The ACS Virgo and Fornax Cluster Surveys were unprecedented surveys of early-type galaxies belonging to two benchmark clusters in the local universe, Virgo and Fornax. The surveys were based on ACS imaging from the Hubble Space Telescope (HST), and helped change the way astronomers think about galaxy formation.
Left: An intermediate-luminosity galaxy (VCC1431) in the Virgo Cluster observed with the Advanced Camera for Surveys (ACS) on HST as part of the ACS Virgo Cluster Survey, which targeted 100 early-type galaxies (Cote et al. 2004). Note the central nucleus or “luminosity excess” (see below). A similar survey of the Fornax Cluster, targeting 43 galaxies, is described in Jordan et al. (2007). Top: The ACS/WFC CCDs before the camera was assembled.
The ACS Virgo and Fornax Cluster Surveys were unprecedented surveys of early-type galaxies belonging to two benchmark clusters in the local universe, Virgo and Fornax. The surveys were based on ACS imaging from the Hubble Space Telescope (HST), and helped change the way astronomers think about galaxy formation.
The ACS Virgo and Fornax Cluster Surveys were unprecedented surveys of early-type galaxies belonging to two benchmark clusters in the local universe, Virgo and Fornax. The surveys were based on ACS imaging from the Hubble Space Telescope (HST), and helped change the way astronomers think about galaxy formation.
The ACS Virgo and Fornax cluster surveys produced more than two dozen publications on topics ranging from the core and global structure of early-type galaxies, to globular cluster systems, new families of hot stellar systems (such as “Ultra Compact Dwarf Galaxies” and “Faint Fuzzies”) and the extragalactic distance scale. Some scientific highlights and data products from the surveys include:
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The first simultaneous characterization of the central and global structure for a large sample of early-type galaxies in the nearby universe (Virgo), made possible by the large field of view of the ACS instrument on HST (Ferrarese et al. 2006a; Cote et al. 2006; Cote et al. 2007).
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The demonstration that the Sersic family of models provides a remarkably accurate description of the brightness profiles of early-type galaxies spanning nearly three orders of magnitude in luminosity (i.e., from “giant” to “dwarf” galaxies, Ferrarese et al. 2006a). These findings build upon pioneering studies by Caon et al. (1993), Graham et al. (2003), Graham & Guzman (2003) and Jerjen & Binggeli (1997).
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The finding of a systematic transition from a central luminosity “deficit” to “excess” in the central regions of galaxies, relative to the global Sersic model fit, and a dramatic upward revision of the frequency of distinct nuclear components in the centers of low- and intermediate-luminosity galaxies (Ferrarese et al. 2006a; Cote et al. 2006; Cote et al. 2007). Once again, see the series of earlier papers by Graham and collaborators, including Graham et al. (2003), Graham & Guzman (2003) and Trujillo et al. (2004), as well as Carolla et al. (1998), Boker et al. (2002, 2004), Lotz et al. (2004) and Grant et al. (2005).
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The realization that these central excesses/nuclei probably arise, for at least some galaxies, through gas inflows and starbursts expected in mergers and accretions, as had been predicted by numerical models (Cote et al. 2006, Cote et al. 2007). See also Mihos & Hernquist (1994), who anticipated these results using pioneering numerical simulations.
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The discovery that the light “excesses” (i.e., compact stellar nuclei) in the faintest galaxies contain roughly the same percentage of the total galaxy mass as do the Supermassive Black Holes (SBHs) in the brightest galaxies, suggesting a possible link between these two components (Ferrarese et al. 2006b, Cote et al. 2006). See the contemporaneous papers by Rossa et al. (2006) and Wehner and Harris (2006), and the comprehensive subsequent study by Seth et al. (2008).
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A demonstration that the early-type galaxy populations do not show a dramatic “dichotomy” in terms of their central brightness profile slopes, as was previously believed; the ACS Virgo Cluster Survey was the first study to show that the previously reported class of “power-law galaxies” actually have a two-component structure on small scales (Ferrarese et al. 2006a; Cote et al. 2007). Once again, see also Jerjen & Binggeli (1997), Graham & Guzman (2003), as well as Rest et al. (2001) and Ravindranath et al. (2001).
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A large and homogeneous catalog of more than ≈ 10,000 globular cluster candidates in early-type galaxies (Jordan et al. 2009).
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The demonstration that the fundamental properties of globular cluster systems show unexpected continuous trends with host galaxy luminosity. Specific examples include their luminosity functions, size distributions, color/metallicity distributions, and formation efficiencies (Jordan et al.2005, 2006, 2007; Peng et al. 2006a,b, 2008; Mieske et al. 2006, 2010; Sivakoff et al. 2007; Masters et al. 2010; Villegas et al. 2010). These results build upon a number of previous studies by other researchers, including Gebhardt & Kissler-Patig (1998), Larsen et al. (2001) and Kundu et al. (2001).
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The discovery of Ultra-Compact Dwarf (UCD) galaxies in the Virgo Cluster, the first measurements for the dynamical masses of these systems, and the discovery of an apparently fundamental transition between globular clusters and UCDs at ≈ 2-3 million solar masses (Hasegan et al. 2005).
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The measurement of accurate SBF distances (i.e., typical errors of ≈ 0.5 Mpc) for a large sample of galaxies in both the Virgo and Fornax Clusters, the direct measurement of the line-of-sight depth of Virgo and a precise measurement of the relative distance of the two clusters (Mei et al. 2005a,b, 2007; Blakeslee et al. 2009).
Please see the science highlights to learn more about these and other topics. To read or download individual papers, see the publications section.
The Survey Teams
The Survey Teams
The ACS Virgo and Fornax Cluster Surveys were unprecedented surveys of early-type galaxies belonging to two benchmark clusters in the local universe, Virgo and Fornax. The surveys were based on ACS imaging from the Hubble Space Telescope (HST), and helped change the way astronomers think about galaxy formation.
Program Galaxies
Left: An intermediate-luminosity galaxy (VCC1431) in the Virgo Cluster observed with the Advanced Camera for Surveys (ACS) on HST as part of the ACS Virgo Cluster Survey, which targeted 100 early-type galaxies (Cote et al. 2004). Note the central nucleus or “luminosity excess” (see below). A similar survey of the Fornax Cluster, targeting 43 galaxies, is described in Jordan et al. (2007). Top: The ACS/WFC CCDs before the camera was assembled.
The Virgo Cluster is the rich cluster nearest to the Milky Way, and the dominant mass concentration in the local universe. It also represents the nearest large collection of early-type (red sequence) galaxies within ~50 Mpc. At a distance of ≈16.5 Mpc, it has historically played a central role in furthering our understanding of galaxy evolution, supermassive black holes, the extragalactic distance scale, the intracluster medium, extragalactic star clusters, and countless other topics in modern astrophysics.
The Virgo Cluster is the rich cluster nearest to the Milky Way, and the dominant mass concentration in the local universe. It also represents the nearest large collection of early-type (red sequence) galaxies within ~50 Mpc. At a distance of ≈16.5 Mpc, it has historically played a central role in furthering our understanding of galaxy evolution, supermassive black holes, the extragalactic distance scale, the intracluster medium, extragalactic star clusters, and countless other topics in modern astrophysics.
The Fornax Cluster is smaller and more compact than Virgo. At a slightly larger distance of ≈20.0 Mpc, it offers an unique opportunity to study the fossil record of galaxy formation in rather different environment than the Virgo Cluster.
The ACS Virgo and Fornax Cluster Surveys
Science Highlights
The ACS Virgo and Fornax Surveys were highly successful in terms of the output by the survey teams, the widespread use of the survey data products (and archival HST data) by the astronomical community, and the large number of unexpected results to have emerged from the surveys. Here we describe some of the key scientific highlights from the surveys.
Unfortunately, a subsequent paper by Kormendy et al. (2009), which reanalyzed ACSVCS data and reached a number of the same conclusions as the original ACSVCS papers, failed to give appropriate credit to the ACSVCS and ACSFCS team (and numerous other researchers).
I. The Simultaneous Characterization of the Core and Global Structure of Early-Type Galaxies.
Prior to the ACS Virgo Cluster Survey, much of what was known about the central properties of early-type galaxies was based on HST imaging carried out with the WFPC1, WFPC2 or NICMOS instruments. Because these cameras have a rather limited field of view, the resulting surface brightness profiles typically extended to distances of ≈ 5-20˝, making a simultaneous characterization of their central and global structure difficult or impossible. The ACS Virgo Cluster Survey capitalized on the wider field available with the ACS/WFC instrument ― and the addition of wide-field data from the literature or from SDSS ― to simultaneously explore the core and global structure of a large sample of early-type galaxies in the nearby universe (i.e., Virgo): the first such study of its kind (see Ferrarese et al. 2006abc, Côté et al. 2006, 2007).
II. The Sersic Law: An Excellent Parameterization from “Giants” to Dwarfs”.
Since the early 1990s, the usual choice for parameterizing the surface brightness profiles of bright (i.e., “giant”) early-type galaxies as observed by HST was the so-called “Nuker Law” ― essentially a broken power-law that has an inflection at some “break radius”. In a point of departure from most previous studies, the ACS Virgo Cluster Survey used Sersic models to parameterize the brightness profiles for a large sample of early-type galaxies. [As noted in Côté et al. (2007): “To first order, Sérsic models provide accurate representations of the global, azimuthally-averaged brightness profiles for almost all galaxies in our sample, largely independent of luminosity, morphological type, prior classification as giant or dwarf, and the presence or absence of morphological peculiarities such as rings, shells, and bars.”]
However, on small scales, two types of deviations from the fitted Sersic model were found. Among bright galaxies, the profiles flatten to form nearly constant brightness “cores”, as many previous studies had found. In the ACS Virgo Cluster Survey, these galaxies were fitted with “core-Sersic” models, following Graham & Guzman (2003). Among the fainter galaxies, however, central excesses relative to the fitted Sersic models were found to be commonplace, including many galaxies that had previously been classified as having “power-law” profiles. In these galaxies, a second, “nuclear”, component was added ― in the form of either a King or Sersic model ― to represent the observed profiles (see Ferrarese et al. 2006abc, Cote et al. 2006, 2007, 2008ab).
III. Two Dichotomies Disappear:Transition from Central Luminosity “Deficit” (Cores) to “Excess” (Nuclei).
At the outset of the ACS Virgo Cluster Survey, it was widely believed that early-type galaxies exhibited two distinct “dichotomies”. At high luminosities, galaxies were thought to divide neatly into two populations based on the slope of their central surface brightness profile ― galaxies brighter than MB = -20.5 mag were observed as having shallow profiles and were classified as “cores”; fainter galaxies showed steeply rising profiles and were referred to as “power-laws”. At somewhat lower luminosities, early-type galaxies were also thought to divide sharply into two populations ― “normal” or “giant” galaxies at the bright (MB < -17.5) end, and “dwarf elliptical” or “spheroidal” galaxies at the faint (MB > -17.5) end ― based both on central and global properties (such as central and mean effective surface brightness, and effective radius).
Thanks to the precision and large radial extent of the ACS brightness profiles ― and to the homogeneous sample of early-type galaxies having a well understood selection function ― it was shown that these “dichotomies” do not in fact exist, and that the properties of early-type galaxies vary smoothly and systematically across the entire luminosity sequence. The ACSVCS data showed that three separate factors led to the erroneous view that galaxies divide into “cores” and ”power-laws”: (1) the limited radial coverage of the brightness profiles available with previous HST imaging; (2) the breakdown of the “power-law” parameterization adopted in most previous studies of early-type galaxies; and (3) the incompleteness and biases built into the galaxy samples observed prior to the ACSVCS. The latter is also the main culprit for the popularly held, but incorrect, notion that early-type galaxies divide into “normal” and “dwarf “ varieties. For more details, see Ferrarese et al. (2006abc), Côté et al. (2006, 2007, 2008ab).
We note that several investigators had arrived at similar conclusions based on smaller and/or more hetereogenous datasets (e.g., Jerjen & Binggeli 1997, Graham & Guzman 2003, Graham et al. 2003, Gavazzi et al. 2005).
IV. Evidence for Gas Inflows and Central Starbursts in Post-Merger Remnants?
The properties of the central excesses (compact stellar nuclei) detected in the ACS Virgo Cluster Survey were examined in detail and found to correlate in several ways with the global properties of the galaxies themselves (and, at the faint end of the sample, to closely resemble the “nuclear star clusters” observed near the photocenters of late-type galaxies; e.g. Carollo et al. 1998, Boker 2000, 2001, Walcher et al. 2005). The nuclei were found to obey size-magnitude and colour-magnitude relations (in the sense that brighter galaxies typically show brighter, larger, and redder nuclei).
Several lines of argument suggest that delayed, and perhaps ongoing, star formation from chemically-enriched gas is likely to have been important in their formation (although other processes, most notably the infall of pre-existing star clusters by dynamical friction, may also have been significant). In Côté et al. (2006), it was argued that some of “...these nuclei may be candidates for the "dense stellar cores" that form in numerical simulations (Mihos & Hernquist 1994) when (chemically enriched) gas is driven inward, perhaps as a result of mergers.” This initial identification was reinforced in Côté et al. (2007; see above) who compared directly the ACS brightness profile for a typical, low-luminosity “power-law” galaxy (which shows an unmistakable two-component structure) to the predictions from the merger simulations of Mihos & Hernquist (1994).
V. A Connection Between Nuclei and Supermassive Black Holes.
The central luminosity excesses (nuclei) obey several tight scaling relations with their host galaxies. This fact, coupled with their location at the bottom of the gravitational potential wells of the galaxies, provides some intriguing evidence for a possible correspondence with the supermassive black holes (SBHs) that are believed to lie at the centers of most (and possible all) bright galaxies. In Côté et al. (2006), it was shown that the fractional luminosity contribution of the nuclei to their hosts (0.32 ± 0.07%) was similar to the mass fraction of SHBs in bright galaxies (0.25 ± 0.04%).
In Ferrarese et al. (2006b), this similarity was explored in more detail. Dynamical masses for the host galaxies and stellar masses for the nuclei (derived from population synthesis modeling) were used to show that the nuclei and SBHs appear to contain a nearly identical mass fraction of 0.18% (with a ±1σ range of 0.06%–0.52%). Two other groups, studying the nuclei of dwarf elliptical and late-type spiral galaxies, reached a similar conclusion at nearly the same time (see, Wehner & Harris 2006, Rossa et al. 2006). For more details, see Ferrarese et al. (2006b), Côté et al. (2006).
VI. A State-of-the-art Catalog for Globular Clusters in Early-Type Galaxies.
One of the three primary science goals of the surveys was an improved understanding of globular cluster systems. Broadly speaking, different galaxy formation scenarios make different predictions for the properties of their associated globular clusters. In practice, the expected differences are often subtle, and testing the models requires not only large cluster samples but only accurate and homogeneous measurements of their sizes, luminosities, masses, positions, ages, metallicities, etc.
At the distances of the Virgo and Fornax Clusters, globular clusters are marginally resolved in HST imaging. A data reduction pipeline optimized for the detection of globular clusters and the measurement of their photometric and structural parameters was therefore developed for the two ACS surveys. The cluster catalogs generated in this way have formed the basis for nearly a dozen papers published by the survey teams on the globular cluster systems of early-type galaxies. The final catalogs are now available to the public, either from the Data Products page or from the scientific journals. For more details, see Jordán et al. (2004, 2007, 2009).
VII. Characterizing the Properties of Globular Cluster Properties in Early-Type Galaxies.
At the time of writing, the globular cluster catalogs described above have analyzed in a continuing series of more than a dozen papers that explore both the properties of globular clusters (e.g., colors, ages, metallicities, luminosity functions, formation efficiencies, structural parameters, low-mass X-ray binary populations, color-magnitude relations, etc) and their relationship to composite stellar systems (i.e., compact stellar nuclei, diffuse star clusters, ultra-compact dwarf galaxies).
A brief overview of the scientific highlights in these broad areas include: (1) the conclusion that the frequency of low-mass X-ray binaries in globular clusters probably has a dynamical origin in the sense that denser clusters are more likely to form LMXBs, presumably due to tidal captures and binary neutron star exchanges (Jordán et al. 2004, Sivakoff et al. 2007); (2) the demonstration that the half-light radii of globular clusters have a nearly “universal” distribution which may be useful in distance estimation (Jordán et al. 2005); (3) the discovery of a population of faint, diffuse star clusters in ~10% of the ACS Virgo Cluster Survey galaxies, possible analogs of the old open clusters seen in the Milky Way (Peng et al. 2006b); (4) the measurement of globular cluster color (metallicity) distributions for galaxies spanning a factor of more than 500 in luminosity, and the first clear evidence that the observed colour bimodality persists well into the “dwarf” galaxy regime (Peng et al. 2006a); (5) the finding that the globular cluster luminosity function in not truly universal, but varies systematically with galaxy luminosity so that fainter galaxies have fainter “turnovers” and narrower distributions (Jordán et al. 2006, 2007); (6) the direct measurement of the efficiency of globular cluster formation along the red sequence, and the discovery that two important environment effects ― biasing and tidal stripping ― have likely played a role in determining the observed globular cluster specific frequencies (Peng et al. 2008); and (7) the confirmation that metal-poor globular clusters in the most luminous Virgo and Fornax galaxies often show a color-magnitude relation that may be the signature of self enrichment (Mieske et al. 2006).
Taken together, these results provide strong support for the unexpected conclusion that early-type galaxies along the red sequence vary slowly but systematically in their properties, with no dramatic dichotomies in terms of their central or global structure, nor in the properties of their globular cluster systems.
For more details, see Jordán et al. (2004, 2005, 2006, 2007, 2009), Peng et al. (2006ab, 2008), Sivakoff et al. (2007), Côté et al. (2006), and Mieske et al. (2006, 2008).
VIII. UCDs in the Virgo Cluster: A Transition from Globulars at 2-3 Million Solar Masses?
Beginning in the late 1990s and early 2000s, a handful of faint, unusually compact stellar systems were discovered in the Fornax Cluster (Hilker 1998; Phillipps et al. 1999; Drinkwater et al. 2000). These systems, identified as cluster members by their radial velocities, appeared unresolved in typical ground-based seeing which implies effective diameters of less than ~100 pc. Since this is considerably smaller than “dwarf” galaxies of comparable brightness, these unusual systems were called Ultra-Compact Dwarf galaxies (UCDs).
The ACS Virgo Cluster Survey discovered the first UCDs (also called Dwarf Globular Transition Objects, or DGTOs) in the much more massive Virgo Cluster. Rather than relying on radial velocity measurements, though, the survey identified UCDs on the basis of their structural parameters (see above) which were clearly distinct from those of globular clusters: i.e., UCDs were observed to be brighter, larger, and more diffuse than the brightest globular clusters. The first dynamical mass measurements for such systems were presented in Hasegan et al. (2005), who noted that the mass-to-light ratios were slightly elevated with respect to those of globular clusters. This may be evidence for the presence of dark matter, unusual stellar populations or IMFs, or a simple failure of the underlying assumption that the UCDs are in virial equilibrium. At the same time, it was noted for the first time in Hasegan et al. (2005) that the dynamical scaling relations of globular clusters and UCDs seem to undergo a transition at ~ 2-3 million solar masses. Subsequent observations have confirmed these findings, although the origin of UCDs remains unclear. The leading explanation, however, probably involves stripping or “threshing” of larger (often nucleated) galaxies, leaving a compact object after the outer envelope is removed. This would be consistent with the observation that UCDs ― as well as “compact elliptical” galaxies ― show a strong preference for occupying dense environments in close proximity to a massive galaxy. For more details, see Hasegan et al. (2005) and also Mieske et al. (2008).
IX. SBF Distances: The 3D Structure of Virgo and a Relative Distance for Fornax
During the 1980s and 1990s, one of the most hotly debated topics in astronomy was the distance ― and the global (3D) structure ― of the Virgo Cluster. Claims in the literature differed significantly on this latter issue, with reports varying from a nearly spherical structure with little dispersion along the line of sight, to very extended, cigar-like distributions with depths of ± 8 Mpc or more.
One of the primary aims of the ACS Virgo Cluster Survey was to map out the three-dimensional structure of Virgo by measuring accurate (σd ~ 0.5 Mpc) surface brightness fluctuation (SBF) distances for a large sample of early-type galaxies. Distances with this level of precision were measured for 85 program galaxies using the Mz‒(g-z)0 relation, which was presented for the first time in the survey. Virgo was found to have a mildly triaxial structure [1.0:0.7:0.5] and a line-of-sight depth of 2.4 ± 0.4 Mpc (±2σ). The “A” and “B” subclusters (centered on M87 and M49, respectively) were found to lie at virtually the same distance (≈16.5 Mpc) and there was some evidence from the distance-velocity diagram that the cluster velocity distribution has not yet virialized (Mei et al. 2007).
By contrast, the Fornax Cluster appears compact and well relaxed. SBF distances for 43 galaxies from the ACS Fornax Cluster Survey were combined with those from the Virgo survey to yield a precise measurement of the relative distance of the Fornax Cluster: d(Fornax)/d(Virgo) = 1.214 ± 0.017. For full details on the SBF aspects of the surveys, see Mei et al. (2005ab, 2007) and Blakeslee et al. (2009).
(Left Panels, click on the figure to enlarge) ACS images of the central 10˝x10˝ regions for the 26th to 50th brightest galaxies from the ACS Virgo Cluster Survey. (Right Panels, click to enlarge) Sersic model parameterization of the surface brightness profiles for these galaxies, showing the accuracy of the Sersic parameterization for both “giants” and “dwarfs”. Most of these galaxies show excess light (i.e., compact stellar nuclei) in their innermost regions; in Côté et al. (2006), it was argued that “These nuclei may be candidates for the "dense stellar cores" that form in numerical simulations (Mihos & Hernquist 1994) when (chemically enriched) gas is driven inward, perhaps as a result of mergers.” These conclusions were further explored and reaffirmed in Côté et al. (2007; see also below).
(Left Panel) Representative surface brightness profiles for six galaxies in the ACS Virgo Cluster Survey, from the brightest (VCC 1226) to the faintest (VCC 1661) galaxy in the sample. The galaxies are arranged, from top to bottom, in order of increasing total B-band magnitude (decreasing luminosity). They have been shifted in the vertical direction for clarity. (Right Panels) A compilation of the best-fit models for the g-band surface brightness profiles of all ACSVCS galaxies. The models exclude a nuclear component when present and are shown before convolution by the ACS point spread function. For each galaxy, the abscissa shows the isophotal radius normalized to the effective radius derived from the best fit. The two panels differ only in the range of surface brightness shown: to the right, the scale is shown to highlight difference in the innermost region. In both panels, galaxies best fit by a core-Sersic profile are shown in red. For more details see Ferrarese et al. (2006a).
Projection of the three-dimensional structure of the Virgo Cluster from Mei et al. (2007). The cluster is embedded in a rectangular parallelepiped of dimensions 4 × 4 × 9.5 Mpc. The red spheres show galaxies with BT < 12 mag. The direction of the Milky Way is indicated by the arrow. Note the five members of the W’ Cloud at ≈ 23 Mpc.
(Left) Surface brightness profiles for nine representative galaxies from the ACS Virgo Cluster Survey, showing the transition from central luminosity “deficit” to “excess” with respect to the inward extrapolation of the Sersic model that best fits the outer galaxy profile. (see Côté et al. 2006, 2007). (Right) Magnified view of the central regions of six galaxies from the ACS Virgo Cluster Survey, showing the transition from shallow profiles (cores or deficits) at the bright end of the luminosity function, to a two-component structure at the faint end (i.e., a dense stellar nucleus and a shallow galaxy profile underlying it).
(Left Panel) Surface brightness profile for a galaxy previously classified as a “power-law galaxy” showing the central component (compact stellar nucleus) that went unrecognized in previous HST/WFPC1 studies due to the limited field of view (see, e.g., Lauer et al. 2007). (Middle Panel) Simulation from Mihos and Hernquist (1994) of a post-merger remnant showing a “dense stellar core” that forms as a result of gas inflow and star formation. (Right Panel). The same “power-law” galaxy shown in the first panel. Note the close correspondence between the central nucleus and the dense stellar core in the middle panel. For more details, see Côté et al. (2006, 2007).
Scaling relations from Ferrarese et al. (2006b) showing the mass of the “central massive object” (defined here as either the supermassive black hole or compact stellar nucleus) plotted against galaxy magnitude, velocity dispersion and mass. The nuclear component in fainter galaxies contributes very nearly the same fraction of the total galaxy mass as the supermassive black holes in luminous systems.
(Left Panels) Images showing the steps involved in identifying and measuring the properties of globular clusters in ACS Virgo and Fornax Cluster Survey galaxies (Jordán et al. 2004, 2007). (Right Panel) Observed distribution of sources in the size-magnitude plane for four galaxies from the ACS Virgo Cluster Survey and for the “custom” control fields constructed for each of these galaxies (Peng et al. 2006, Jordán et al. 2009).
(Left Panel) Magnitude of the globular cluster luminosity function (GCLF) “turnover”, showing the tendency for fainter galaxies to have fainter globular clusters in the mean (Jordán et al. 2006, 2007). (Right Panel) GCLF dispersion as a function of galaxy magnitude; fainter galaxies also show narrower GCLFs (Jordán et al. 2006,2007).
(Left Panel) Globular cluster specific frequency (i.e., formation efficiency) as a function of galaxy magnitude from Peng et al. (2008). (Right Panel) Globular cluster specific frequency for low-mass galaxies plotted against three-dimensional distance from the center of Virgo, as marked by M87. Note the tendency for specific frequency to increase toward the cluster center, a possible signature of “biased” globular cluster formation (Peng et al. 2008).
(Left Panel) Smoothed colour distributions for globular clusters in early-type galaxies of differing magnitudes from Peng et al. (2008). Even faint galaxies appear to show evidence for red (metal-rich) cluster populations. (Right Panel) Demonstration that the blue (metal-poor) globular clusters in luminous galaxies often show a “blue tilt”, a possible sign of self enrichment (Mieske et al. 2006).
(Left Panel) Dynamical mass-to-light ratios for ultra-compact dwarf galaxies (UCDs) in the Virgo Cluster. UCDs are found to have elevated mass-to-light ratios compared to globular clusters (Hasegan et al. 2005, Mieske et al. 2008) (Right Panel) Dynamical scaling relations for globular clusters and UCDs. Note the apparent transition in scaling relations at a mass of roughly 2-3 million solar masses (Hasegan et al. 2005).
(Left Panel) “Hubble diagram” for the Virgo Cluster based on the SBF distances from the ACS Virgo Cluster Survey (Mei et al. 2007). (Right Panel) Determination of the relative distance of the Fornax and Virgo Clusters using the ACS SBF measurements (see Blakeslee et al. 2009).
