Gretchen L. H. Harris

Dynamics of the Globular Cluster System Associated with M87 (NGC 4486). II. Analysis

We present a dynamical analysis of the globular cluster system associated with M87 (=NGC 4486), the cD galaxy near the dynamical center of the Virgo Cluster. The analysis utilizes a new spectroscopic and photometric database, which is described in a companion paper. Using a sample of 278 globular clusters with measured radial velocities and metallicities and new surface density profiles based on wide-field Washington photometry, we study the dynamics of the M87 globular cluster system both globally (for the entire cluster sample) and separately (for the metal-rich and metal-poor globular cluster samples). This constitutes the largest sample of radial velocities for pure Population II tracers yet assembled for any external galaxy. Our principal findings are summarized as follows:

The Halo Stars in NGC 5128. II. An Outer Halo Field and a New Metallicity Distribution

We present new HST/WFPC2 (V,I) photometry for the red-giant 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. 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 metal-poor gas, after which the infall dies away exponentially. A comparison with the MDF for the NGC 5128 globular clusters indicates that there is a clear decrease of specific frequency

The Distance to NGC 5128 (Centaurus A)

S_N

DEEP ACS IMAGING OF THE HALO OF NGC 5128: REACHING THE HORIZONTAL BRANCH

(number of clusters per unit halo light) with increasing metallicity, from S_N ~ 4-8 at [Fe/H]<-1.6 down to S_N = 1.5 at [Fe/H]>-1. This trend may indicate that globular cluster formation efficiency is a strong function of the metallicity of the protocluster gas.

The metallicity distribution in the Halo stars of NGC 5128: implications for galaxy formation

In this paper we review the various high precision methods that are now available to determine the distance to NGC 5128. These methods include: Cepheids, TRGB (tip of the red giant branch), PNLF (planetary nebula luminosity function), SBF (surface brightness fluctuations), and Long Period Variable (LPV) Mira stars. From an evaluation of these methods and their uncertainties, we derive a best-estimate distance of 3.8 ± 0.1 Mpc to NGC 5128 and find that this mean is now well supported by the current data. We also discuss the role of NGC 5128 more generally for the extragalactic distance scale as a testbed for the most direct possible comparison among these key methods.

Structural parameters for globular clusters in M31 and generalizations for the Fundamental Plane

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 [M/H] = -0.64 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 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 ages, metallicities, and alpha element enhancements of globular clusters in the elliptical NGC 5128: A homogeneous spectroscopic study with gemini/gemini multi-object spectrograph

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 mag down the red giant branch (RGB). From the sharply defined RGB tip at I = 24.1 ± 0.1 and MI(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 globular clusters 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 Z0 = 0 and the second component with Z0 0.25 Z⊙. Two-thirds of the stars belong to the metal-richer component, and one-third to the metal-poorer one; the mean metallicity of the entire sample is [Fe/H] = -0.41, consistent with the colors of the integrated halo light. The metal-rich component also coincides strikingly in mean and dispersion with the metal-rich peak of the halo globular clusters in NGC 5128, suggesting that both of these halo subsystems formed contemporaneously. A discussion of various models of E galaxy formation leads us to suggest that a basic in situ formation picture with two distinct epochs of star formation best fits the observations; other models involving major contributions from accretions or mergers are less satisfactory. The timing of the events we suggest is that the first, more metal-poor star-forming epoch took place while the protogalaxy was still in a clumpy, fragmented state, leaving most of the gas unused. The second and larger star formation epoch took place after the majority of the now preenriched gas had recollected into the fully formed potential well of the new giant elliptical galaxy.

A 2dF spectroscopic study of globular clusters in NGC 5128: probing the formation history of the nearest giant elliptical

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.

The Leo Elliptical NGC 3379: A Metal-Poor Halo Emerges*

We present new integrated light spectroscopy of globular clusters (GCs) in NGC 5128, a nearby giant elliptical galaxy less than 4 Mpc away, in order to measure radial velocities and derive ages, metallicities, and alpha-element abundance ratios. Using the Gemini South 8 meter telescope with the instrument Gemini Multi-Object Spectrograph, we obtained spectroscopy in the range of ~3400-5700 ? for 72 GCs with a signal-to-noise ratio greater than 30 ??1; and we have also discovered 35 new GCs within NGC 5128 from our radial velocity measurements. We measured and compared the Lick indices from H? A through Fe5406 with the single stellar population models of Thomas et?al. in order to derive age, metallicity, and [?/Fe] values. We also measure Lick indices for 41 Milky Way GCs from Puzia et?al. and Schiavon et?al. with the same methodology for direct comparison. Our results show that 68% of the NGC 5128 GCs have old ages (>8 Gyr), 14% have intermediate ages (5-8 Gyr), and 18% have young ages ( 8 Gyr, indicating that the majority of both metallicity subpopulations of GCs formed earlier, with a significant population of young and metal-rich GCs forming later. Our metallicity distribution function generated directly from spectroscopic Lick indices is clearly bimodal, as is the color distribution of the same set of GCs. Thus, the metallicity bimodality is real and not an artifact of the color to metallicity conversion. However, the metallicity distribution function obtained from comparison with the single stellar population models is consistent with a unimodal, bimodal, or multimodal shape. The [?/Fe] values are supersolar with a mean value of 0.14 ? 0.04, indicating a fast formation timescale. However, the GCs in NGC 5128 are not as [?/Fe] enhanced as the Milky Way GCs also examined in this study. Our measured indices also indicate that the GCs in NGC 5128 may have a slight overabundance in nitrogen and a wider range of calcium strength compared to the Milky Way GCs. Our results support a rapid, early formation of the GC system in NGC 5128, with subsequent major accretion and/or GC and star-forming events in more recent times.

Metal abundances from Washington photometry of globular clusters in NGC 5128

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.