107 research outputs found
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
Differential protein expression profiling by ITRAQ-2DLC-MS/MS of human bladder cancer EJ138 cells transfected with the metastasis suppressor kiss-1 gene
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