1,190 research outputs found
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 10 dwarf irregular galaxies
We examine the stellar content of 10 dwarf irregular galaxies of which
broad-band CCD photometry was published in Hopp & Schulte-Ladbeck (1995). We
also present Halpha images for several of these galaxies. The galaxies in the
sample are located outside of the Local Group. Yet, they are still close enough
to be resolved into single stars from the ground but only the brightest stars
(or star clusters) are detected and there is severe crowding. The sample
galaxies were selected to be isolated from massive neighbors; about half of
them are (mostly peripheral) members of groups, the other half is located in
the field. We discuss the vicinity of the sample galaxies to other dwarf
galaxies.
In order to interpret single-star photometry and draw conclusions about the
stellar content or other distance-dependent quantities, it is crucial that
accurate distances to the galaxies be known. The distances to the sample
galaxies are not well known since all but one have not had a primary distance
indicator measured. We make an attempt to constrain the distances by
identifying the envelope of the brightest supergiants in B, B-R and R, B-R
color-magnitude diagrams, but the results are not very accurate (we estimate
the minimal error on the distance modulus is 1.36 mag). Nevertheless, the fact
that the sample galaxies are resolved with direct ground-based imaging
indicates that they are sufficiently nearby to represent good candidates for
observations with instruments that provide high spatial resolution, e.g.,
adaptive optics systems on large ground-based telescopes, or the Hubble Space
Telescope. ...Comment: 56 pages, 4 tables, 35 figures, The Astronomical Journal, accepte
The Dwarf Spheroidal Companions to M31: WFPC2 Observations of Andromeda III
The Hubble Space Telescope WFPC2 camera has been used to image Andromeda III,
a dwarf spheroidal companion (dSph) to M31. The resulting color-magnitude (c-m)
diagrams reveal the morphology of the horizontal branch (HB) in this dwarf
galaxy. We find that like Andromeda I and Andromeda II, and like most of the
Galactic dSph companions, the HB morphology of And III is predominantly red,
redder than that of both And I and And II despite And III having a lower mean
metallicity. We interpret this red HB morphology as indicating that the bulk of
the And III population is ~3 Gyr younger than the age of the majority of
Galactic globular clusters. Nevertheless, the And III c-m diagram does reveal a
few blue HB stars, and a number of RR Lyrae variables are also evident in the
data. This indicates that And III does contain an `old' population of age
comparable to that of the Galactic globular clusters. There is no evidence,
however, for any young stars in And III despite a claimed association between
this dSph and an HI cloud. As was the case for And II, but not And I, no radial
gradient was detected in the And III HB morphology. And III is ~75 kpc from the
center of M31, comparable to the Galactocentric distances of Sculptor and
Draco. Comparison with standard globular cluster red giant branches indicates
= -1.88 +/-0.11, consistent with the absolute-magnitude - mean
abundance relation followed by dSph galaxies. The same comparison yields an
intrinsic abundance dispersion of sigma([Fe/H]) = 0.12, a low value compared to
the Galactic dSphs of comparable luminosity to And III. The list of candidate
variables reveals one definite and one probable Anomalous Cepheid variables.Comment: 30 pages including 1 table, 10 figures, Fig 1 as jpeg to save space.
Accepted for publication in the Astronomical Journal, July 2002 issu
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
The interaction of lean and building information modeling in construction
Lean construction and Building Information Modeling are quite different initiatives, but both are having profound impacts on the construction industry. A rigorous analysis of the myriad specific interactions between them indicates that a synergy exists which, if properly understood in theoretical terms, can be exploited to improve construction processes beyond the degree to which it might be improved by application of either of these paradigms independently. Using a matrix that juxtaposes BIM functionalities with prescriptive lean construction principles, fifty-six interactions have been identified, all but four of which represent constructive interaction. Although evidence for the majority of these has been found, the matrix is not considered complete, but rather a framework for research to
explore the degree of validity of the interactions. Construction executives, managers, designers and developers of IT systems for construction can also benefit from the framework as an aid to recognizing the potential synergies when planning their lean and BIM adoption strategies
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
Superheavy dark matter and ultrahigh energy cosmic rays
The phase of inflationary expansion in the early universe produces superheavy
relics in a mass window between 10^{12} GeV and 10^{14} GeV. Decay or
annihilation of these superheavy relics can explain the observed ultrahigh
energy cosmic rays beyond the Greisen-Zatsepin-Kuzmin cutoff. We emphasize that
the pattern of cosmic ray arrival directions with energies beyond 20 EeV will
decide between the different proposals for the origin of ultrahigh energy
cosmic rays.Comment: Based on an invited talk given by RD at Theory Canada 1, Vancouver,
June 2-5, 200
The FORS Deep Field: Field selection, photometric observations and photometric catalog
The FORS Deep Field project is a multi-colour, multi-object spectroscopic
investigation of an approx. 7 times 7 region near the south galactic pole based
mostly on observations carried out with the FORS instruments attached to the
VLT telescopes. It includes the QSO Q 0103-260 (z = 3.36). The goal of this
study is to improve our understanding of the formation and evolution of
galaxies in the young Universe. In this paper the field selection, the
photometric observations, and the data reduction are described. The source
detection and photometry of objects in the FORS Deep Field is discussed in
detail. A combined B and I selected UBgRIJKs photometric catalog of 8753
objects in the FDF is presented and its properties are briefly discussed. The
formal 50% completeness limits for point sources, derived from the co-added
images, are 25.64, 27.69, 26.86, 26.68, 26.37, 23.60 and 21.57 in U, B, g, R,
I, J and Ks (Vega-system), respectively. A comparison of the number counts in
the FORS Deep Field to those derived in other deep field surveys shows very
good agreement.Comment: 15 pages, 11 figures (included), accepted for publication in A&
Properties of M31. II: A Cepheid disk sample derived from the first year of PS1 PAndromeda data
We present a sample of Cepheid variable stars towards M31 based on the first
year of regular M31 observations of the PS1 survey in the r_P1 and i_P1
filters. We describe the selection procedure for Cepheid variable stars from
the overall variable source sample and develop an automatic classification
scheme using Fourier decomposition and the location of the instability strip.
We find 1440 fundamental mode (classical \delta) Cep stars, 126 Cepheids in the
first overtone mode, and 147 belonging to the Population II types. 296 Cepheids
could not be assigned to one of these classes and 354 Cepheids were found in
other surveys. These 2009 Cepheids constitute the largest Cepheid sample in M31
known so far and the full catalog is presented in this paper. We briefly
describe the properties of our sample in its spatial distribution throughout
the M31 galaxy, in its age properties, and we derive an apparent
period-luminosity relation (PLR) in our two bands. The Population I Cepheids
nicely follow the dust pattern of the M31 disk, whereas the 147 Type II
Cepheids are distributed throughout the halo of M31. We outline the time
evolution of the star formation in the major ring found previously and find an
age gradient. A comparison of our PLR to previous results indicates a curvature
term in the PLR
More evidence for an intracluster planetary nebulae population in the Virgo cluster
We surveyed a 50 sq arcmin region in the Virgo cluster core to search for
intergalactic planetary nebulae, and found 11 candidates in the surveyed area.
The measured fluxes of these unresolved sources are consistent with these
objects being planetary nebulae from an intracluster population of stars. We
compute the cumulative luminosity function of these 11 planetary nebula
candidates. If we assume that they belong to the Virgo cluster, their
cumulative luminosity function is in good agreement with planetary nebula
luminosity function simulations. This comparison allows us to estimate the
surface mass density of the intracluster stellar population at the surveyed
field in the cluster core.Comment: ApJ Letters, in press. A .ps file is also available at:
http://www.usm.uni-muenchen.de:8001/people/mendez/preprints/preprints.htm
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