1,885 research outputs found
The Lyman Break Galaxies: their Progenitors and Descendants
We study the evolution of Lyman Break Galaxies (LBGs) from z=5 to z=0 by
tracing the merger trees of galaxies in a large-scale hydrodynamic simulation
based on a Lambda cold dark matter model. In particular, we emphasize on the
range of properties of the sample selected by the rest-frame V band luminosity,
in accordance with recent near-IR observations. The predicted rest-frame V band
luminosity function agrees well with the observed one when dust extinction is
taken into account. The stellar content and the star formation histories of
LBGs are also studied. We find that the LBGs intrinsically brighter than
Mv=-21.0 at z=3 have stellar masses of at least 10^9\Msun, with a median of
10^{10}h^{-1}\Msun. The brightest LBGs (Mv<-23) at z=3 merge into
clusters/groups of galaxies at z=0, as suggested from clustering studies of
LBGs. Roughly one half of the galaxies with -23<Mv<-22 at z=3 fall into
groups/clusters, and the other half become typical L* galaxies at z=0 with
stellar mass of ~10^{11}\Msun. Descendants of LBGs at the present epoch have
formed roughly 30% of their stellar mass by z=3, and the half of their current
stellar population is 10 Gyr old, favoring the scenario that LBGs are the
precursors of the present day spheroids. We find that the most luminous LBGs
have experienced a starburst within 500 Myr prior to z=3, but also have formed
stars continuously over a period of 1 Gyr prior to z=3 when all the star
formation in progenitors is coadded. We also study the evolution of the mean
stellar metallicity distribution of galaxies, and find that the entire
distribution shifts to lower metallicity at higher redshift. The observed
sub-solar metallicity of LBGs at z=3 is naturally predicted in our simulation.Comment: 29 pages, including 11 figures, ApJ in press. One reference adde
On the Virialization of Disk Winds: Implications for the Black Hole Mass Estimates in AGN
Estimating the mass of a supermassive black hole (SMBH) in an active galactic
nucleus (AGN) usually relies on the assumption that the broad line region (BLR)
is virialized. However, this assumption seems invalid in BLR models that
consists of an accretion disk and its wind. The disk is likely Keplerian and
therefore virialized. However, the wind material must, beyond a certain point,
be dominated by an outward force that is stronger than gravity. Here, we
analyze hydrodynamic simulations of four different disk winds: an isothermal
wind, a thermal wind from an X-ray heated disk, and two line-driven winds, one
with and the other without X-ray heating and cooling. For each model, we check
whether gravity governs the flow properties, by computing and analyzing the
volume-integrated quantities that appear in the virial theorem: internal,
kinetic, and gravitational energies, We find that in the first two models, the
winds are non-virialized whereas the two line-driven disk winds are virialized
up to a relatively large distance. The line-driven winds are virialized because
they accelerate slowly so that the rotational velocity is dominant and the wind
base is very dense. For the two virialized winds, the so-called projected
virial factor scales with inclination angle as . Finally, we
demonstrate that an outflow from a Keplerian disk becomes unvirialized more
slowly when it conserves the gas specific angular momentum -- as in the models
considered here, than when it conserves the angular velocity -- as in the
so-called magneto-centrifugal winds.Comment: Accepted to Ap
Muons tomography applied to geosciences and volcanology
Imaging the inner part of large geological targets is an important issue in
geosciences with various applications. Dif- ferent approaches already exist
(e.g. gravimetry, electrical tomography) that give access to a wide range of
informations but with identified limitations or drawbacks (e.g. intrinsic
ambiguity of the inverse problem, time consuming deployment of sensors over
large distances). Here we present an alternative and complementary tomography
method based on the measurement of the cosmic muons flux attenuation through
the geological structures. We detail the basics of this muon tomography with a
special emphasis on the photo-active detectors.Comment: Invited talk at the 6th conference on New Developments In
Photodetection (NDIP'11), Lyon-France, July 4-8, 2011; Nuclear Instruments
and Methods in Physics Research Section A, 201
Cosmic Mach Number as a Function of Overdensity and Galaxy Age
We carry out an extensive study of the cosmic Mach number (\mach) on scales
of R=5, 10 and 20h^-1Mpc using an LCDM hydrodynamical simulation. We
particularly put emphasis on the environmental dependence of \mach on
overdensity, galaxy mass, and galaxy age. We start by discussing the difference
in the resulting \mach according to different definitions of \mach and
different methods of calculation. The simulated Mach numbers are slightly lower
than the linear theory predictions even when a non-linear power spectrum was
used in the calculation, reflecting the non-linear evolution in the simulation.
We find that the observed \mach is higher than the simulated mean by more than
2-standard deviations, which suggests either that the Local Group is in a
relatively low-density region or that the true value of \Omega_m is ~ 0.2,
significantly lower than the simulated value of 0.37. We show from our
simulation that the Mach number is a weakly decreasing function of overdensity.
We also investigate the correlations between galaxy age, overdensity and \mach
for two different samples of galaxies --- DWARFs and GIANTs. Older systems
cluster in higher density regions with lower \mach, while younger ones tend to
reside in lower density regions with larger \mach, as expected from the
hierarchical structure formation scenario. However, for DWARFs, the correlation
is weakened by the fact that some of the oldest DWARFs are left over in
low-density regions during the structure formation history. For giant systems,
one expects blue-selected samples to have higher \mach than red-selected ones.
We briefly comment on the effect of the warm dark matter on the expected Mach
number.Comment: 43 pages, including 15 figures. Accepted version in ApJ. Included
correlation function of different samples of galaxies, and the cumulative
number fraction distribution as a fcn. of overdensity. Reorganized figures
and added some reference
Impact of astrophysical effects on the dark matter mass constraint with 21cm intensity mapping
We present an innovative approach to constraining the non-cold dark matter
model using a convolutional neural network (CNN). We perform a suite of
hydrodynamic simulations with varying dark matter particle masses and generate
mock 21cm radio intensity maps to trace the dark matter distribution. Our
proposed method complements the traditional power spectrum analysis. We compare
our CNN classification results with those from the power spectrum of the
differential brightness temperature map of 21cm radiation, and find that the
CNN outperforms the latter. Moreover, we investigate the impact of baryonic
physics on the dark matter model constraint, including star formation,
self-shielding of HI gas, and UV background model. We find that these effects
may introduce some contamination in the dark matter constraint, but they are
insignificant when compared to the realistic system noise of the SKA
instruments.Comment: 17 pages, 12 figure
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
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