The ACS Virgo Cluster Survey. VII. Resolving the Connection between Globular Clusters and Ultracompact Dwarf Galaxies

By Monica Haşegan, A. Jord'an, P. Côté, S. George Djorgovski, D. McLaughlin, J. Blakeslee, S. Mei, M. West, E. Peng, L. Ferrarese and 3 more. 2005

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We investigate the connection between globular clusters and ultracompact dwarf galaxies (UCDs) by examining the properties of 10 compact, high-luminosity (-11.8 ≲ MV ≲ -10.8) objects associated with M87 (NGC 4486, VCC 1316), the cD galaxy in the Virgo Cluster. These objects, most of which were previously classified as M87 globular clusters, were selected from a combination of ground- and space-based imaging surveys. Our observational database for these objects—which we term DGTOs, or "dwarf-globular transition objects"—includes Advanced Camera for Surveys (ACS) F475W and F850LP imaging from the ACS Virgo Cluster Survey, integrated-light spectroscopy from Keck/ESI, and archival F606W WFPC2 imaging. We also present a search for DGTOs associated with other galaxies based on ACS imaging for 100 early-type galaxies in Virgo. Our main findings can be summarized as follows: 1. Of the six DGTOs in M87 with both ground-based spectroscopy and HST imaging, we find two objects to have half-light radii, velocity dispersions, and mass-to-light ratios that are consistent with the predictions of population synthesis models for old, metal-rich, high-luminosity globular clusters. 2. Three other DGTOs are much larger, with half-light radii rh ∼ 20 pc, and have V-band mass-to-light ratios in the range 6 ≲ M/LV ≲ 9. These objects, which we consider to be UCDs, resemble the nuclei of nucleated dwarf elliptical galaxies in the Virgo Cluster, having similar mass-to-light ratios, luminosities, and colors. 3. The classification of the sixth object is more uncertain, but it bears a strong resemblance to simulated "stellar superclusters," which are presumed to form through the amalgamation of multiple young massive clusters. 4. In general, the UCDs in M87 are found to follow the extrapolated scaling relations of galaxies more closely than those of globular clusters. There appears to be a transition between the two types of stellar systems at a mass of ≈ 2 × 106 M⊙. We suggest that the presence of dark matter is the fundamental property distinguishing globular clusters from UCDs. 5. We identify a sample of 13 DGTO candidates from the complete ACS Virgo Cluster Survey, selecting on the basis of half-light radius, magnitude, and color. For a number of these objects, membership in Virgo can be established through radial velocities or surface brightness fluctuation measurements with our ACS images. Three of these DGTO candidates are embedded in low-surface brightness envelopes. 6. Five of the 13 DGTOs in Virgo are associated with a single galaxy: M87. This finding suggests that proximity to the Virgo center may be of critical importance for the formation of these objects, although we find M87 to be more abundant in DGTOs than would be expected on the basis of its luminosity, the size of its globular cluster system, or the local galaxy density. These results show that distinguishing bona fide UCDs from high-luminosity globular clusters requires a careful analysis of their detailed structural and dynamical properties, particularly their mass-to-light ratios. In general, the properties of the UCDs in our sample are consistent with models in which these objects form through tidal stripping of nucleated dwarf galaxies.

Subfields: Astronomy and Astrophysics, Instrumentation

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THE RELATIONSHIPS AMONG COMPACT STELLAR SYSTEMS: A FRESH VIEW OF ULTRACOMPACT DWARFS (2011, background): “Previous studies have identified 16 objects around M87 that fit our criteria for UCDs (Haşegan et al. 2005; Jones et al. 2006; Haşegan 2007; Evstigneeva et al. 2007, 2008).”
The ACS Virgo Cluster Survey. XIII. SBF Distance Catalog and the Three-dimensional Structure of the Virgo Cluster (2007, background): “…profiles for early-type galaxies (Ferrarese et al. 2006a = Paper VI), the connection between globular clusters and ultra-compact dwarf galaxies (Haşegan et al. 2005 = Paper VII), the nuclei of earlytype galaxies (Côté et al. 2006 = Paper VIII) the color distributions of globular clusters…”
From Supermassive Black Holes to Dwarf Elliptical Nuclei: A Mass Continuum (2006, background): “Although galactic bulges and GCs are both “hot” stellar systems, they follow quite different fundamental plane relations (e.g., Hasegan et al. 2005; McLaughlin & van der Marel 2005; Kissler-Patig et al. 2006; and numerous additional papers cited there): for E galaxies and bulges, the Faber-Jackson…”
THE NEXT GENERATION VIRGO CLUSTER SURVEY (NGVS). I. INTRODUCTION TO THE SURVEY (2012, methodology): “…al. 2006; Peng et al. 2009); the shape of the cluster luminosity/mass function across galaxies spanning a range in mean age and morphology; and the relation of star clusters to UCDs from photometric, dynamical, and structural scaling relations (see Figure 10 of Haşegan et al. 2005; Section 5.1.4).”
The ACS Virgo Cluster Survey. X. Half-Light Radii of Globular Clusters in Early-Type Galaxies: Environmental Dependencies and a Standard Ruler for Distance Estimation (2005, background): “In any case, there appears to be no sudden upturn in the distribution for r̂h <∼ 1 pc or r̂h >∼ 10 pc (see, e.g., Figure 12 of Haşegan et al. 2005), so it is clear that the bulk of the distribution lies in the regime accesible to us.”
WIDE-FIELD PRECISION KINEMATICS OF THE M87 GLOBULAR CLUSTER SYSTEM (2011, methodology): “Jones et al. (2006), Firth et al. (2008), and Paudel et al. (2010) present low-resolution spectra13; we also include higher resolution studies of a small number of objects, using Keck/ESI, undertaken by Haşegan et al. (2005), Haşegan (2007), and Evstigneeva et al. (2007).”
The AIMSS Project I: bridging the star cluster, galaxy divide (2014, background): “…a tight mass–size relation for the UCD and cE populations that extend from the massive GC (i.e. stellar mass >2 × 106 M¯) to elliptical galaxy regime (Haşegan et al. 2005; Kissler-Patig, Jordán & Bastian 2006; Dabringhausen, Hilker & Kroupa 2008; Murray 2009; Misgeld & Hilker 2011; Norris &…”
DISSIPATIONLESS FORMATION AND EVOLUTION OF THE MILKY WAY NUCLEAR STAR CLUSTER (2011, background): “Fainter than a few million solar masses, the nuclei and globular clusters have comparable sizes (Haşegan et al. 2005).”
RADIATION PRESSURE FROM MASSIVE STAR CLUSTERS AS A LAUNCHING MECHANISM FOR SUPER-GALACTIC WINDS (2010, background): “Clusters withMcl & 106M⊙ do show an increase in radius with increasing mass, of the formrcl ∼ M3/5cl (Haşegan et al. 2005; Murray 2009); see Figure 1.”
Hubble Space Telescope STIS Spectra of Nuclear Star Clusters in Spiral Galaxies: Dependence of Age and Mass on Hubble Type (2006, background): “These similarities extend also to related objects such as the nuclei of dwarf elliptical galaxies (Geha et al. 2002) and the so-called “ultra-compact dwarf” galaxies (Haşegan et al. 2005).”
From star clusters to dwarf galaxies: the properties of dynamically hot stellar systems (2008, background): “Haşegan et al. (2005) find that some of the stellar systems they discuss have M/LV ratios that imply extreme stellar populations in these objects.”
THE NUCLEUS OF THE SAGITTARIUS DSPH GALAXY AND M54: A WINDOW ON THE PROCESS OF GALAXY NUCLEATION (2008, background): “…discovered Ultra Compact Dwarf (UCD) galaxies (Drinkwater et al. 2003) clearly suggests possible links among these classes of stellar systems (Freeman 1993; Lotz et al. 2001; Lotz, Miller & Ferguson 2004; Bassino, Muzzio & Rabolli 1994; Hasegan et al. 2005; Federici et al. 2007; Böker 2007).”
The ACS Virgo Cluster Survey. XII. The Luminosity Function of Globular Clusters in Early-Type Galaxies (2007, background): “These data have been used previously to analyze the surface-brightness profilesof the galaxies and their nuclei (Ferrarese et al. 2006ab, Côté et al. 2006), their surface brightness fluctuations (Mei et al. 2005ab; 2007), and the properties of their populations of star clusters, mainly GCs (Jordán e”
Globular Clusters in Virgo Ellipticals: Unexpected Results for Giants and Dwarfs from Advanced Camera for Surveys Imaging (2005, background): “…profiles of these bright GCs resemble those of other GCs and do not have the exponential profiles seen in some ultracompact dwarf galaxies (De Propris et al. 2005), although we note that our size selection criterion for GCs would exclude some Virgo ultracompact dwarfs (Hasegan et al. 2005).”
THE ACS VIRGO CLUSTER SURVEY XVI. SELECTION PROCEDURE AND CATALOGS OF GLOBULAR CLUSTER CANDIDATES (2008, background): “L relation emerges (Haşegan et al. 2005; Mieske et al. 2006b, Kissler-Patig et al. 2006).”