Globular cluster systems as clues to galaxy evolution

We investigate the properties of systems of globular clusters in light of the hypothesis that galaxy mergers play a major role in galaxy evolution. In a previous paper, we presented a model in which the formation of globular clusters occurs during galaxy interactions and mergers. We discussed several predictions of the model, including the existence of young globular clusters in currently merging galaxies and the presence of two or more metallicity peaks in the globular clusters systems of normal elliptical galaxies. Here, we present recent observational evidence which supports both of these predictions and suggests that mergers may have a significant influence on the formation and evolution of galaxies and their globular clusters

Globular Cluster Formation

The discovery of young globular clusters in merging galaxies and other environments provides an opportunity to study directly the process of globular cluster formation. Empirically it appears that globular cluster formation occurs preferentially in regions in which star formation occurs at a high rate and efficiency. Further, the interstellar medium in such regions is likely to be at a higher pressure than less active star-forming environments. An additional observational clue to the globular cluster formation process is that young globular clusters have little or no mass-radius relationship. In this paper I argue that high pressure and high star-formation efficiency are responsible for current globular cluster formation. I suggest that the precursors to globular clusters are molecular clouds and that the mass-radius relationship exhibited by such clouds is wiped out by a variable star formation efficiency.Comment: 13 pages. Review to appear in "Extragalactic Globular Cluster Systems," ed. M. Kissler-Patig, Springer-Verla

Detecting Bimodality in Astronomical Datasets

We discuss statistical techniques for detecting and quantifying bimodality in astronomical datasets. We concentrate on the KMM algorithm, which estimates the statistical significance of bimodality in such datasets and objectively partitions data into sub-populations. By simulating bimodal distributions with a range of properties we investigate the sensitivity of KMM to datasets with varying characteristics. Our results facilitate the planning of optimal observing strategies for systems where bimodality is suspected. Mixture-modeling algorithms similar to the KMM algorithm have been used in previous studies to partition the stellar population of the Milky Way into subsystems. We illustrate the broad applicability of KMM by analysing published data on globular cluster metallicity distributions, velocity distributions of galaxies in clusters, and burst durations of gamma-ray sources. PostScript versions of the tables and figures, as well as FORTRAN code for KMM and instructions for its use, are available by anonymous ftp from kula.phsx.ukans.edu.Comment: 32 page

Some Revised Observational Constraints on the Formation and Evolution of the Galactic Disk

A set of 76 open clusters with abundances based upon DDO photometry and/or moderate dispersion spectroscopy has been transformed to a common [Fe/H] scale and used to study the local structure and evolution of the galactic disk. The metallicity distribution of clusters with R_GC is best described by two distinct zones. Between R_GC = 6.5 and 10 kpc, the distribution has a mean [Fe/H] = 0.0 and a dispersion of 0.1 dex; there is only weak evidence for a shallow abundance gradient over this distance range. Beyond R_GC = 10 kpc, the metallicity distribution has a dispersion between 0.10 and 0.15 dex, but with a mean [Fe/H] = -0.3, implying a sharp discontinuity at R_GC = 10 kpc. After correcting for the discontinuity, no evidence is found for a gradient perpendicular to the plane. Adopting the clusters interior to 10 kpc as a representative sample of the galactic disk over the last 7 Gyr, the cluster metallicity range is found to be about half that of the field stars. When coupled with the discontinuity in the galactocentric gradient, the discrepancy in the metallicity distribution is interpreted as an indication of significant diffusion of field stars into the solar neighborhood from beyond 10 kpc. These results imply that the sun is NOT atypical of the stars formed in the solar circle 4.6 Gyr ago. It is suggested that the discontinuity reflects the edge of the initial galactic disk as defined by the disk globular cluster system and the so-called thick disk; the initial offset in [Fe/H] created by the differences in the chemical history on either side of the discontinuity has carried through to the current stage of galactic evolution. If correct, diffusion coupled with the absence of an abundance gradient could make the separation of field stars on the basis of galactocentric origin difficult.Comment: 41 pages, 9 figure files, LaTex. Appendix section and tables (tex or postscript) available at http://kubarb.phsx.ukans.edu/ ~twarog/ Submitted to Astronomical Journal July 199

Constraints on the Formation History of the Elliptical Galaxy NGC 3923 from the Colors of Its Globular Clusters

We present a study of the colors of globular clusters associated with the elliptical galaxy NGC 3923. Our final sample consists of Washington system C and T_1 photometry for 143 globular cluster candidates with an expected contamination of no more than 10%. We find that the color distribution of the NGC 3923 globular cluster system (GCS) is broad and appears to have at least two peaks. A mixture modeling analysis of the color distribution indicates that a two component model is favored over a single component one at a high level of confidence (>99%). This evidence for more than one population in the GCS of NGC 3923 is similar to that previously noted for the four other elliptical galaxies for which similar data have been published. Furthermore, we find that the NGC 3923 GCS is redder than the GCSs of previously studied elliptical galaxies of similar luminosity. The median metallicity inferred from our (C-T_1)_0 colors is [Fe/H]_{med} = -0.56, with an uncertainty of 0.14 dex arising from all sources of uncertainty in the mean color. This is more metal rich than the median metallicity found for the GCS of M87 using the same method, [Fe/H]_{med} = -0.94. Since M87 is more luminous than NGC 3923, this result points to significant scatter about any trend of higher GCS metallicity with increasing galaxy luminosity. We also show that there is a color gradient in the NGC 3923 GCS corresponding to about -0.5 dex in Delta[Fe/H]\Delta(log r). We conclude that the shape of the color distribution of individual GCSs and the variation in mean color among the GCSs of ellipticals are difficult to understand if elliptical galaxies are formed in a single protogalacticComment: 26 pages + 4 figures, uuencoded, tar, compressed postscript file. To be published in the Astrophysical Journa

Some Constraints on The Formation of Globular Clusters

We explore the constraints on globular cluster formation provided by the observed conditions in starbursts where globulars are currently forming, and by the observed properties of young and old globular clusters. We note that the pressure in the ISM of starbursts and mergers implies that molecular clouds in these environments have radii similar to those of globular clusters. Such molecular clouds are therefore viable precursors to globular clusters if the star formation efficiency in the clouds is high. A high star formation efficiency may be a consequence of the high density and associated high binding energy and short dynamical timescale of molecular clouds in such environments. We also note that the apparent lack of a mass-radius relationship in young and old globular cluster systems places important constraints on globular cluster formation models. This is because molecular clouds are observed to follow a virial scaling relation between mass and radius. We suggest that a variable star formation efficiency may weaken or eliminate the mass-radius relation of molecular clouds as they fragment to form globular clusters. We attribute the absence of young globular clusters in the disks of ordinary galaxies such as the Milky Way to the relatively low ambient pressures in such systems.Comment: 12 pages. Accepted for publication in the Astronomical Journal, Oct 1st issu

HST Imaging of Globular Clusters in the Edge--on Spiral Galaxies NGC 4565 and NGC 5907

We present a study of the globular cluster systems of two edge-on spiral galaxies, NGC4565 and NGC5907, from WFPC2 images in the F450W and F814W filters. The globular cluster systems of both galaxies appear to be similar to the Galactic globular cluster system. In particular, we derive total numbers of globular clusters of N_{GC}(4565)= 204+/-38 {+87}{-53} and N_{GC}(5907)=170+/-41 {+47}{-72} (where the first are statistical, the second potential systematic errors) for NGC4565 and NGC5907, respectively. This determination is based on a comparison to the Milky Way system, for which we adopt a total number of globular clusters of 180+/-20. The specific frequency of both galaxies is S_N~0.6: indistinguishable from the value for the Milky Way. The similarity in the globular cluster systems of the two galaxies is noteworthy since they have significantly different thick disks and bulge-to-disk ratios. This would suggest that these two components do not play a major role in the building up of a globular cluster system around late-type galaxies.Comment: Accepted for publication in the Astronomical Journal, 17 pages incl 5 figures, AAS style two columns. Also available at http://www.eso.org/~mkissler, Color version of figure 1 only available at http://www.eso.org/~mkissler (B/W version included

The Globular Cluster System in the Inner Region of M87

1057 globular cluster candidates have been identified in a WFPC2 image of the inner region of M87. The Globular Cluster Luminosity Function (GCLF) can be well fit by a Gaussian profile with a mean value of m_V^0=23.67 +/- 0.07 mag and sigma=1.39 +/- 0.06 mag (compared to m_V^0=23.74 mag and sigma=1.44 mag from an earlier study using the same data by Whitmore it et al. 1995). The GCLF in five radial bins is found to be statistically the same at all points, showing no clear evidence of dynamical destruction processes based on the luminosity function (LF), in contradiction to the claim by Gnedin (1997). Similarly, there is no obvious correlation between the half light radius of the clusters and the galactocentric distance. The core radius of the globular cluster density distribution is R_c=56'', considerably larger than the core of the stellar component (R_c=6.8''). The mean color of the cluster candidates is V-I=1.09 mag which corresponds to an average metallicity of Fe/H = -0.74 dex. The color distribution is bimodal everywhere, with a blue peak at V-I=0.95 mag and a red peak at V-I=1.20 mag. The red population is only 0.1 magnitude bluer than the underlying galaxy, indicating that these clusters formed late in the metal enrichment history of the galaxy and were possibly created in a burst of star/cluster formation 3-6 Gyr after the blue population. We also find that both the red and the blue cluster distributions have a more elliptical shape (Hubble type E3.5) than the nearly spherical galaxy. The average half light radius of the clusters is ~2.5 pc which is comparable to the 3 pc average effective radius of the Milky Way clusters, though the red candidates are ~20% smaller than the blue ones.Comment: 40 pages, 17 figures, 4 tables, latex, accepted for publication in the Ap