1,674 research outputs found
Tracing early structure formation with massive starburst galaxies and their implications for reionization
Cosmological hydrodynamic simulations have significantly improved over the
past several years, and we have already shown that the observed properties of
Lyman-break galaxies (LBGs) at z=3 can be explained well by the massive
galaxies in the simulations. Here we extend our study to z=6 and show that we
obtain good agreement for the LBGs at the bright-end of the luminosity function
(LF). Our simulations also suggest that the cosmic star formation rate density
has a peak at z= 5-6, and that the current LBG surveys at z=6 are missing a
significant number of faint galaxies that are dimmer than the current magnitude
limit. Together, our results suggest that the universe could be reionized at
z=6 by the Pop II stars in ordinary galaxies. We also estimate the LF of
Lyman-alpha emitters (LAEs) at z=6 by relating the star formation rate in the
simulation to the Ly-alpha luminosity. We find that the simulated LAE LFs agree
with the observed data provided that the net escape fraction of Ly-alpha photon
is f_{Ly-alpha} <= 0.1. We investigate two possible scenarios for this effect:
(1) all sources in the simulation are uniformly dimmer by a factor of 10
through attenuation, and (2) one out of ten LAEs randomly lights up at a given
moment. We show that the correlation strength of the LAE spatial distribution
can possibly distinguish the two scenarios.Comment: 9 pages, 4 figures. Summary of the talk given at the "First Light &
Reionization" workshop at UC Irvine, May 2005. The published article is
available from http://dx.doi.org/10.1016/j.newar.2005.11.00
Luminosity Distribution of Gamma-Ray Burst Host Galaxies at redshift z=1 in Cosmological Smoothed Particle Hydrodynamic Simulations: Implications for the Metallicity Dependence of GRBs
We study the relationship between the metallicity of gamma-ray burst (GRB)
progenitors and the probability distribution function (PDF) of GRB host
galaxies as a function of luminosity using cosmological hydrodynamic
simulations of galaxy formation. We impose a maximum limit to the gas
metallicity in which GRBs can occur, and examine how the predicted luminosity
PDF of GRB host galaxies changes in the simulation. We perform the
Kolmogorov-Smirnov test, and show that the result from our simulation agrees
with the observed luminosity PDF of core-collapse supernovae (SNe) host
galaxies when we assume that the core-collapse SNe trace star formation. When
we assume that GRBs occur only in a low-metallicity environment with Z\lesssim
0.1 \Zsun, GRBs occur in lower luminosity galaxies, and the simulated
luminosity PDF becomes quantitatively consistent with the observed luminosity
PDF. The observational bias against the host galaxies of optically dark GRBs
owing to dust extinction may be another reason for the lower luminosities of
GRB host galaxies, but the observed luminosity PDF of GRB host galaxies cannot
be reproduced solely by the dust bias in our simulation.Comment: 11 pages, 14 figures, minor revisions, one added figure, accepted for
publication in Ap
Hyperfine Populations Prior to Muon Capture
It is shown that the 1S level hyperfine populations prior to muon capture
will be statistical when either target or beam are unpolarised independent of
the atomic level at which the hyperfine interaction becomes appreciable. This
assertion holds in the absence of magnetic transitions during the cascade and
is true because of minimal polarisation after atomic capture and selective
feeding during the cascade.Comment: (revtex, 6 preprint pages, no figures
Evolution of the Luminosity Function and Colors of Galaxies in a Lambda-CDM Universe
The luminosity function of galaxies is derived from a cosmological
hydrodynamic simulation of a Lambda cold dark matter (CDM) universe with the
aid of a stellar population synthesis model. At z=0, the resulting B band
luminosity function has a flat faint end slope of \alpha \approx -1.15 with the
characteristic luminosity and the normalization in a fair agreement with
observations, while the dark matter halo mass function is steep with a slope of
\alpha \approx -2. The colour distribution of galaxies also agrees well with
local observations. We also discuss the evolution of the luminosity function,
and the colour distribution of galaxies from z=0 to 5. A large evolution of the
characteristic mass in the stellar mass function due to number evolution is
compensated by luminosity evolution; the characteristic luminosity increases
only by 0.8 mag from z=0 to 2, and then declines towards higher redshift, while
the B band luminosity density continues to increase from z=0 to 5 (but only
slowly at z>3).Comment: 6 pages, including 4 figures, mn2e style. Accepted to MNRAS pink
page
Astrophysics: Most distant cosmic blast seen
The most distant -ray burst yet sighted is the earliest astronomical object
ever observed in cosmic history. This ancient beacon offers a glimpse of the
little-known cosmic dark ages.Comment: Published in Nature News & View
The use of cosmic muons in detecting heterogeneities in large volumes
The muon intensity attenuation method to detect heterogeneities in large
matter volumes is analyzed. Approximate analytical expressions to estimate the
collection time and the signal to noise ratio, are proposed and validated by
Monte Carlo simulations. Important parameters, including point spread function
and coordinate reconstruction uncertainty are also estimated using Monte Carlo
simulations.Comment: 8 pages, 11 figures, submetted to NIM
The K20 survey. VI. The Distribution of the Stellar Masses in Galaxies up to z~2
We present a detailed analysis of the stellar mass content of galaxies up to
z=2.5 in the K20 galaxy sample, that has a 92% spectroscopic completeness and a
complete multicolor coverage. We find that the M/L ratio decreases
with redshift: in particular, the average M/L ratio of early type galaxies
decreases with , with a scatter that is indicative of a range of
star--formation time-scales and redshift of formation. More important, the
typical M/L of massive early type galaxies is larger than that of less massive
ones, suggesting that their stellar population formed at higher z. The final
K20 galaxy sample spans a range of stellar masses from M*=10^9Msun to
M*=10^12Msun, with massive galaxies ($M*>10^11Msun) detected up to z~2. We
compute the Galaxy Stellar Mass Function at various z, of which we observe only
a mild evolution (i.e. by 20-30%) up to z~1. At z>1, the evolution of the GSMF
appears to be much faster: at z~2, about 35% of the present day stellar mass in
objects with M*~10^11Msun appear to have assembled. We also detect a change in
the physical nature of the most massive galaxies, since at z>1 a population of
massive star--forming galaxies progressively appears. We finally analyze our
results in the framework of Lambda-CDM hierarchical models. First, we show that
the large number of massive galaxies detected at high z does not violate any
fundamental Lambda-CDM constraint based on the number of massive DM halos.
Then, we compare our results with the predictions of renditions of both
semianalytic and hydro-dynamical models, that range from severe underestimates
to slight overestimates of the observed mass density at z<~2. We discuss how
the differences among these models are due to the different implementation of
the main physical processes. (Abridged)Comment: Accepted for publication on Astronomy & Astrophysic
A new measurement of the evolving near-infrared galaxy luminosity function out to z~4: a continuing challenge to theoretical models of galaxy formation
We present the most accurate measurement to date of cosmological evolution of
the near-infrared galaxy luminosity function, from the local Universe out to
z~4. The analysis is based on a large and highly complete sample of galaxies
selected from the first data release of the UKIDSS Ultra Deep Survey.
Exploiting a master catalogue of K- and z-band selected galaxies over an area
of 0.7 square degrees, we analyse a sample of ~50,000 galaxies, all with
reliable photometry in 16-bands from the far-ultraviolet to the mid-infrared.
The unique combination of large area and depth provided by the Ultra Deep
Survey allows us to trace the evolution of the K-band luminosity function with
unprecedented accuracy. In particular, via a maximum likelihood analysis we
obtain a simple parameterization for the luminosity function and its
cosmological evolution, including both luminosity and density evolution, which
provides an excellent description of the data from z =0 up to z~4. We find
differential evolution for galaxies dependent on galaxy luminosity, revealing
once again the ``down-sizing behaviour'' of galaxy formation. Finally, we
compare our results with the predictions of the latest theoretical models of
galaxy formation, based both on semi-analytical prescriptions, and on full
hydrodynamical simulations.Comment: 11 pages, 6 figures, submitted to MNRA
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