1,757 research outputs found
The Globular Cluster System of the Spiral Galaxy NGC7814
We present the results of a wide-field photometric study of the globular
cluster (GC) system of the edge-on Sab spiral NGC7814. This is the first spiral
to be fully analyzed from our survey of the GC systems of a large sample of
galaxies beyond the Local Group. NGC7814 is of particular interest because a
previous study estimated that it has 500-1000 GCs, giving it the largest
specific frequency (S_N) known for a spiral. Understanding this galaxy's GC
system is important in terms of our understanding of the GC populations of
spirals in general and has implications for the formation of massive galaxies.
We observed the galaxy in BVR filters with the WIYN 3.5-m telescope, and used
image classification and three-color photometry to select GC candidates. We
also analyzed archival HST WFPC2 images of NGC7814, both to help quantify the
contamination level of the WIYN GC candidate list and to detect GCs in the
inner part of the galaxy halo. Combining HST data with high-quality
ground-based images allows us to trace the entire radial extent of this
galaxy's GC system and determine the total number of GCs directly through
observation. We find that rather than being an especially high-S_N spiral,
NGC7814 has <200 GCs and S_N ~ 1, making it comparable to the two most
well-studied spirals, the Milky Way and M31. We explore the implications of
these results for models of the formation of galaxies and their GC systems. The
initial results from our survey suggest that the GC systems of typical
ellipticals can be accounted for by the merger of two or more spirals, but that
for highly-luminous ellipticals, additional physical processes may be needed.Comment: 28 pages, incl. 4 figures; accepted for publication in The
Astronomical Journal, November 2003 issu
Bimodal Infrared Colors of the M87 Globular Cluster System: Peaks in the Metallicity Distribution
The globular cluster (GC) systems of many galaxies reveal bimodal optical
color distributions. Based on stellar evolutionary models and the bimodal
colors and metallicities of Galactic GCs this is thought to reflect an
underlying bimodal metallicity distribution. However, stars at many different
phases of stellar evolution contribute to optical light. The I-H color is a
much cleaner tracer of metallicity because it primarily samples the metallicity
sensitive giant branch. Therefore, we use deep HST-NICMOS H, and WFPC2 optical
observations, of M87 GCs to study their metallicity distribution. The M87
clusters are bimodal in I-H, for which there is no known physical explanation
other than a bimodal metallicity distribution. Moreover, the two modes defined
by the B-I and I-H colors are comprised of roughly the same two sets of
objects, confirming that optical colors also primarily trace the metallicity.
This is inconsistent with a recent suggestion based on one model of metallicity
effects on the horizontal branch that bimodality arises from an underlying
unimodal metallicity distribution due to a specific color-metallicity relation.
We also find no discernable variation in the peak colors of the M87 GCs out to
roughly 75 kpc due to the declining ratio of red-to-blue GCs, as implied by
this model. Similarly, there is no evidence that the bimodal peaks are bluer
for systems with large blue-to-red GC ratio. Our observations confirm that the
primary cause of bimodality in cluster systems is an underlying bimodal
metallicity distribution, and not the specific color-metallicity relationship
defined by this horizontal branch model.Comment: Accepted for publication in ApJ Letters. 5 pages, 4 figs. Version 2
is identical to version
Wide-Field Imaging from Space of Early-Type Galaxies and Their Globular Clusters
Wide-field imaging from space will reveal a wealth of information about the
globular cluster systems of any galaxies in the local universe that are
observed by such a mission. Individual globular clusters around galaxies in the
local universe have compact sizes that are ideal for the excellent spatial
resolution afforded by space-based imaging, while systems of these globular
clusters have large spatial extent that can only be fully explored by
wide-field imaging. One example of the science return from such a study is the
determination of the major formation epoch(s) of galaxies from the ages of
their globular clusters determined via their optical to near-infrared colors. A
second example is determining the sites of metal-poor globular cluster
formation from their cosmological bias, which constrains the formation of
structures early in the universe.Comment: To appear in the conference proceedings of "Wide Field Imaging from
Space" published in New Astronomy Reviews, eds. T. McKay, A. Fruchter, & E.
Linde
Global Properties of the Globular Cluster Systems of Four Spiral Galaxies
We present results from a wide-field imaging study of the globular cluster
(GC) systems of a sample of edge-on, Sb-Sc spiral galaxies ~7-20 Mpc away. This
study is part of a larger survey of the ensemble properties of the GC
populations of giant galaxies. We imaged the galaxies in BVR filters with
large-format CCD detectors on the WIYN 3.5-m telescope, to projected radii of
~20-40 kpc. For four galaxies (NGC 2683, NGC 3556, NGC 4157, and NGC 7331), we
quantify the radial distributions of the GC systems and estimate the total
number, luminosity- and mass-normalized specific frequencies (S_N and T), and
blue (metal-poor) fraction of GCs. A fifth galaxy (NGC 3044) was apparently too
distant for us to have detected its GC system. Our S_N for NGC 2683 is 2.5
times smaller than the previously-published value, likely due in part to
reduced contamination from non-GCs. For the spiral galaxies analyzed for the
survey to date, the average number of GCs is 170+/-40 and the weighted mean
values of S_N and T are 0.8+/-0.2 and 1.4+/-0.3. We use the survey data to
derive a relationship between radial exent of the GC system and host galaxy
mass over a factor of 20 in mass. Finally, we confirm the trend, identified in
previous survey papers, of increasing specific frequency of metal-poor GCs with
increasing galaxy mass. We compare the data with predictions from a simple
model and show that carefully quantifying the numbers of metal-poor GCs in
galaxies can constrain the formation redshifts of the GCs and their host
galaxies.Comment: 30 pages, including 14 figures and 13 tables; accepted for
publication in The Astronomical Journal, Oct 2007 issu
Signatures of multiple stellar populations in unresolved extragalactic globular/ young massive star clusters
We present an investigation of potential signatures of the formation of
multiple stellar populations in recently formed extragalactic star clusters.
All of the Galactic globular clusters for which good samples of individual
stellar abundances are available show evidence for multiple populations. This
appears to require that multiple episodes of star formation and light element
enrichment are the norm in the history of a globular cluster. We show that
there are detectable observational signatures of multiple formation events in
the unresolved spectra of massive, young extragalactic star clusters. We
present the results of a pilot program to search for one of the cleanest
signatures that we identify - the combined presence of emission lines from a
very recently formed population and absorption lines from a somewhat older
population. A possible example of such a system is identified in the Antennae
galaxies. This source's spectrum shows evidence of two stellar populations with
ages of 8 Myr and 80 Myr. Further investigation shows that these populations
are in fact physically separated, but only by a projected distance of 59 pc. We
show that the clusters are consistent with being bound and discuss the
possibility that their coalescence could result in a single globular cluster
hosting multiple stellar populations. While not the prototypical system
proposed by most theories of the formation of multiple populations in clusters,
the detection of this system in a small sample is both encouraging and
interesting. Our investigation suggests that expanded surveys of massive young
star clusters should detect more clusters with such signatures.Comment: 11 pages, 7 figures: accepted for publication in Ap
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