1,581 research outputs found
Stellar structures in the outer regions of M33
We present Subaru/Suprime-Cam deep V and I imaging of seven fields in the
outer regions of M33. Our aim is to search for stellar structures corresponding
to extended HI clouds found in a recent 21-cm survey of the galaxy. Three
fields probe a large HI complex to the southeastern (SE) side of the galaxy. An
additional three fields cover the northwestern (NW) side of the galaxy along
the HI warp. A final target field was chosen further north, at a projected
distance of approximately 25 kpc, to study part of the large stellar plume
recently discovered around M33. We analyse the stellar population at R > 10 kpc
by means of V, I colour magnitude diagrams reaching the red clump. Evolved
stellar populations are found in all fields out to 120' (~ 30 kpc), while a
diffuse population of young stars (~ 200 Myr) is detected out to a
galactocentric radius of 15 kpc. The mean metallicity in the southern fields
remains approximately constant at [M/H] = -0.7 beyond the edge of the optical
disc, from 40' out to 80'. Along the northern fields probing the outer \hi
disc, we also find a metallicity of [M/H] = -0.7 between 35' and 70' from the
centre, which decreases to [M/H] = -1.0 at larger angular radii out to 120'. In
the northernmost field, outside the disc extent, the stellar population of the
large stellar feature possibly related to a M33-M31 interaction is on average
more metal-poor ([M/H] = -1.3) and older (> 6 Gyr). An exponential disc with a
large scale-length (~ 7 kpc) fits well the average distribution of stars
detected in both the SE and NW regions from a galactocentric distance of 11 kpc
out to 30 kpc. The stellar distribution at large radii is disturbed and,
although there is no clear correlation between the stellar substructures and
the location of the HI clouds, this gives evidence for tidal interaction or
accretion events.Comment: 13 pages, 13 figures. Accepted for publications in Astronomy and
Astrophysics; minor revisions of the tex
Probing the Cosmic Star Formation History by Brightness Distribution of Gamma-Ray Bursts
Brightness distribution of Gamma-Ray Bursts (GRBs) is studied in detail under
the assumption that GRB rate is related to cosmic star formation rate. The two
populations of the long- and short-duration bursts in the 4B BATSE catalog are
analyzed separately. Taking account of current uncertainties in the
observational estimate of star formation rate (SFR), we have tried various
models of the cosmic star formation history and we find that the SFR evolution
in = 0--1 is strongly constrained by the GRB distribution if the standard
candle approximation is valid. The strong SFR evolution by a factor of
15 from = 0 to 1 inferred from UV observations is too steep to be
consistent with the GRB distribution for any distance scale of GRBs. Some
possibilities to reconcile this discrepancy are discussed, including the
intrinsic luminosity dispersion of GRBs and/or modification of star formation
history estimated by UV observations. We argue that SFR increase factor from
= 0 to 1 may be as low as about 4 if we choose different sets of
cosmological parameters and/or take account of the evolution of metallicity and
dust extinction in the UV data, and this would significantly remedy the
discrepancy.Comment: Accepted version by ApJ. Discussion on luminosity dispersion of GRBs
is newly added. 30 pages including 16 figure
Evolution of the Luminosity Density in the Universe: Implications for the Nonzero Cosmological Constant
We show that evolution of the luminosity density of galaxies in the universe
provides a powerful test for the geometry of the universe. Using reasonable
galaxy evolution models of population synthesis which reproduce the colors of
local galaxies of various morphological types, we have calculated the
luminosity density of galaxies as a function of redshift . Comparison of the
result with recent measurements by the Canada-France Redshift Survey in three
wavebands of 2800{\AA}, 4400{\AA}, and 1 micron at z<1 indicates that the
\Lambda-dominated flat universe with \lambda_0 \sim 0.8 is favored, and the
lower limit on \lambda_0 yields 0.37 (99% C.L.) or 0.53 (95% C.L.) if
\Omega_0+\lambda_0=1. The Einstein-de Sitter universe with (\Omega_0,
\lambda_0)=(1, 0) and the low-density open universe with (0.2, 0) are however
ruled out with 99.86% C.L. and 98.6% C.L., respectively. The confidence levels
quoted apply unless the standard assumptions on galaxy evolution are
drastically violated. We have also calculated a global star formation rate in
the universe to be compared with the observed rate beyond z \sim 2. We find
from this comparison that spiral galaxies are formed from material accretion
over an extended period of a few Gyrs, while elliptical galaxies are formed
from initial star burst at z >~ 5 supplying enough amount of metals and
ionizing photons in the intergalactic medium.Comment: 11 pages including 3 figures, LaTeX, uses AASTeX. To Appear in ApJ
Letter
Galaxy number counts in the Hubble Deep Field as a strong constraint on a hierarchical galaxy formation model
Number counts of galaxies are re-analyzed using a semi-analytic model (SAM)
of galaxy formation based on the hierarchical clustering scenario. We have
determined the astrophysical parameters in the SAM that reproduce observations
of nearby galaxies, and used them to predict the number counts and redshifts of
faint galaxies for three cosmological models for (1) the standard cold dark
matter (CDM) universe, (2) a low-density flat universe with nonzero
cosmological constant, and (3) a low-density open universe with zero
cosmological constant. The novelty of our SAM analysis is the inclusion of
selection effects arising from the cosmological dimming of surface brightness
of high-redshift galaxies, and also from the absorption of visible light by
internal dust and intergalactic \ion{H}{1} clouds. Contrary to previous SAM
analyses which do not take into account such selection effects, we find, from
comparison with observed counts and redshifts of faint galaxies in the Hubble
Deep Field (HDF), that the standard CDM universe is {\it not} preferred, and a
low-density universe either with or without cosmological constant is favorable,
as suggested by other recent studies. Moreover, we find that a simple
prescription for the time scale of star formation (SF), being proportional to
the dynamical time scale of the formation of the galactic disk, is unable to
reproduce the observed number- redshift relation for HDF galaxies, and that the
SF time scale should be nearly independent of redshift, as suggested by other
SAM analyses for the formation of quasars and the evolution of damped
Ly- systems.Comment: 16 pages, 13 figures, LaTeX, using emulateapj5.st
The Tully-Fisher relation of intermediate redshift field and cluster galaxies from Subaru spectroscopy
We have carried out spectroscopic observations in 4 cluster fields using
Subaru's FOCAS multi-slit spectrograph and obtained spectra for 103 bright disk
field and cluster galaxies at . Seventy-seven of these
show emission lines, and 33 provide reasonably-secure determinations of the
galaxies' rotation velocity. The rotation velocities, luminosities, colours and
emission-line properties of these galaxies are used to study the possible
effects of the cluster environment on the star-formation history of the
galaxies. Comparing the Tully-Fisher relations of cluster and field galaxies at
similar reshifts we find no measurable difference in rest-frame -band
luminosity at a given rotation velocity (the formal difference is mag). The colours of the cluster emission line galaxies are only marginally
redder in rest-frame (by mag) than the field galaxies in
our sample. Taken at face value, these results seem to indicate that bright
star-forming cluster spirals are similar to their field counterparts in their
star-formation properties. However, we find that the fraction of disk galaxies
with absorption-line spectra (i.e., with no current star formation) is larger
in clusters than in the field by a factor of --5. This suggests that the
cluster environment has the overall effect of switching off star formation in
(at least) some spiral galaxies. To interpret these observational results, we
carry out simulations of the possible effects of the cluster environment on the
star-formation history of disk galaxies and thus their photometric and
spectroscopic properties. Finally, we evaluate the evolution of the rest-frame
absolute -band magnitude per unit redshift at fixed rotation velocity.Comment: 21 pages, 13 figures, accepted for publication in MNRA
A Molecular Tidal Tail in the Medusa Minor Merger
We have detected CO 1-0 emission along the tidal tail of the NGC 4194 (the
Medusa) merger. It is the first CO detection in the optical tail of a minor
merger. Emission is detected both in the centre of the tail and at its tip. The
molecular mass in the 33'' Onsala 20m beam is estimated to be >= 8.5 x 10^7
M_{sun} which is at least 4% of the total molecular mass measured so far in
this system. We suggest that the emission is a molecular tidal tail which is
part of the extended structure of the main body, and that the molecular gas was
thrown out by the collision instead of having formed in situ from condensing
atomic material. We find it unlikely that the emission is associated with a
tidal dwarf galaxy (even if the future formation of such an object is
possible), but high resolution HI, CO and optical observations are necessary to
resolve the issue. The Medusa is very likely the result of an elliptical+spiral
collison and our detection supports the notion that molecular gas in minor
mergers can be found at great distances from the merger centre.Comment: 4 Pages, 2 figures included, accepted for A&A letter
Galaxy Number Counts in the Subaru Deep Field: Multi-band Analysis in a Hierarchical Galaxy Formation Model
Number counts of galaxies are re-analyzed using a semi-analytic model (SAM)
of galaxy formation based on the hierarchical clustering scenario. Faint
galaxies in the Subaru Deep Field (SDF) and the Hubble Deep Field (HDF) are
compared with our model galaxies. We have determined the astrophysical
parameters in the SAM that reproduce observations of nearby galaxies, and used
them to predict the number counts and redshifts of faint galaxies for three
cosmological models, the standard cold dark matter (CDM) universe, a flat
lambda-CDM, and an open CDM. The novelty of our SAM analysis is the inclusion
of selection effects arising from the cosmological dimming of surface
brightness of high-z galaxies, and from the absorption of visible light by
internal dust and intergalactic HI clouds. As was found in our previous work,
in which the UV/optical HDF galaxies were compared with our model galaxies, we
find that our SAM reproduces counts of near-IR SDF galaxies in low-density
models, and that the standard CDM universe is not preferred, as suggested by
other recent studies. Moreover, we find that simple prescriptions for (1) the
timescale of star formation being proportional to the dynamical time scale of
the formation of galactic disks, (2) the size of galactic disks being
rotationally supported with the same specific angular momentum as that of
surrounding dark halo, and (3) the dust optical depth being proportional to the
metallicity of cold gas, cannot completely explain all of observed data.
Improved prescriptions incorporating mild z-dependence for those are suggested
from our SAM analysis.Comment: 16 pages, 13 figures, to appear in Ap
Ideal barriers to polarization reversal and domain-wall motion in strained ferroelectric thin films
The ideal intrinsic barriers to domain switching in c-phase PbTiO_3 (PTO),
PbZrO_3 (PZO), and PbZr_{1-x}Ti_xO_3 (PZT) are investigated via
first-principles computational methods. The effects of epitaxial strain on the
atomic structure, ferroelectric response, barrier to coherent domain reversal,
domain-wall energy, and barrier to domain-wall translation are studied. It is
found that PTO has a larger polarization, but smaller energy barrier to domain
reversal, than PZO. Consequentially the idealized coercive field is over two
times smaller in PTO than PZO. The Ti--O bond length is more sensitive to
strain than the other bonds in the crystals. This results in the polarization
and domain-wall energy in PTO having greater sensitivity to strain than in PZO.
Two ordered phases of PZT are considered, the rock-salt structure and a (100)
PTO/PZO superlattice. In these simple structures we find that the ferroelectric
properties do not obey Vergard's law, but instead can be approximated as an
average over individual 5-atom unit cells.Comment: 9 pages, 13 figure
Unavoidable Selection Effects in the Analysis of Faint Galaxies in the Hubble Deep Field: Probing the Cosmology and Merger History of Galaxies
(Abridged) We present a detailed analysis of the number count and photometric
redshift distribution of faint galaxies in the Hubble Deep Field (HDF), paying
a special attention to the selection effects including the cosmological dimming
of surface brightness of galaxies. We find a considerably different result from
previous studies ignoring the selection effects, and these effects should
therefore be taken into account in the analysis. We find that the model of pure
luminosity evolution (PLE) of galaxies in the Einstein-de Sitter (EdS) universe
predicts much smaller counts than those observed at faint magnitude limits by a
factor of more than 10, so that a very strong number evolution of galaxies with
\eta > 3-4 must be invoked to reproduce the I_{814} counts, when parametrized
as \phi^* \propto (1+z)^\eta. However we show that such a strong number
evolution under realistic merging processes of galaxies can not explain the
steep slope of the B_{450} and V_{606} counts, and it is seriously inconsistent
with their photometric redshift distribution. We find that these difficulties
still persist in an open universe with \Omega_0 > 0.2, but are resolved only
when we invoke a -dominated flat universe, after examining various
systematic uncertainties in modeling the formation and evolution of galaxies.
The present analysis revitalizes the practice of using faint number counts as
an important cosmological test, giving one of the arguments against the EdS
universe and suggests acceleration of the cosmic expansion by vacuum energy
density. While a modest number evolution of galaxies with \eta ~ 1 is still
necessary even in a Lambda-dominated universe, a stronger number evolution with
\eta > 1 is rejected from the HDF data, giving a strong constraint on the
merger history of galaxies.Comment: 24 pages, 15 figures, final version matching publication in ApJ. Some
references added. The complete ps file of Table 3 is available at
http://th.nao.ac.jp/~totani/images/paper/ty2000-table3.p
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