1,807 research outputs found
The Luminosity Functions of Old and Intermediate-Age Globular Clusters in NGC 3610
The WFPC2 Camera on board HST has been used to obtain high-resolution images
of NGC 3610, a dynamically young elliptical galaxy. These observations
supersede shorter, undithered HST observations where an intermediate-age
population of globular clusters was first discovered. The new observations show
the bimodal color distribution of globular clusters more clearly, with peaks at
(V-I)o = 0.95 and 1.17. The luminosity function (LF) of the blue, metal-poor
population of clusters in NGC 3610 turns over, consistent with a Gaussian
distribution with a peak Mv ~= -7.0, similar to old globular-cluster
populations in ellipticals. The red, metal-rich population of clusters has a LF
that is more extended toward both the bright and faint ends, as expected for a
cluster population of inter-mediate age. It is well fit by a power law with an
exponent of alpha = -1.78 +-0.05, or -1.90+-0.07 when corrected for
observational scatter. A Kolmogorov-Smirnov test confirms the significant
difference between the LFs of the red and blue clusters, with a probability of
less than 0.1% that they come from the same population. A comparison with the
Fall & Zhang cluster disruption models shows marginal agreement with the
observed LF, although there are differences in detail. In particular, there is
no clear evidence of the predicted turnover at the faint end. A by-product of
the analysis is the demonstration that, at any given metallicity, the peak of
the LF should remain nearly constant from 1.5 Gyr to 12 Gyr, since the effect
of the disruption of faint clusters is almost perfectly balanced by the fading
of the clusters. This may help explain the apparent universality of the peak of
the globular cluster luminosity function. (Abridged)Comment: 32 pages, LaTeX, 13 PS figures, 1 table; to appear in AJ (July 2002
The Globular Cluster Luminosity Function and Specific Frequency in Dwarf Elliptical Galaxies
The globular cluster luminosity function, specific globular cluster
frequency, S_N, specific globular cluster mass, T_MP, and globular cluster mass
fraction in dwarf elliptical galaxies are explored using the full 69 galaxy
sample of the HST WFPC2 Dwarf Elliptical Galaxy Snapshot Survey. The GCLFs of
the dEs are well-represented with a t_5 function with a peak at
M_{V,Z}^0(dE,HST) = -7.3 +/- 0.1. This is ~0.3 magnitudes fainter than the GCLF
peaks in giant spiral and elliptical galaxies, but the results are consistent
within the uncertainties. The bright-end slope of the luminosity distribution
has a power-law form with slope alpha = -1.9 +/- 0.1. The trend of increasing
S_N or T_MP with decreasing host galaxy luminosity is confirmed. The mean value
for T_MP in dE,N galaxies is about a factor of two higher than the mean value
for non-nucleated galaxies and the distributions of T_MP in dE,N and dE,noN
galaxies are statistically different. These data are combined with results from
the literature for a wide range of galaxy types and environments. At low host
galaxy masses the distribution of T_MP for dE,noN and dI galaxies are similar.
This supports the idea that one pathway for forming dE,noN galaxies is by the
stripping of dIs. The formation of nuclei and the larger values of T_MP in dE,N
galaxies may be due to higher star formation rates and star cluster formation
efficiencies due to interactions in galaxy cluster environments.Comment: 53 pages, 13 figures, 12 tables, accepted by the Astrophysical
Journa
ISM Abundances in Sculptor Group Dwarf Irregular Galaxies
Using the CTIO 4-m telescope, we have obtained optical spectra of HII regions
in five Sculptor Group dwarf irregular galaxies. We derive oxygen, nitrogen,
and sulfur abundances from the HII region spectra. Oxygen abundances are
derived via three different methods (the ``direct'' method, the empirical
method guided by photoionization modeling of McGaugh (1991), and the purely
empirical method of Pilyugin (2000)) and are compared. Significant systematic
differences are found between the three methods, and we suggest that a
recalibration of the empirical abundance scale is required. The N/O ratio for
the metal-poor dI ESO 473-G24 of log (N/O) = -1.43 +/- 0.03 lies well above the
plateau of log (N/O) = -1.60 +/- 0.02 found by Izotov & Thuan (1999) for a
collection of metal-poor blue compact galaxies. This shows that not all
galaxies with 12 + log (O/H) < 7.6 have identical elemental abundance ratios,
and this implies that the Izotov & Thuan scenario for low metallicity galaxies
is not universal. Measurements of the HII regions in NGC 625 yield log (N/O) =
-1.25. Assuming N production by intermediate mass stars, this relatively high
N/O ratio may be indicative of a long quiescent period prior to the recent
active burst of star formation. The oxygen abundances in the Sculptor Group dIs
are in good agreement with the relationship between metallicity and luminosity
observed in the Local Group dIs. The Sculptor Group dIs, in general, lie closer
to the simple closed box model evolutionary path than the Local Group dIs. The
higher gas contents, lower average star formation rates, and closer resemblance
to closed box evolution could all be indicative of evolution in a relatively
low density environment.Comment: 40 pages, 9 figures, accepted for Feb 2003 A
The Luminosity Function of Young Star Clusters In "The Antennae" Galaxies (NGC 4038/4039)
The WFPC2 of the HST has been used to obtain high-resolution images of NGC
4038/4039 that go roughly 3 magnitudes deeper in V than previous observations
made during Cycle 2 (-14 < M_V < -6). To first order the luminosity function
(LF) is a power law, with exponent \alpha = -2.12 +/- 0.04. However, after
decoupling the cluster and stellar LFs, which overlap in the range -9 < M_V <
-6, we find an apparent bend in the young cluster LF at approximately M_V =
-10.4. The LF has a power law exponent -2.6 +/- 0.2 in the brightward and -1.7
+/- 0.2 in the faintward. The bend corresponds to a mass ~ 10^5 M_{\odot}, only
slightly lower than the characteristic mass of globular clusters in the Milky
Way (~2x10^5 M_{\odot}). The star clusters of the Antennae appear slightly
resolved, with median effective radii of 4 +/- 1 pc, similar to or perhaps
slightly larger than those of globular clusters in our Galaxy. However, the
radial extents of some of the very young clusters (ages < 10 Myr) are much
larger than those of old globular clusters. A combination of the UBVI colors,
\Halpha morphology, and GHRS spectra enables us to age-date the clusters in
different regions of The Antennae. We find two groups of young star clusters
with ages <~ 20Myr and ~100Myr, as well as an intermediate-age group (~500 Myr)
and a handful of old globular clusters from the progenitor galaxies. Age
estimates derived from GHRS spectroscopy yield 3 +/- 1 Myr for Knot K (just
south of the nucleus of NGC 4038) and 7 +/- 1 Myr for Knot S in the Western
Loop, in good agreement with ages derived from the UBVI colors. Effective
gas-outflow velocities from Knots S and K are estimated to be about 25-30 km/s.
However, the measured widths of the interstellar absorption lines suggest
dispersion velocities of ~400 km/s along the lines of sight to Knots S and K.Comment: 56 pages, 4 tables and 23 figures, texts in AAS style, to be
published in A
The Specific Globular Cluster Frequencies of Dwarf Elliptical Galaxies from the Hubble Space Telescope
The specific globular cluster frequencies (S_N) for 24 dwarf elliptical (dE)
galaxies in the Virgo and Fornax Clusters and the Leo Group imaged with the
Hubble Space Telescope are presented. Combining all available data, we find
that for nucleated dEs --- which are spatially distributed like giant
ellipticals in galaxy clusters --- S_N(dE,N)=6.5 +- 1.2 and S_N increases with
M_V, while for non-nucleated dEs --- which are distributed like late-type
galaxies --- S_N(dE,noN)=3.1 +- 0.5 and there is little or no trend with M_V.
The S_N values for dE galaxies are thus on average significantly higher than
those for late-type galaxies, which have S_N < 1. This suggests that dE
galaxies are more akin to giant Es than to late-type galaxies. If there are
dormant or stripped irregulars hiding among the dE population, they are likely
to be among the non-nucleated dEs. Furthermore, the similarities in the
properties of the globular clusters and in the spatial distributions of dE,Ns
and giant Es suggest that neither galaxy mass or galaxy metallicity is
responsible for high values of S_N. Instead, most metal-poor GCs may have
formed in dwarf-sized fragments that merged into larger galaxies.Comment: 12 pages (uses aaspp4.sty), 2 figures, 1 table, to appear in the
Astrophysical Journa
Statistical Computations with AstroGrid and the Grid
We outline our first steps towards marrying two new and emerging
technologies; the Virtual Observatory (e.g, AstroGrid) and the computational
grid. We discuss the construction of VOTechBroker, which is a modular software
tool designed to abstract the tasks of submission and management of a large
number of computational jobs to a distributed computer system. The broker will
also interact with the AstroGrid workflow and MySpace environments. We present
our planned usage of the VOTechBroker in computing a huge number of n-point
correlation functions from the SDSS, as well as fitting over a million CMBfast
models to the WMAP data.Comment: Invited talk to appear in "Proceedings of PHYSTAT05: Statistical
Problems in Particle Physics, Astrophysics and Cosmology
Gemini Spectroscopic Survey of Young Star Clusters in Merging/Interacting Galaxies. II. NGC 3256 Clusters
We present Gemini optical spectroscopy of 23 young star clusters in NGC3256.
We find that the cluster ages range are from few Myr to ~150 Myr. All these
clusters are relatively massive (2--40)x 10^{5} \msun$ and appear to be of
roughly 1.5 \zo metallicity. The majority of the clusters in our sample follow
the same rotation curve as the gas and hence were presumably formed in the
molecular-gas disk. However, a western subsample of five clusters has
velocities that deviate significantly from the gas rotation curve. These
clusters may either belong to the second spiral galaxy of the merger or may
have formed in tidal-tail gas falling back into the system. We discuss our
observations in light of other known cluster populations in merging galaxies,
and suggest that NGC 3256 is similar to Arp 220, and hence may become an
Ultra-luminous Infrared Galaxy as the merger progresses and the star-formation
rate increases.
Some of the clusters which appeared as isolated in our ground-based images
are clearly resolved into multiple sub-components in the HST-ACS images. The
same effect has been observed in the Antennae galaxies, showing that clusters
are often not formed in isolation, but instead tend to form in larger groups or
cluster complexes.Comment: 20 pages, 10 figures, 3 tables; Accepted Ap
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