1,573 research outputs found
The Dwarf Spheroidal Companions to M31: WFPC2 Observations of Andromeda I
Images have been obtained with the Hubble Space Telescope WFPC2 camera of
Andromeda I, a dwarf spheroidal (dSph) galaxy that lies in the outer halo of
M31. The resulting color-magnitude diagrams reveal for the first time the
morphology of the horizontal branch in this system. We find that, in a similar
fashion to many of the galactic dSph companions, the horizontal branch (HB) of
And~I is predominantly red. Combined with the metal abundance of this dSph,
this red HB morphology indicates that And I can be classified as a ``second
parameter'' system in the outer halo of M31. This result then supports the
hypothesis that the outer halo of M31 formed in the same extended chaotic
manner as is postulated for the outer halo of the Galaxy.Comment: 26 pages using aas2pp4.sty, including 2 tables and 7 figures, to be
published in AJ. Figure 1 is in gif form. To include in main ps file, use xv
to create a ps file called Da_Costa.fig1.ps and uncomment appropriate lines
in .tex fil
Stellar Populations in the Andromeda V Dwarf Spheroidal Galaxy
Using archival imaging from the Wide Field Planetary Camera 2 aboard the
Hubble Space Telescope, we investigate the stellar populations of the Local
Group dwarf spheroidal Andromeda V - a companion satellite galaxy of M31. The
color-magnitude diagram (CMD) extends from above the first ascent red giant
branch (RGB) tip to approximately one magnitude below the horizontal branch
(HB). The steep well-defined RGB is indicative of a metal-poor system while the
HB is populated predominantly redward of the RR Lyrae instability strip.
Utilizing Galactic globular cluster fiducial sequences as a reference, we
calculate a mean metallicity of [Fe/H] = -2.20 +/- 0.15 and a distance of
(m-M)0 = 24.57 +/- 0.04 after adopting a reddening of E(B-V) = 0.16. This metal
abundance places And V squarely in the absolute magnitude - metallicity diagram
for dwarf spheroidal galaxies. In addition, if we attribute the entire
error-corrected color spread of the RGB stars to an abundance spread, we
estimate a range of ~0.5 dex in the metallicities of And V stars. Our analysis
of the variable star population of And V reveals the presence of 28 potential
variables. Of these, at least 10 are almost certainly RR Lyrae stars based on
their time sequence photometry.Comment: 4 pages, 6 figures Accepted to the Astronomical Journa
Phantom Friedmann Cosmologies and Higher-Order Characteristics of Expansion
We discuss a more general class of phantom () cosmologies with
various forms of both phantom () matter. We
show that many types of evolution which include both Big-Bang and Big-Rip
singularities are admitted and give explicit examples. Among some interesting
models, there exist non-singular oscillating (or "bounce") cosmologies, which
appear due to a competition between positive and negative pressure of variety
of matter content. From the point of view of the current observations the most
interesting cosmologies are the ones which start with a Big-Bang and terminate
at a Big-Rip. A related consequence of having a possibility of two types of
singularities is that there exists an unstable static universe approached by
the two asymptotic models - one of them reaches Big-Bang, and another reaches
Big-Rip. We also give explicit relations between density parameters
and the dynamical characteristics for these generalized phantom models,
including higher-order observational characteristics such as jerk and "kerk".
Finally, we discuss the observational quantities such as luminosity distance,
angular diameter, and source counts, both in series expansion and explicitly,
for phantom models. Our series expansion formulas for the luminosity distance
and the apparent magnitude go as far as to the fourth-order in redshift
term, which includes explicitly not only the jerk, but also the "kerk" (or
"snap") which may serve as an indicator of the curvature of the universe.Comment: REVTEX 4, 23 pages, references updated, to appear in Annals of
Physics (N.Y.
Spinor Dark Energy and Cosmological Coincidence Problem
Recently, the so-called Elko spinor field has been proposed to be a candidate
of dark energy. It is a non-standard spinor and has unusual properties. When
the Elko spinor field is used in cosmology, its unusual properties could bring
some interesting consequences. In the present work, we discuss the cosmological
coincidence problem in the spinor dark energy models by using the dynamical
system method. Our results show that the cosmological coincidence problem
should be taken to heart in the investigations of spinor dark energy models.Comment: 9 pages, revtex4; v2: major revision, title changed, Phys. Lett. B in
press; v3: published versio
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
Dwarf elliptical galaxies in Centaurus A group: stellar populations in AM 1339-445 and AM 1343-452
We study the red giant populations of two dE galaxies, AM 1339-445 and AM
1343-452, with the aim of investigating the number and luminosity of any upper
asymptotic giant branch (AGB) stars present. The galaxies are members of the
Centaurus A group (D~3.8 Mpc) and are classified as outlying (R~350 kpc)
satellites of Cen A. The analysis is based on near-IR photometry for individual
red giant stars, derived from images obtained with ISAAC on the VLT. The
photometry, along with optical data derived from WFPC2 images retrieved from
the HST science archive, enable us to investigate the stellar populations of
the dEs in the vicinity of the red giant branch (RGB) tip. In both systems we
find stars above the RGB tip, which we interpret as intermediate-age upper-AGB
stars. The presence of such stars is indicative of extended star formation in
these dEs similar to that seen in many, but not all, dEs in the Local Group.
For AM 1339-445, the brightest of the upper-AGB stars have Mbol~-4.5 while
those in AM 1343-452 have Mbol~-4.8 mag. These luminosities suggest ages of
approximately 6.5+/-1 and 4+/-1 Gyr as estimates for the epoch of the last
episode of significant star formation in these systems. In both cases the
number of upper-AGB stars suggests that ~15% of the total stellar population is
in the form of intermediate-age stars, considerably less than is the case for
outlying dE satellites of the Milky Way such as Fornax and LeoI.Comment: 18 pages, 19 figures, A&A accepted; high resolution version available
from: http://www.eso.org/~mrejkuba/CenA_dEs_I.pd
Ancient stars beyond the Local Group: RR Lyrae variables and Blue Horizontal Branch stars in Sculptor Group Dwarf Galaxies
We have used Hubble Space Telescope ACS images to generate color-magnitude
diagrams that reach below the magnitude of the horizontal branch in the
Sculptor Group dwarf galaxies ESO294-010 and ESO410-005. In both diagrams blue
horizontal branch stars are unambiguously present, a signature of the existence
of an ancient stellar population whose age is comparable to that of the
Galactic halo globular clusters. The result is reinforced by the discovery of
numerous RR Lyrae variables in both galaxies. The occurrence of these stars in
the first direct confirmation of the existence of ancient stellar populations
beyond the Local Group and indicates that star formation can occur at the
earliest epochs even in low density environments.Comment: 11 pages, 2 figures, accepted for publication in Astrophysical
Journal Letter
Spectroscopic versus Photometric Metallicities: Milky Way Dwarf Spheroidal Companions as a Test Case
Aims. The method of deriving photometric metallicities using red giant branch
stars is applied to resolved stellar populations under the common assumption
that they mainly consist of single-age old stellar populations. We explore the
effect of the presence of mixed-age stellar populations on deriving photometric
metallicities. Methods. We use photometric data sets for the five Galactic
dwarf spheroidals Sculptor, Sextans, Carina, Fornax, and Leo II in order to
derive their photometric metallicity distribution functions from their resolved
red giant branches using isochrones of the Dartmouth Stellar Evolutionary
Database. We compare the photometric metallicities with published spectroscopic
metallicities based on the analysis of the near-infrared Ca triplet (Ca T),
both on the metallicity scale of Carretta & Gratton and on the scale defined by
the Dartmouth isochrones. In addition, we compare the photometric metallicities
with published spectroscopic metallicities based on spectral synthesis and
medium-resolution spectroscopy, and on high resolution spectra where available.
Results. The mean properties of the spectroscopic and photometric metallicity
samples are comparable within the intrinsic scatter of each method although the
mean metallicities of dSphs with pronounced intermediate-age population
fractions may be underestimated by the photometric method by up to a few tenths
of dex in [Fe/H]. The star-by-star differences of the spectroscopic minus the
photometric metallicities show a wide range of values along the fiducial
spectroscopic metallicity range, with the tendency to have systematically lower
photometric metallicities for those dwarf spheroidals with a higher fraction of
intermediate-age populations. Such discrepancies persist even in the case of
the purely old Sculptor dSph, where one would na\"ively expect a very good
match when comparing with medium or low resolution metallicity measurements.
Overall, the agreement between Ca T metallicities and photometric metallicities
is very good in the metallicity range from ~ -2 dex to ~ -1.5 dex. We find that
the photometric method is reliable in galaxies that contain small (less than
15%) intermediate-age stellar fractions. Therefore, in the presence of
mixed-age stellar populations, one needs to quantify the fraction of the
intermediate-age stars in order to assess their effect on determining
metallicities from photometry alone. Finally, we note that the comparison of
spectroscopic metallicities of the same stars obtained with different methods
reveals similarly large discrepancies as the comparison with photometric
metallicities.Comment: 17 pages, 12 figures; A&A accepte
Preliminary Limits on the WIMP-Nucleon Cross Section from the Cryogenic Dark Matter Search (CDMS)
We are conducting an experiment to search for WIMPs, or weakly-interacting
massive particles, in the galactic halo using terrestrial detectors. This
generic class of hypothetical particles, whose properties are similar to those
predicted by extensions of the standard model of particle physics, could
comprise the cold component of non-baryonic dark matter. We describe our
experiment, which is based on cooled germanium and silicon detectors in a
shielded low-background cryostat. The detectors achieve a high degree of
background rejection through the simultaneous measurement of the energy in
phonons and ionization. Using exposures on the order of one kilogram-day from
initial runs of our experiment, we have achieved (preliminary) upper limits on
the WIMP-nucleon cross section that are comparable to much longer runs of other
experiments.Comment: 5 LaTex pages, 5 eps figs, epsf.sty, espcrc2dsa2.sty. Proceedings of
TAUP97, Gran Sasso, Italy, 7-11 Sep 1997, Nucl. Phys. Suppl., A. Bottino, A.
di Credico and P. Monacelli (eds.). See also http://cfpa.berkeley.ed
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