We present a catalog of 422 galaxies with published measurements of their globular cluster (GC) populations. Of these, 248 are E galaxies, 93 are S0 galaxies, and 81 are spirals or irregulars. Among various correlations of the total number of GCs with other global galaxy properties, we find that N GC correlates well though nonlinearly with the dynamical mass of the galaxy bulge M dyn = 4σ 2 e R e /G, where σ e is the central velocity dispersion and R e the effective radius of the galaxy light profile. We also present updated versions of the GC specific frequency S N and specific mass S M versus host galaxy luminosity and baryonic mass. These graphs exhibit the previously known U-shape: highest S N or S M values occur for either dwarfs or supergiants, but in the midrange of galaxy size (10 9 − 10 10 L ⊙ ) the GC numbers fall along a well defined baseline value of S N ≃ 1 or S M = 0.1, similar among all galaxy types. Along with other recent discussions, we suggest that this trend may represent the effects of feedback, which systematically inhibited early star formation at either very low or very high galaxy mass, but which had its minimum effect for intermediate masses. Our results strongly reinforce recent proposals that GC formation efficiency appears to be most nearly proportional to the galaxy halo mass M halo . The mean "absolute" efficiency ratio for GC formation that we derive from the catalog data is M GCS /M halo = 6 × 10 −5 . We suggest that the galaxy-to-galaxy scatter around this mean value may arise in part because of differences in the relative timing of GC formation versus field-star formation. Finally, we find that an excellent empirical predictor of total GC population for galaxies of all luminosities is N GC ∼ (R e σ e ) 1.3 , a result consistent with Fundamental Plane scaling relations.

The structures of globular clusters (GCs) reflect their dynamical states and past histories. High-resolution imaging allows the exploration of morphologies of clusters in other galaxies. Surface brightness profiles from new Hubble Space Telescope observations of 34 GCs in M31 are presented, together with fits of several different structural models to each cluster. M31 clusters appear to be adequately fit by standard King models and do not obviously require alternate descriptions with relatively stronger halos, such as are needed to fit many GCs in other nearby galaxies. The derived structural parameters are combined with corrected versions of those measured in an earlier survey in order to construct a comprehensive catalog of structural and dynamical parameters for M31 GCs with a sample size similar to that for the Milky Way. Clusters in M31, the Milky Way, Magellanic Clouds, the Fornax dwarf spheroidal, and NGC 5128 define a very tight fundamental plane with identical slopes. The combined evidence for these widely different galaxies strongly reinforces the view that old GCs have near-universal structural properties, regardless of host environment.

We present new Hubble Space Telescope WFPC2 (V, I) photometry for field stars in NGC 5128 at a projected distance of 8 kpc from the galaxy center, which probe a mixture of its inner halo and outer bulge. The color-magnitude diagram shows an old red-giant branch which is even broader in color than our two previously studied outer-halo fields (at 21 and 31 kpc), with significant numbers of stars extending to Solar metallicity and higher. The peak frequency of the metallicity distribution function (MDF) is at [m/H] ≃ −0.4, with even fewer metal-poor stars than in the outer-halo fields. If we use the 21-and 31-kpc fields to define template "halo" MDFs and subtract these from the 8-kpc field, the residual "bulge" population has a mean [m/H] ≃ −0.2, similar to the bulges of other large spirals and ellipticals. We find that the main features of the halo MDF can be reproduced by a simple chemical evolution model in which early star formation goes on simultaneously with an initial stage of rapid infall of very metalpoor gas, after which the infall dies away exponentially. Finally, by comparison with the MDFs for the NGC 5128 globular clusters, we find that in all the halo fields we have studied, there is a clear decrease of specific frequency S N (number of clusters per unit halo light) with increasing metallicity. At the lowest-metallicity range ([Fe/H] < −1.6) S N is ∼ 4 − 8, while at metallicities [Fe/H] > −1 it has dropped to ≃ 1.5.

Using the Hubble Space Telescope (HST ) Wide Field Camera (WFC) of the Advanced Camera for Surveys (ACS), we have obtained deep (V, I ) photometry of an outer halo field in NGC 5128, to a limiting magnitude of I ' 29. Our photometry directly reveals the core helium burning stellar population (the ''red clump'' or horizontal branch) in a giant E/S0 galaxy for the first time. The color-magnitude diagram displays a very wide red giant branch (RGB), an asymptotic giant branch (AGB) bump, and the red clump; no noticeable population of blue horizontal branch stars is present, confirming previous suggestions that old, very metal-poor population is not ubiquitous in the halo of this galaxy. From the upper RGB we derive the metallicity distribution, which we find to be very broad and moderately metal-rich, with average ½M/H ¼ À0:64 and dispersion 0.49 dex. The metallicity distribution function is virtually identical to that found in other halo fields observed previously with HST, but with an enhanced metal-rich population that was partially missed in the previous surveys due to V-band incompleteness for these very red stars. Combining the metallicity-sensitive colors of the RGB stars with the metallicity-and age-sensitive features of the AGB bump and the red clump, we infer the average age of the halo stars to be $8 þ3 À3:5 Gyr. As part of our study, we present an empirical calibration of the ACS F606W and F814W filters to the standard V and I bands, achieved with ground-based observations of the same field made from the EMMI camera of the New Technology Telescope of the ESO La Silla Observatory.

The total mass M GCS in the globular cluster (GC) system of a galaxy is empirically a near-constant fraction of the total mass M h ≡ M bary + M dark of the galaxy, across a range of 10 5 in galaxy mass. This trend is radically unlike the strongly nonlinear behavior of total stellar mass M versus M h . We discuss extensions of this trend to two more extreme situations: (a) entire clusters of galaxies, and (b) the Ultra-Diffuse Galaxies (UDGs) recently discovered in Coma and elsewhere. Our calibration of the ratio η M = M GCS /M h from normal galaxies, accounting for new revisions in the adopted massto-light ratio for GCs, now gives η M = 2.9 × 10 −5 as the mean absolute mass fraction. We find that the same ratio appears valid for galaxy clusters and UDGs. Estimates of η M in the four clusters we examine tend to be slightly higher than for individual galaxies, but more data and better constraints on the mean GC mass in such systems are needed to determine if this difference is significant. We use the constancy of η M to estimate total masses for several individual cases; for example, the total mass of the Milky Way is calculated to be M h = 1.1 × 10 12 M . Physical explanations for the uniformity of η M are still descriptive, but point to a picture in which massive, dense star clusters in their formation stages were relatively immune to the feedback that more strongly influenced lower-density regions where most stars form.

We have used the Hubble Space Telescope to obtain WFPC2 (V, I) photometry for stars in the halo of NGC 5128, the nearest giant elliptical galaxy. The resulting color-magnitude diagram (CMD) of this field, which lies ∼ 21 kpc from the center of the galaxy, contains more than 10,000 stars and reaches almost 3 magnitudes down the red-giant branch (RGB). From the sharply defined RGB tip at I = 24.1 ± 0.1 and M I (tip) = −4.1, we obtain a distance to NGC 5128 of 3.9 Mpc. Comparison with the fiducial RGBs of Milky Way globular clusters and model isochrones demonstrates that this outer-halo population of NGC 5128 is completely dominated by old stars, with an extremely broad metallicity range extending from the most metal-poor Galactic globulars at [Fe/H] < ∼ − 2 up to above-solar abundance. The relative contribution from any younger, bright asymptotic-branch component is negligible. The shape of the metallicity distribution function (MDF), derived from the CMD by interpolation within the isochrones, can be remarkably well matched by a simple two-component model of closed-box chemical enrichment, where the first component starts with an initial abundance Z 0 = 0 and the second component with Z 0 ≃ 0.18Z ⊙ . Two-thirds of the stars belong to the metal-richer component

We have performed a spectroscopic study of globular clusters (GCs) in the nearest giant elliptical NGC 5128 using the 2dF facility at the Anglo-Australian Telescope. We obtained integrated optical spectra for a total of 254 GCs, 79 of which are newly confirmed on the basis of their radial velocities and spectra. In addition, we obtained an integrated spectrum of the galaxy starlight along the southern major axis. We derive an empirical metallicity distribution function (MDF) for 207 GCs (∼14 per cent of the estimated total GC system) based upon Milky Way GCs. This MDF is multimodal at high statistical significance with peaks at [Z/H] ∼ −1.3 and −0.5. A comparison between the GC MDF and that of the stellar halo at 20 kpc (∼4R e ) reveals close coincidence at the metal-rich ends of the distributions. However, an inner 8-kpc stellar MDF shows a clear excess of metal-rich stars when compared to the GCs. We compare a higher signal-to-noise ratio (S/N) subsample (147 GCs) with two stellar population models which include non-solar abundance ratio corrections. The vast majority of our sample (∼90 per cent) appears old, with ages similar to the Milky Way GC system. There is evidence for a population of intermediate-age (∼4-8 Gyr) GCs ( 15 per cent of the sample) which are on average more metal-rich than the old GCs. We also identify at least one younger cluster (∼1-2 Gyr) in the central regions of the galaxy. Our observations are consistent with a picture where NGC 5128 has undergone at least two mergers and/or interactions involving star formation and limited GC formation since z = 1, however the effect of non-canonical hot stellar populations on the integrated spectra of GCs remains an outstanding uncertainty in our GC age estimates.

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