Globular Cluster Population of Hickson Compact Group 22A And
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Globular Cluster Population of Hickson Compact Group 22a and 90c1 Wayne A. Barkhouse Department of Astronomy & Astrophysics, University of Toronto, Toronto, Ontario, Canada, M5S 3H8 [email protected] Michael J. West Department of Physics & Astronomy, University of Hawaii at Hilo, Hilo, HI 96720 [email protected] and Gregory D. Bothun University of Oregon, Department of Physics, Eugene, OR, USA, 97403 [email protected] ABSTRACT We present the first measurement of the globular cluster populations of galaxies in Hickson compact groups, in order to investigate the effect of these high density environments on the formation and evolution of globular cluster systems. Based on V and R band images that we obtained of HCG 22a and HCG 90c with the ESO New Technology Telescope (NTT), we find a total globular cluster population of 1590 854 for HCG 22a and 2136 718 for 90c. The specific ± ± frequency for HCG 22a was found to be SN = 1.9 1.0 and SN = 3.4 1.1 for HCG 90c. A power-law fit to the globular cluster radial profile of± HCG 22a yields σ± R−2.01±0.30 and for HCG 90c we found σ R−1.20±0.16. A comparison of the globular cluster∼ radial profiles with the surface brightness∼ of the parent galaxy shows that the globular cluster systems are at least arXiv:astro-ph/0107491v1 25 Jul 2001 as extended as the halo light. The measured values for the specific frequency are consistent with a scenario in which the host galaxies were in a low density “field-like” environment when they formed their globular cluster systems. Subject headings: galaxies: compact groups—galaxies: individual (NGC 1199, NGC 7173)—globular clusters: general 1. Introduction studied in various environments in the hope of gaining further insight into galaxy formation and Tremendous progress has been made in re- evolution. Studies have shown that the number cent years in the detection of globular cluster of globular clusters associated with any particu- (GC) populations around galaxies beyond the Lo- lar galaxy varies widely, from none for M32 to cal Group (e.g., Harris 1991; Blakeslee 1999). The 20, 000 for the cD galaxy M87 (e.g., Harris globular cluster populations of well over 100 galax- 1991).∼ ies up to distances & 100h−1 Mpc have now been A useful parameter for quantifying the total 1Based on observations collected at the European number of globular clusters per galaxy is the spe- 0.4(M +15) Southern Observatory, Chile. cific frequency, SN = Ntot10 V , which is 1 defined as the total number of globular clusters, 1.1. Hickson Compact Group 22 N , per unit galaxy luminosity (normalized to tot HCG 22 initially was thought to contain five M = 15; Harris & van den Bergh 1981). Previ- V members (three ellipticals and two spirals) but ous studies− have shown that globular cluster pop- later redshift measurements determined that two ulations vary systematically with galaxy type and of the elliptical galaxies (HCG 22d and 22e) were environment (Harris 1991; West 1994). Charac- most likely background galaxies since their helio- teristic values of S range from S 1 for field N N centric radial velocities are 7000 km s−1 greater spiral galaxies, to S 2 4 for field≤ ellipticals, N than the remaining group members∼ (Hickson et al. to S 5 7 for ellipticals≃ − in dense environments, N 1992). The median heliocentric radial velocity of and up≃ to S− 10 20 for supergiant ellipticals at N the three bona fide group members is 2700 km s−1, the centers of≃ rich clusters.− (We note that normal- with a velocity dispersion of 43.7 km s−1. ization with respect to bulge luminosity for spiral From Table 1, the heliocentric velocity of HCG galaxies would increase the value of SN , see Cˆote −1 et al. 2000.) Yet, which factor has the greatest in- 22ais 2705kms which indicates that this galaxy fluence on globular cluster formation—galaxy type is most likely located near the group center of or environment—is unclear. mass. Hickson et al. (1992) estimated the mass-to- −1 light ratio of HCG 22 to be M/L 1.3h M⊙/L⊙ Several competing theories have been proposed and a projected median galaxy-to-galaxy∼ separa- to explain the origin of the observed variations tion of 26.9h−1kpc. The group crossing time, de- in globular cluster systems of different galaxies. fined as the ratio of the crossing time to the age Among these are: galaxy mergers (Schweizer 1986; of the universe, is Ht = 0.1905 which is greater Ashman & Zepf 1992), accretion via tidal stripping c than the median value of Ht =0.016 determined or merging (Cˆote, Marzke, & West 1998; Cˆote et c for all of the Hickson compact groups combined al. 2000), and two-stage collapse models (Forbes, (Hickson et al. 1992). This suggests that galaxy Brodie, & Grillmair 1997). At present, it is un- mergers and interactions may be less important clear whether a single mechanism, or a combina- for HCG 22 than for other compact groups with tion of proposed formation scenarios, can explain shorter crossing times. the properties of globular cluster systems (Kissler- Patig 2000). Determining which scenario most ac- 1.2. Hickson Compact Group 90 curately describes the formation of globular clus- ters will require the investigation of GC systems HCG 90 consists of four bright galaxies (two el- in a wide range of environments. lipticals, a spiral, and an irregular) with a median −1 In this paper we present observations of the heliocentric group radial velocity of 2640 km s and a velocity dispersion of 100∼ km s−1 (Hick- globular cluster population of two galaxies in Hick- ∼ son compact groups; HCG 22a (NGC 1199) and son et al. 1992). The mass-to-light ratio of HCG HCG 90c (NGC 7173). Compact groups are in- 90 is given by Hickson et al. (1992) as M/L −1 ∼ teresting because, although they are small (typ- 12.3h M⊙/L⊙ and a median projected galaxy −1 ically 3–7 member galaxies per group), they are separation of 29.5h kpc. Table 1 lists the helio- −1 extremely dense systems, comparable in density centric radial velocity of HCG 90c as 2696 km s , to the cores of rich Abell clusters (e.g., Hickson which indicates that this galaxy is probably close 1982; Hickson et al. 1992). This makes them a to the center of mass of the group. useful tool for probing the influence of environ- The group crossing time was estimated by Hick- ment on globular cluster formation. son et al. (1992) to be Htc = 0.0224 , which is only 12% of the estimated value for HCG 22. Figure 1 and 2 show V band images of HCG 22a ∼ and 90c, respectively. Both HCG 22 and 90 have This suggests that galaxy mergers and interactions been detected by ROSAT and each group has been are likely to have been more important in HCG shown to be a weak X-ray source (Ponman et al. 90 than for HCG 22 and it is interesting to note 1996). Table 1 summarizes the general properties that HCG 90 contains a galaxy (HCG 90d) that of these systems. We discuss each in detail below. is clearly interacting with other group members (Longo et al. 1994; Plana et al. 1999). In this study, we have measured the globular 2 cluster specific frequency of HCG 22a and HCG a higher S/N image. Instrumental magnitudes 90c. In 2 we present the observations and data were transformed to the standard UBVRI system reductions.§ Section 3 presents the results for HCG by measuring several standard stars from Landolt 22a and 90c in terms of their globular cluster lumi- (1992). nosity functions, radial distributions, and specific At the distance of the target galaxies ( frequencies. In 4 we discuss the results of this ∼ § 54 Mpc), and with the typical seeing of the com- study in the context of globular cluster formation bined frames ( 0.9′′), individual globular clusters and the nature of Hickson compact groups. are unresolved∼ and appear as an apparent excess Throughout this paper, unless otherwise speci- of star-like objects around their parent galaxy. In −1 −1 fied, we use H0 =50 km s Mpc . order to maximize the detection of individual glob- ular clusters and to perform accurate photometry, 2. Observations and Data Reductions the light from each host galaxy was removed. This was done by using the STSDAS package in IRAF Observations of HCG 22a and 90c were ob- to model the light distribution of each galaxy using tained on the nights of October 9 and 10, 1993 the tasks ellipse and bmodel, which fit isophotes using the 3.5m New Technology Telescope (NTT) and then generate models of each galaxy’s surface operated by the European Southern Observatory brightness profile. The constructed galaxy models (ESO) in La Silla, Chile. All images were taken were then subtracted from the appropriate image. with the ESO Multi-Mode Instrument (EMMI) The galaxy-subtracted image of HCG 22a re- with a Loral 2048 CCD mounted on the red arm. vealed a large, central, diffuse structure which is The Loral 2048 CCD chip consists of 2048 2048 the residual of a known central dust lane (Franx, 15µm pixels, giving a scale of 0.29′′ per pixel× at the Illingworth, & Heckman 1989). In order to remove f/11 Nasmyth focus. Technical limitations during this feature, the galaxy-subtracted image was me- the observing run limited the effective area of the dian filtered using a ring median filter (Secker chip to 1700 1700 pixels, resulting in a projected 1995) having an inner radius of 6.36 pixels and sky coverage× of 8.2′ 8.2′.