70 research outputs found
The Cosmological Dependence of Cluster Density Profiles
We use N-body simulations to study the shape of mean cluster density and
velocity profiles formed via gravitational instability. The dependence of the
final structure on both cosmology and initial density field is examined, using
a grid of cosmologies and scale-free initial power spectra P\propto k^n. For
each model, we stack clusters to define an average density profile in the
non-linear regime. The profiles are well fit by a power law over 99% of the
cluster volume, with a clear trend toward steeper slopes with both increasing n
and decreasing Omega_o. For models with a Omega_o = 0.2, the profile slopes are
consistently higher than those for Omega-1.0. Cluster density profiles are thus
potentially useful cosmological diagnostics. We find no evidence for a constant
density core in any of the models, although the density profiles do tend to
flatten at small radii. Much of the flattening is due to the force softening
required by the simulations, and an attempt is made to recover the unsoftened
profiles assuming angular momentum invariance. The recovered profiles in the
Omega=1 cosmologies are consistent with a pure power law up to the highest
density contrasts (10^6) accessible with our resolution. The low density models
show significant deviations from a power law above density contrasts \sim 10^5.
We interpret this curvature as reflecting the non scale-invariant nature of the
background cosmology in these models.Comment: uuencoded, 22 pages + 13 figs. Astrophysical Journal, in pres
The Correlation Dimension of Young Stars in Dwarf Galaxies
We present the correlation dimension of resolved young stars in four actively
star-forming dwarf galaxies that are sufficiently resolved and transparent to
be modeled as projections of three-dimensional point distributions. We use data
in the Hubble Space Telescope archive; photometry for one of them, UGCA 292, is
presented here for the first time. We find that there are statistically
distinguishable differences in the nature of stellar clustering among the
sample galaxies. The young stars of VII Zw 403, the brightest galaxy in the
sample, have the highest value for the correlation dimension and also the most
dramatic decrease with logarithmic scale, falling from to
over less than a factor of ten in . This decrease is
consistent with the edge effect produced by a projected Poisson distribution
within a 2:2:1 ellipsoid. The young stars in UGC 4483, the faintest galaxy in
the sample, exhibit very different behavior, with a constant value of about 0.5
over this same range in , extending nearly to the edge of the distribution.
This behavior may indicate either a scale-free distribution with an unusually
low correlation dimension, or a two-component (not scale-free) combination of
cluster and field stars.Comment: 25 pages, 10 figures, accepted by A
Baade's red sheet resolved into stars with HST in the Blue Compact Dwarf Galaxy VII Zw 403
HST WFPC2 observations of the nearby Blue Compact Dwarf Galaxy VII~Zw~403 (=
UGC 6456) resolve single stars down to M-2.5, deep enough to
identify red giants. This population has a more uniform spatial distribution
than the young main-sequence stars and supergiants, forming the structure known
as "Baade's red sheet". We conclude that VII~Zw~403 is not a primeval galaxy.Comment: submitted to: ApJ Letter
The Oldest Stars of the Extremely Metal-Poor Local Group Dwarf Irregular Galaxy Leo A
We present deep Hubble Space Telescope single-star photometry of Leo A in B,
V, and I. Our new field of view is offset from the centrally located field
observed by Tolstoy et al. (1998) in order to expose the halo population of
this galaxy. We report the detection of metal-poor red horizontal branch stars,
which demonstrate that Leo A is not a young galaxy. In fact, Leo A is as least
as old as metal-poor Galactic Globular Clusters which exhibit red horizontal
branches, and are considered to have a minimum age of about 9 Gyr. We discuss
the distance to Leo A, and perform an extensive comparison of the data with
stellar isochrones. For a distance modulus of 24.5, the data are better than
50% complete down to absolute magnitudes of 2 or more. We can easily identify
stars with metallicities between 0.0001 and 0.0004, and ages between about 5
and 10 Gyr, in their post-main-sequence phases, but lack the detection of
main-sequence turnoffs which would provide unambiguous proof of ancient (>10
Gyr) stellar generations. Blue horizontal branch stars are above the detection
limits, but difficult to distinguish from young stars with similar colors and
magnitudes. Synthetic color-magnitude diagrams show it is possible to populate
the blue horizontal branch in the halo of Leo A. The models also suggest ~50%
of the total astrated mass in our pointing to be attributed to an ancient (>10
Gyr) stellar population. We conclude that Leo A started to form stars at least
about 9 Gyr ago. Leo A exhibits an extremely low oxygen abundance, of only 3%
of Solar, in its ionized interstellar medium. The existence of old stars in
this very oxygen-deficient galaxy illustrates that a low oxygen abundance does
not preclude a history of early star formation.Comment: 44 pages, 18 figures, accepted for publication in the August 2002
issue of AJ. High resolution figures is available at
http://www.astro.spbu.ru/staff/dio/preprints.htm
A Compact Population of Red Giants in the Blue Compact Dwarf Galaxy UGCA 290
We present HST/WFPC2 single-star photometry for the blue dwarf galaxy UGCA
290, whose morphology is intermediate between classic iE Blue Compact Dwarfs
and blue dwarfs which exhibit no red background sheet of older stars. The
color-magnitude diagram of this galaxy in V and I, extending over six
magnitudes, is remarkably similar to that of the star-forming region in the iE
Blue Compact Dwarf VII Zw 403. There is no evidence for gaps in its
star-formation history over the last billion years, and the color of its red
giant branch indicates a very metal-poor stellar population. From the magnitude
of the tip of the red giant branch, we derive a distance of 6.7 Mpc, more than
twice the distance estimated from the brightest blue supergiants.Comment: 10 pages, 3 color figures, LaTeX2e. Accepted for publication in ApJ
Letter
A Stellar Population Gradient in VII Zw 403 - Implications for the Formation of Blue Compact Dwarf Galaxies
We present evidence for the existence of an old stellar halo in the Blue
Compact Dwarf galaxy VII Zw 403. VII Zw 403 is the first Blue Compact Dwarf
galaxy for which a clear spatial segregation of the resolved stellar content
into a "core-halo" structure is detected. Multicolor HST/WFPC2 observations
indicate that active star formation occurs in the central region, but is
strikingly absent at large radii. Instead, a globular-cluster-like red giant
branch suggests the presence of an old (> 10 Gyr) and metal poor
(=-1.92) stellar population in the halo. While the vast majority of
Blue Compact Dwarf galaxies has been recognized to possess halos of red color
in ground-based surface photometry, our observations of VII Zw 403 establish
for the first time a direct correspondence between a red halo color and the
presence of old, red giant stars. If the halos of Blue Compact Dwarf galaxies
are all home to such ancient stellar populations, then the fossil record
conflicts with delayed-formation scenarios for dwarfs.Comment: Accepted for publication in the Ap
The Stellar Content of NGC 6789, A Blue Compact Dwarf Galaxy in the Local Void
We find that NGC6789 is the most nearby example of a Blue Compact Dwarf
galaxy known to date. With the help of WFPC2 aboard the Hubble Space Telescope,
we resolve NGC6789 into over 15,000 point sources in the V and I bands. The
young stars of NGC6789 are found exclusively near the center of the galaxy. The
red giant population identified at large galacticentric radii yields a distance
of about 3.6 Mpc, a stellar metallicity [Fe/H] of about -2, and a minimum age
of about 1 Gyr. Despite its isolated location in the Local Void,its low
metallicity, and its active star formation, the properties of NGC6789 are
clearly not those of a galaxy in formation.Comment: 8 pages, 4 figures, ApJL Accepte
A Near-Infrared Stellar Census of the Blue Compact Dwarf Galaxy VII~Zw~403
We present near-infrared single-star photometry for the low-metallicity Blue
Compact Dwarf galaxy VII~Zw~403. We achieve limiting magnitudes of
F110W~~25.5 and F160W~~24.5 using one of the NICMOS cameras
with the HST equivalents of the ground-based J and H filters. The data have a
high photometric precision (0.1 mag) and are % complete down to magnitudes
of about 23, far deeper than previous ground-based studies in the near-IR. The
color-magnitude diagram contains about 1000 point sources. We provide a
preliminary transformation of the near-IR photometry into the ground system...Comment: Accepted for publication by the AJ, preprint has 49 pages, 2 tables,
and 16 figure
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