1,048 research outputs found
Galaxy Clustering and Large-Scale Structure from z = 0.2 to z = 0.5 in Two Norris Redshift Surveys
(abridged) We present a study of the nature and evolution of large-scale
structure based on two independent redshift surveys of faint field galaxies
conducted with the 176-fiber Norris Spectrograph on the Palomar 200-inch
telescope. The two surveys together sparsely cover ~20 sq. degrees and contain
835 r < 21 mag galaxies with redshifts 0.2 < z < 0.5. Both surveys have a
median redshift of z = 0.30. In order to obtain a rough estimate of the cosmic
variance, we analyze the two surveys independently. We measure the comoving
correlation length to be 3.70 +/- 0.13 h^-1 Mpc at z = 0.30 with a power-law
slope gamma = 1.77 +/- 0.05. Dividing the sample into low (0.2 < z < 0.3) and
high (0.32 < z < 0.5) redshift intervals, we see no evidence for a change in
the comoving correlation length over the redshift range 0.2 < z < 0.5. Similar
to the well-established results in the local universe, we find that
intrinsically bright galaxies are more strongly clustered than intrinsically
faint galaxies and that galaxies with little ongoing star formation, as judged
from the rest-frame equivalent width of the [OII]3727, are more strongly
clustered than galaxies with significant ongoing star formation. The rest-frame
pairwise velocity dispersion of the sample is 326^+67_-52 km s^-1, ~25% lower
than typical values measured locally. The appearance of the galaxy
distribution, particularly in the more densely sampled Abell 104 field, is
quite striking. The pattern of sheets and voids which has been observed locally
continues at least to z ~ 0.5. A friends-of-friends analysis of the galaxy
distribution supports the visual impression that > 90% of all galaxies at z <
0.5 are part of larger structures with overdensities of > 5.Comment: 40 pages including 26 Postscript figures; revised version to match
version accepted by Ap
Protostellar Collapse with Various Metallicities
The thermal and chemical evolution of gravitationally collapsing protostellar
clouds is investigated, focusing attention on their dependence on metallicity.
Calculations are carried out for a range of metallicities spanning the local
interstellar value to zero. During the time when clouds are transparent to
continuous radiation, the temperatures are higher for those with lower
metallicity, reflecting lower radiative ability. However, once the clouds
become opaque, in the course of the adiabatic contraction of the transient
cores, their evolutionary trajectories in the density-temperature plane
converge to a unique curve that is determined by only physical constants. The
trajectories coincide with each other thereafter. Consequently, the size of the
stellar core at the formation is the same regardless of the gas composition of
the parent cloud.Comment: 30 pages. The Astrophysical Journal, 533, in pres
The Gemini Deep Deep Survey: II. Metals in Star-Forming Galaxies at Redshift 1.3<z<2
The goal of the Gemini Deep Deep Survey (GDDS) is to study an unbiased sample
of K<20.6 galaxies in the redshift range 0.8<z<2.0. Here we determine the
statistical properties of the heavy element enrichment in the interstellar
medium (ISM) of a subsample of 13 galaxies with 1.34<z<1.97 and UV absolute
magnitude M_2000 < -19.65. The sample contains 38% of the total number of
identified galaxies in the first two fields of the survey with z>1.3. The
selected objects have colors typical of irregular and Sbc galaxies. Strong
[OII] emission indicates high star formation activity in the HII regions
(SFR~13-106 M_sun/yr). The high S/N composite spectrum shows strong ISM MgII
and FeII absorption, together with weak MnII and MgI lines. The FeII column
density, derived using the curve of growth analysis, is logN_FeII =
15.54^{+0.23}_{-0.13}. This is considerably larger than typical values found in
damped Ly-alpha systems (DLAs) along QSO sight lines, where only 10 out of 87
(~11%) have logN_FeII > 15.2. High FeII column densities are observed in the
z=2.72 Lyman break galaxy cB58 (logN_FeII ~ 15.25) and in gamma-ray burst host
galaxies (logN_FeII ~ 14.8-15.9). Given our measured FeII column density and
assuming a moderate iron dust depletion (delta_Fe ~ 1 dex), we derive an
optical dust extinction A_V ~ 0.6. If the HI column density is log N(HI)<21.7
(as in 98% of DLAs), then the mean metallicity is Z/Z_sun > 0.2. The high
completeness of the GDDS sample implies that these results are typical of
star-forming galaxies in the 1<z<2 redshift range, an epoch which has
heretofore been particularly challenging for observational programs.Comment: ApJ in press, corrected HI column density estimat
Real-time Analysis and Selection Biases in the Supernova Legacy Survey
The Supernova Legacy Survey (SNLS) has produced a high-quality, homogeneous
sample of Type Ia supernovae (SNe Ia) out to redshifts greater than z=1. In its
first four years of full operation (to June 2007), the SNLS discovered more
than 3000 transient candidates, 373 of which have been confirmed
spectroscopically as SNe Ia. Use of these SNe Ia in precision cosmology
critically depends on an analysis of the observational biases incurred in the
SNLS survey due to the incomplete sampling of the underlying SN Ia population.
This paper describes our real-time supernova detection and analysis procedures,
and uses detailed Monte Carlo simulations to examine the effects of Malmquist
bias and spectroscopic sampling. Such sampling effects are found to become
apparent at z~0.6, with a significant shift in the average magnitude of the
spectroscopically confirmed SN Ia sample towards brighter values for z>0.75. We
describe our approach to correct for these selection biases in our three-year
SNLS cosmological analysis (SNLS3), and present a breakdown of the systematic
uncertainties involved.Comment: Accepted for publication in A
SiFTO: An Empirical Method for Fitting SNe Ia Light Curves
We present SiFTO, a new empirical method for modeling type Ia supernovae (SNe
Ia) light curves by manipulating a spectral template. We make use of
high-redshift SN observations when training the model, allowing us to extend it
bluer than rest frame U. This increases the utility of our high-redshift SN
observations by allowing us to use more of the available data. We find that
when the shape of the light curve is described using a stretch prescription,
applying the same stretch at all wavelengths is not an adequate description.
SiFTO therefore uses a generalization of stretch which applies different
stretch factors as a function of both the wavelength of the observed filter and
the stretch in the rest-frame B band. We compare SiFTO to other published
light-curve models by applying them to the same set of SN photometry, and
demonstrate that SiFTO and SALT2 perform better than the alternatives when
judged by the scatter around the best fit luminosity distance relationship. We
further demonstrate that when SiFTO and SALT2 are trained on the same data set
the cosmological results agree.Comment: Modified to better match published version in Ap
Room temperature degradation of YBa2Cu3O(7-x) superconductors in varying relative humidity environments
An aging study was performed to determine the stability of YBa2Cu3O(7-x) ceramics in humid environments at 20 C. In this study, fired ceramic specimens were exposed to humidity levels ranging from 30.5 to 100 percent for 2-, 4-, and 6-week time intervals. After storage under these conditions, the specimens were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrical resistance measurements. At every storage condition evaluated, the fired ceramics were found to interact with H2O present in the surrounding environment, resulting in the decomposition of the YBa2Cu3O(7-x) phase. XRD data showed that BaCO3, CuO, and Y2BaCuO5 were present after aging and that the peak intensities of these impurity phases increased both with increasing humidity level and with increasing time of exposure. Additionally, SEM analyses of the ceramic microstructures after aging revealed the development of needle-like crystallites along the surface of the test specimens after aging. Furthermore, the superconducting transition temperature T(sub c) was found to decrease both with increasing humidity level and with increasing time of exposure. All the specimens aged at 30.5, 66, and 81 percent relative humidity exhibited superconducting transitions above 80 K, although these values were reduced by the exposure to the test conditions. Conversely, the specimens stored in direct contact with water (100 percent relative humidity) exhibited no superconducting transitions
Shape and orientation of stellar velocity ellipsoids in spiral galaxies
We present a numerical study of the properties of the stellar velocity
distribution in stellar discs which have developed a saturated, two-armed
spiral structure. We follow the growth of the spiral structure deeply into the
non-linear regime by solving the Boltzmann moment equations up to second order.
By adopting the thin-disc approximation, we restrict our study of the stellar
velocity distribution to the plane of the stellar disc. We find that the outer
(convex) edges of stellar spiral arms are characterized by peculiar properties
of the stellar velocity ellipsoids, which make them distinct from most other
galactic regions. In particular, the ratio \sigma_1:\sigma_2 of the smallest
versus largest principal axes of the stellar velocity ellipsoid can become
abnormally small (as compared to the rest of the disc) near the outer edges of
spiral arms. Moreover, the epicycle approximation fails to reproduce the ratio
of the tangential versus radial velocity dispersions in these regions. These
peculiar properties of the stellar velocity distribution are caused by
large-scale non-circular motions of stars, which in turn are triggered by the
non-axisymmetric gravitational field of stellar spiral arms. The magnitude of
the vertex deviation appears to correlate globally with the amplitude of the
spiral stellar density perturbations. However, locally there is no simple
correlation between the vertex deviation and the density perturbations.
(Abstract abridged).Comment: 16 pages, 18 figures, Accepted for publication in MNRA
On the Mass of Population III Stars
Performing 1D hydrodynamical calculations coupled with non-equilibrium
processes for H2 formation, we pursue the thermal and dynamical evolution of
filamentary primordial clouds and attempt to make an estimate on the mass of
population III stars. It is found that, almost independent of initial
conditions, a filamentary cloud continues to collapse nearly isothermally due
to H_2 cooling until the cloud becomes optically thick against the H_2 lines.
During the collapse the cloud structure separates into two parts, i.e., a
denser spindle and a diffuse envelope. The spindle contracts quasi-statically,
and thus the line mass of the spindle keeps a characteristic value determined
solely by the temperature ( K). Applying a linear theory, we find
that the spindle is unstable against fragmentation during the collapse. The
wavelength of the fastest growing perturbation lessens as the collapse
proceeds. Consequently, successive fragmentation could occur. When the central
density exceeds , the successive fragmentation may
cease since the cloud becomes opaque against the H_2 lines and the collapse
decelerates appreciably. The mass of the first star is then expected to be
typically , which may grow up to by accreting
the diffuse envelope. Thus, the first-generation stars are anticipated to be
massive but not supermassive.Comment: 23 pages, 6 figures, accepted by ApJ (April 10
On the Initial Mass Function of Population III Stars
The collapse and fragmentation of filamentary primordial gas clouds are
explored using 1D and 2D hydrodynamical simulations coupled with the
nonequilibrium processes of H2 formation. The simulations show that depending
upon the initial density,there are two occasions for the fragmentation of
primordial filaments. If a filament has relatively low initial density, the
radial contraction is slow due to less effective H2 cooling. This filament
tends to fragment into dense clumps before the central density reaches
cm, where H2 cooling by three-body reactions is effective and
the fragment mass is more massive than some tens . In contrast, if a
filament is initially dense, the more effective H2 cooling with the help of
three-body reactions allows the filament to contract up to
cm. After the density reaches cm, the filament
becomes optically thick to H2 lines and the radial contraction subsequently
almost stops. At this final hydrostatic stage, the fragment mass is lowered
down to because of the high density of the filament. The
dependence of the fragment mass upon the initial density could be translated
into the dependence on the local amplitude of random Gaussian density fields or
the epoch of the collapse of a parent cloud. Hence, it is predicted that the
initial mass function of Population III stars is likely to be bimodal with
peaks of and , where the relative
heights could be a function of the collapse epoch.Comment: Accepted by Ap
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