Globular Cluster Luminosity Functions and the Hubble Constant from WFPC2 Imaging: Galaxies in the Coma I Cloud

The membership of some galaxies in the nearby (d ~ 12 Mpc) Coma I cloud is uncertain. Here we present globular cluster luminosity functions (GCLFs) from the HST for two bright ellipticals which may belong to this group. After fitting the GCLF, we find a turnover magnitude of m_V^0 = 23.23 +/- 0.11 for NGC 4278 and m_V^0 = 23.07 +/- 0.13 for NGC 4494. Our limiting magnitude is about two magnitudes fainter than these values, making this data among the most complete GCLFs published to date. The fitted GCLF dispersions (~ 1.1 mag.) are somewhat smaller than typical values for other ellipticals. Assuming an absolute turnover magnitude of M_V^0 = -7.62, and after applying a small metallicity correction, we derive distance modulii of (m -- M) = 30.61 +/- 0.14 for NGC 4278 and 30.50 +/- 0.15 for NGC 4494. These distance estimates are compared to other methods, and lie within the published range of values. We conclude that both galaxies lie at the same distance and are both members of the Coma I cloud.Comment: 13 pages, Latex. Full paper also available at http://www.ucolick.org/~forbes/home.htm

Globular Clusters and Galaxy Formation

Globular clusters provide a unique probe of galaxy formation and evolution. Here I briefly summarize the known observational properties of globular cluster systems. One re-occurring theme is that the globular cluster systems of spirals and ellipticals are remarkably similar. Photometry, and the limited spectra available, are consistent with metal-poor clusters forming before the main spheroid component is established and the metal-rich ones forming at the same time as the spheroid in a burst of star formation. These observations are compared to a model for globular cluster formation in a LCDM hierarchical universe. One model result reported here is that S_N is determined at early times and little affected by late epoch mergers.Comment: 2 pages, Latex, 2 figures, To appear in the proceedings of Galaxy Evolution: Theory and Observations, ed. V. Avila-Reese, C. Firmani, C. Frenk, C. Allen, RevMexA

The Globular Cluster Systems of Ellipticals and Spirals

In this overview of the key properties of globular cluster (GC) systems I show that the GCs in elliptical and spiral host galaxies have more in common than previously thought. After contrasting these properties I briefly comment on GC formation.Comment: 6 pages, Latex, 4 figures, To appear in the proceedings of IAU Symp. 207, "Extragalactic Star Clusters", eds. Grebel, Geisler, Minnit

A New Method for Estimating Dark Matter Halo Masses using Globular Cluster Systems

All galaxies are thought to reside within large halos of dark matter, whose properties can only be determined from indirect observations. The formation and assembly of galaxies is determined from the interplay between these dark matter halos and the baryonic matter they host. Although statistical relations can be used to approximate how massive a galaxy's halo is, very few individual galaxies have direct measurements of their halo masses. We present a method to directly estimate the total mass of a galaxy's dark halo using its system of globular clusters. The link between globular cluster systems and halo masses is independent of a galaxy's type and environment, in contrast to the relationship between galaxy halo and stellar masses. This trend is expected in models where globular clusters form in early, rare density peaks in the cold dark matter density field and the epoch of reionisation was roughly coeval throughout the Universe. We illustrate the general utility of this relation by demonstrating that a galaxy's supermassive black hole mass and global X-ray luminosity are directly proportional to their host dark halo masses, as inferred from our new method.Comment: 6 pages, 4 colour figures. Accepted by MNRAS Letters. Data catalogue available from the first autho

Observational Properties of Extragalactic Globular Cluster Systems

The superior resolution of HST and the light gathering power of large 8-10m class telescopes are now providing information on distant globular clusters (GCs) that is comparable to that obtained in early 1990s for Local Group systems. Here I summarise what has been learnt from the imaging and limited spectroscopy of GCs in other galaxies. The GC systems of spirals and ellipticals reveal remarkable similarities. The vast bulk of GCs appear to have formed at early epochs, with mergers making a limited contribution to the overall GC system at later epochs. These observational findings are placed in the context of galaxy formation.Comment: 8 pages, Latex, 3 figures, To appear in the proceedings of New Horizons in Globular Cluster Astronomy ed. G. Piotto, G. Meylan, G. Djorgovski, M. Riello, ASP Conference serie

Globular Cluster Luminosity Functions and the Hubble Constant from WFPC2 Imaging: The Giant Elliptical NGC 4365

The turnover, or peak, magnitude in a galaxy's globular cluster luminosity function (GCLF) may provide a standard candle for an independent distance estimator. Here we examine the GCLF of the giant elliptical NGC 4365 using photometry of ~ 350 globular clusters from the HST's WFPC2. The WFPC2 data have several advantages over equivalent ground--based imaging. The membership of NGC 4365 in the Virgo cluster has been the subject of recent debate. We have fit a Gaussian and t_5 profile to the GCLF and find that it can be well represented by a turnover magnitude of m_V^0 = 24.2 +/- 0.3 and a dispersion sigma = 1.28 +/- 0.15. After applying a small metallicity correction to the `universal' globular cluster turnover magnitude, we derive a distance modulus of (m -- M) = 31.6 +/- 0.3 which is in reasonable agreement with that from surface brightness fluctuation measurements. This result places NGC 4365 about 6 Mpc beyond the Virgo cluster core. For a V_{CMB} = 1592 +/- 24 km/s the Hubble constant is H_o = 72 (-12,+10) km/s/Mpc. We also describe our method for estimating a local specific frequency for the GC system within the central 5 h^{-1} kpc which has fewer uncertain corrections than a total estimate. The resulting value of 6.4 $\pm$ 1.5 indicates that NGC 4365 has a GC richness similar to other early type galaxies.Comment: 12 pages, Latex, full paper also available at http://www.ucolick.org/~forbes/home.htm