1,738 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
Direct Collapse to Supermassive Black Hole Seeds with Radiation Transfer: Cosmological Halos
We have modeled direct collapse of a primordial gas within dark matter halos
in the presence of radiative transfer, in high-resolution zoom-in simulations
in a cosmological framework, down to the formation of the photosphere and the
central object. Radiative transfer has been implemented in the flux-limited
diffusion (FLD) approximation. Adiabatic models were run for comparison. We
find that (a) the FLD flow forms an irregular central structure and does not
exhibit fragmentation, contrary to adiabatic flow which forms a thick disk,
driving a pair of spiral shocks, subject to Kelvin-Helmholtz shear instability
forming fragments; (b) the growing central core in the FLD flow quickly reaches
~10 Mo and a highly variable luminosity of 10^{38}-10^{39} erg/s, comparable to
the Eddington luminosity. It experiences massive recurrent outflows driven by
radiation force and thermal pressure gradients, which mix with the accretion
flow and transfer the angular momentum outwards; and (c) the interplay between
these processes and a massive accretion, results in photosphere at ~10 AU. We
conclude that in the FLD model (1) the central object exhibits dynamically
insignificant rotation and slower than adiabatic temperature rise with density;
(2) does not experience fragmentation leading to star formation, thus promoting
the fast track formation of a supermassive black hole (SMBH) seed; (3)
inclusion of radiation force leads to outflows, resulting in the mass
accumulation within the central 10^{-3} pc, which is ~100 times larger than
characteristic scale of star formation. The inclusion of radiative transfer
reveals complex early stages of formation and growth of the central structure
in the direct collapse scenario of SMBH seed formation.Comment: 19 pages, 16 figures, MNRAS, accepted for publicatio
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
Scalability of spin FPGA: A Reconfigurable Architecture based on spin MOSFET
Scalability of Field Programmable Gate Array (FPGA) using spin MOSFET (spin
FPGA) with magnetocurrent (MC) ratio in the range of 100% to 1000% is discussed
for the first time. Area and speed of million-gate spin FPGA are numerically
benchmarked with CMOS FPGA for 22nm, 32nm and 45nm technologies including 20%
transistor size variation. We show that area is reduced and speed is increased
in spin FPGA owing to the nonvolatile memory function of spin MOSFET.Comment: 3 pages, 7 figure
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