1,262 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
Analysis of linearized inverse problems in ultrasound transmission imaging
The purpose of this paper is to analyze the linearized inverse problem during the iterativesolution process of the ill-posed nonlinear inverse problem of image reconstruction for ultra-sound transmission imaging. We show that the conjugate gradient applied to normal equation(CGNE) method gives more reliable solutions for linearized systems than Tikhonov regular-ization methods. The linearized systems are more sensitive when treated by CGNE than byTikhonov regularization methods. The Tikhonov regularization is less effective at the be-ginning of the outer-loop iteration, where the nonlinearity is dominating while the conjugategradient for the linearized system stops earlier. Only when the linear approximation is goodenough to describe the whole system, Tikhonov regularization can fully play its role and giveslightly better reconstruction results as compared to CGNE in a very noisy case
The star formation rate history in the FORS Deep and GOODS South Fields
We measure the star formation rate (SFR) as a function of redshift z up to z
\~4.5, based on B, I and (I+B) selected galaxy catalogues from the FORS Deep
Field (FDF) and the K-selected catalogue from the GOODS-South field. Distances
are computed from spectroscopically calibrated photometric redshifts accurate
to (Delta_z / (z_spec+1)) ~0.03 for the FDF and ~0.056 for the GOODS-South
field. The SFRs are derived from the luminosities at 1500 Angstroem. We find
that the total SFR estimates derived from B, I and I+B catalogues agree very
well (\lsim 0.1 dex) while the SFR from the K catalogue is lower by ~0.2 dex.
We show that the latter is solely due to the lower star-forming activity of
K-selected intermediate and low luminosity (L<L_*) galaxies. The SFR of bright
(L>L_*) galaxies is independent of the selection band, i.e. the same for B, I,
(I+B), and K-selected galaxy samples. At all redshifts, luminous galaxies
(L>L_*) contribute only ~1/3 to the total SFR. There is no evidence for
significant cosmic variance between the SFRs in the FDF and GOODs-South field,
~0.1 dex, consistent with theoretical expectations. The SFRs derived here are
in excellent agreement with previous measurements provided we assume the same
faint-end slope of the luminosity function as previous works (alpha ~ -1.6).
However, our deep FDF data indicate a shallower slope of alpha=-1.07, implying
a SFR lower by ~0.3 dex. We find the SFR to be roughly constant up to z ~4 and
then to decline slowly beyond, if dust extinctions are assumed to be constant
with redshift.Comment: 6 pages, 2 figures, Accepted for publication in ApJ
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
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
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
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&
Galaxy populations in the Antlia cluster. I. Photometric properties of early-type galaxies
We present the first colour-magnitude relation (CMR) of early-type galaxies
in the central region of the Antlia cluster, obtained from CCD wide-field
photometry in the Washington photometric system. Integrated (C -T1) colours, T1
magnitudes, and effective radii have been measured for 93 galaxies (i.e. the
largest galaxies sample in the Washington system till now) from the FS90
catalogue (Ferguson & Sandage 1990). Membership of 37 objects can be confirmed
through new radial velocities and data collected from the literature. The
resulting colour-magnitude diagram shows that early-type FS90 galaxies that are
spectroscopically confirmed Antlia members or that were considered as definite
members by FS90, follow a well defined CMR (sigma_(C -T1) ~ 0.07 mag) that
spans 9 magnitudes in brightness with no apparent change of slope. This
relation is very tight for the whole magnitude range but S0 galaxies show a
larger dispersion, apparently due to a separation of ellipticals and S0s.
Antlia displays a slope of -13.6 in a T1 vs. (C -T1) diagram, in agreement with
results for clusters like Fornax, Virgo, Perseus and Coma, which are
dynamically different to Antlia. This fact might indicate that the build up of
the CMR in cluster of galaxies is more related to galaxies internal processes
than to the influence of the environment. Interpreting the CMR as a
luminosity-metallicity relation of old stellar systems, the metallicities of
the Antlia galaxies define a global relation down to Mv ~ -13. We also find,
for early-type dwarfs, no clear relation between luminosity and effective
radius, indicating a nearly constant mean effective radius of ~ 1 kpc. This
value is also found in several samples of dwarf galaxies in Virgo and Coma.Comment: 13 pages, 6 figures. Accepted for publication in MNRA
- âŠ