891 research outputs found
Evidence for a Positive Cosmological Constant from Flows of Galaxies and Distant Supernovae
Recent observations of high-redshift supernovae seem to suggest that the
global geometry of the Universe may be affected by a `cosmological constant',
which acts to accelerate the expansion rate with time. But these data by
themselves still permit an open universe of low mass density and no
cosmological constant. Here we derive an independent constraint on the lower
bound to the mass density, based on deviations of galaxy velocities from a
smooth universal expansion. This constraint rules out a low-density open
universe with a vanishing cosmological constant, and together the two favour a
nearly flat universe in which the contributions from mass density and the
cosmological constant are comparable. This type of universe, however, seems to
require a degree of fine tuning of the initial conditions that is in apparent
conflict with `common wisdom'.Comment: 8 pages, 1 figure. Slightly revised version. Letter to Natur
Non-linear Stochastic Galaxy Biasing in Cosmological Simulations
We study the biasing relation between dark-matter halos or galaxies and the
underlying mass distribution, using cosmological -body simulations in which
galaxies are modelled via semi-analytic recipes. The nonlinear, stochastic
biasing is quantified in terms of the mean biasing function and the scatter
about it as a function of time, scale and object properties. The biasing of
galaxies and halos shows a general similarity and a characteristic shape, with
no galaxies in deep voids and a steep slope in moderately underdense regions.
At \sim 8\hmpc, the nonlinearity is typically \lsim 10 percent and the
stochasticity is a few tens of percent, corresponding to percent
variations in the cosmological parameter . Biasing
depends weakly on halo mass, galaxy luminosity, and scale. The time evolution
is rapid, with the mean biasing larger by a factor of a few at
compared to , and with a minimum for the nonlinearity and stochasticity at
an intermediate redshift. Biasing today is a weak function of the cosmological
model, reflecting the weak dependence on the power-spectrum shape, but the time
evolution is more cosmology-dependent, relecting the effect of the growth rate.
We provide predictions for the relative biasing of galaxies of different type
and color, to be compared with upcoming large redshift surveys. Analytic models
in which the number of objects is conserved underestimate the evolution of
biasing, while models that explicitly account for merging provide a good
description of the biasing of halos and its evolution, suggesting that merging
is a crucial element in the evolution of biasing.Comment: 27 pages, 21 figures, submitted to MNRA
Dependence of the Inner DM Profile on the Halo Mass
I compare the density profile of dark matter (DM) halos in cold dark matter
(CDM) N-body simulations with 1 Mpc, 32 Mpc, 256 Mpc and 1024 Mpc box sizes. In
dimensionless units the simulations differ only for the initial power spectrum
of density perturbations. I compare the profiles when the most massive halos
are composed of about 10^5 DM particles. The DM density profiles of the halos
in the 1 Mpc box show systematically shallower cores with respect to the
corresponding halos in the 32 Mpc simulation that have masses, M_{dm}, typical
of the Milky Way and are fitted by a NFW profile. The DM density profiles of
the halos in the 256 Mpc box are consistent with having steeper cores than the
corresponding halos in the 32 Mpc simulation, but higher mass resolution
simulations are needed to strengthen this result. Combined, these results
indicate that the density profile of DM halos is not universal, presenting
shallower cores in dwarf galaxies and steeper cores in clusters. Physically the
result sustains the hypothesis that the mass function of the accreting
satellites determines the inner slope of the DM profile. In comoving
coordinates, r, the profile \rho_{dm} \propto 1/(X^\alpha(1+X)^{3-\alpha}),
with X=c_\Delta r/r_\Delta, r_\Delta is the virial radius and \alpha
=\alpha(M_{dm}), provides a good fit to all the DM halos from dwarf galaxies to
clusters at any redshift with the same concentration parameter c_\Delta ~ 7.
The slope, \gamma, of the outer parts of the halo appears to depend on the
acceleration of the universe: when the scale parameter is a=(1+z)^{-1} < 1, the
slope is \gamma ~ 3 as in the NFW profile, but \gamma ~ 4 at a > 1 when
\Omega_\Lambda ~ 1 and the universe is inflating.[abridged]Comment: Accepted for publication in MNRAS. 13 pages, including 11 figures and
2 tables. The revised version has an additional discussion section and work
on the velocity dispersion anisotrop
Galactic Wind Signatures around High Redshift Galaxies
We carry out cosmological chemodynamical simulations with different strengths
of supernova (SN) feedback and study how galactic winds from star-forming
galaxies affect the features of hydrogen (HI) and metal (CIV and OVI)
absorption systems in the intergalactic medium at high redshift. We find that
the outflows tend to escape to low density regions, and hardly affect the dense
filaments visible in HI absorption. As a result, the strength of HI absorption
near galaxies is not reduced by galactic winds, but even slightly increases. We
also find that a lack of HI absorption for lines of sight (LOS) close to
galaxies, as found by Adelberger et al., can be created by hot gas around the
galaxies induced by accretion shock heating. In contrast to HI, metal
absorption systems are sensitive to the presence of winds. The models without
feedback can produce the strong CIV and OVI absorption lines in LOS within 50
kpc from galaxies, while strong SN feedback is capable of creating strong CIV
and OVI lines out to about twice that distance. We also analyze the mean
transmissivity of HI, CIV, and OVI within 1 h Mpc from star-forming
galaxies. The probability distribution of the transmissivity of HI is
independent of the strength of SN feedback, but strong feedback produces LOS
with lower transmissivity of metal lines. Additionally, strong feedback can
produce strong OVI lines even in cases where HI absorption is weak. We conclude
that OVI is probably the best tracer for galactic winds at high redshift.Comment: 16 pages, 16 figures, ApJ in press. Higher resolution version
available at http://www.ociw.edu/~dkawata/research/papers.htm
Luminescence from highly excited nanorings: Luttinger liquid description
We study theoretically the luminescence from quantum dots of a ring geometry.
For high excitation intensities, photoexcited electrons and holes form Fermi
seas. Close to the emission threshold, the single-particle spectral lines
aquire weak many-body satellites. However, away from the threshold, the
discrete luminescence spectrum is completely dominated by many-body
transitions. We employ the Luttinger liquid approach to exactly calculate the
intensities of all many-body spectral lines. We find that the transition from
single-particle to many-body structure of the emission spectrum is governed by
a single parameter and that the distribution of peaks away from the threshold
is universal.Comment: 10 pages including 2 figure
CGM properties in VELA and NIHAO simulations; the OVI ionization mechanism: dependence on redshift, halo mass and radius
We study the components of cool and warm/hot gas in the circumgalactic medium
(CGM) of simulated galaxies and address the relative production of OVI by
photoionization versus collisional ionization, as a function of halo mass,
redshift, and distance from the galaxy halo center. This is done utilizing two
different suites of zoom-in hydro-cosmological simulations, VELA (6 halos;
) and NIHAO (18 halos; to ), which provide a broad theoretical basis
because they use different codes and physical recipes for star formation and
feedback. In all halos studied in this work, we find that collisional
ionization by thermal electrons dominates at high redshift, while
photoionization of cool or warm gas by the metagalactic radiation takes over
near . In halos of and above, collisions become
important again at , while photoionization remains significant down to
for less massive halos. In halos with , at most of the photoionized OVI is in a
warm, not cool, gas phase (~K). We also find that
collisions are dominant in the central regions of halos, while photoionization
is more significant at the outskirts, around , even in massive
halos. This too may be explained by the presence of warm gas or, in lower mass
halos, by cool gas inflows
Constrained semi-analytical models of Galactic outflows
We present semi-analytic models of galactic outflows, constrained by
available observations on high redshift star formation and reionization.
Galactic outflows are modeled in a manner akin to models of stellar wind blown
bubbles. Large scale outflows can generically escape from low mass halos
(M<10^9 M_sun) for a wide range of model parameters but not from high mass
halos (M> 10^{11} M_sun). The gas phase metallicity of the outflow and within
the galaxy are computed. Ionization states of different metal species are
calculated and used to examine the detectability of metal lines from the
outflows. The global influence of galactic outflows is also investigated.
Models with only atomic cooled halos significantly fill the IGM at z~3 with
metals (with -2.5>[Z/Z_sun]>-3.7), the actual extent depending on the
efficiency of winds, the IMF, the fractional mass that goes through star
formation and the reionization history of the universe. In these models, a
large fraction of outflows at z~3 are supersonic, hot (T> 10^5 K) and have low
density, making metal lines difficult to detect. They may also result in
significant perturbations in the IGM gas on scales probed by the Lyman-alpha
forest. On the contrary, models including molecular cooled halos with a normal
mode of star formation can potentially volume fill the universe at z> 8 without
drastic dynamic effects on the IGM, thereby setting up a possible metallicity
floor (-4.0<[Z/Z_sun]<-3.6). Interestingly, molecular cooled halos with a
``top-heavy'' mode of star formation are not very successful in establishing
the metallicity floor because of the additional radiative feedback, that they
induce. (Abridged)Comment: 27 pages, 31 figures, 2 tables, pdflatex. Accepted for publication in
MNRA
Sequential pivotal mechanisms for public project problems
It is well-known that for several natural decision problems no budget
balanced Groves mechanisms exist. This has motivated recent research on
designing variants of feasible Groves mechanisms (termed as `redistribution of
VCG (Vickrey-Clarke-Groves) payments') that generate reduced deficit. With this
in mind, we study sequential mechanisms and consider optimal strategies that
could reduce the deficit resulting under the simultaneous mechanism. We show
that such strategies exist for the sequential pivotal mechanism of the
well-known public project problem. We also exhibit an optimal strategy with the
property that a maximal social welfare is generated when each player follows
it. Finally, we show that these strategies can be achieved by an implementation
in Nash equilibrium.Comment: 19 pages. The version without the appendix will appear in the Proc.
2nd International Symposium on Algorithmic Game Theory, 200
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