of the clusters in our clear clusters in both spiral and elliptical galaxies have edge-on sample are very similar to those observed shown a remarkably similar trend (Rossa et al. 2006; in other face-on, late-type galaxies (Böker et al. Côtéet al. 2006; Ferrarese et al. 2006; Wehner & Harris 2004) and dE galaxies (Côtéet al. 2006). 2006). For a large sample of nucleated elliptical galaxies, Côtéet al. (2006) have shown that the nucleus ac- 4. The spheroids of the multi-component nuclear clus- counts for 0.30±0.04% of the total galaxy mass. This ters are 0.3-0.6 magnitudes redder than the disks fraction is very similar to the fraction of galaxy mass in F606W-F814W. This color difference implies the found in the SMBHs (Ferrarese et al. 2006). For this disks are populated with stars <1 Gyr in age. Nuclear Star Clusters in Edge-on Spirals 17

5. Fitting single-stellar population models to the cluster. spectral and photometric data of the NGC 4244 nuclear cluster, we find it is best-fit by ages of ∼70 Myr or ∼0.8 Gyr. The implied total mass These observational results strongly support an in situ 6 of the cluster for is ∼3×10 M⊙. formation model for nuclear star clusters. We suggest a specific model in which stars are formed episodically 6. A combination of two or more stellar populations when gas accretes into a compact nuclear disk. After significantly improves the fit to the NGC 4244 nu- formation the stars gradually lose angular momentum clear cluster spectral and photometric data. In the and eventually end up in a spheroidal component. We multi-age fits, the youngest (. 100 Myr) stellar show that a duty cycle for star formation of ∼0.5 Gyr population matches the luminosity of the disk com- is consistent with our and previous observations. The ponent, and appears to make up .5% of the total 7 angular momentum must also be lost on this timescale, mass of the cluster. As much as ∼10 M⊙ of old which we suggest may occur due to interactions of the (10 Gyr) stars may also be present in the cluster. nuclear cluster with the surrounding galaxy potential. A constant star formation rate over the last 12 Gyr Acknowledgments: The authors would like to thank also provides a surprisingly good fit. their anonymous referee, Carl-Jakob Walcher, Thomas 7. Using an HII region 19 pc from the center of the Puzia, Tom Quinn, and Nate Bastian for their help- NGC 4244 NC, we measure the velocity offset of ful comments on improving this manuscript. We also emission and absorption components in our spec- thank Leo Girardi for his help in interpreting models, trum to obtain a lower limit to the dynamical mass Andrew West for his help with SDSS, Kevin Covey for +1.7 6 fruitful discussions, and Roelof de Jong for his ongoing for the nuclear cluster of 2.5− × 10 M⊙. 1.2 support. This work has been supported by HST-AR- 8. Analysis of emission lines in the NGC 4206 nucleus 10309, HST-GO-9765, the Sloan Foundation, and NSF suggest that it may host an AGN as well as a stellar grant CAREER AST 02-38683.

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