1,173 research outputs found
Coulomb effects in semiconductor quantum dots
Coulomb correlations in the optical spectra of semiconductor quantum dots are
investigated using a full-diagonalization approach. The resulting multi-exciton
spectra are discussed in terms of the symmetry of the involved states.
Characteristic features of the spectra like the nearly equidistantly spaced
s-shell emission lines and the approximately constant p-shell transition
energies are explained using simplified Hamiltonians that are derived taking
into account the relative importance of various interaction contributions.
Comparisons with previous results in the literature and their interpretation
are made.Comment: 7 pages, 2 figure
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
Bimodal AGNs in Bimodal Galaxies
By their star content, the galaxies split out into a red and a blue
population; their color index peaked around u-r=2.5 or u-r=1, respectively,
quantifies the ratio of the blue stars newly formed from cold galactic gas, to
the redder ones left over by past generations. On the other hand, upon
accreting substantial gas amounts the central massive black holes energize
active galactic nuclei (AGNs); here we investigate whether these show a
similar, and possibly related, bimodal partition as for current accretion
activity relative to the past. To this aim we use an updated semianalytic
model; based on Monte Carlo simulations, this follows with a large statistics
the galaxy assemblage, the star generations and the black hole accretions in
the cosmological framework over the redshift span from z=10 to z=0. We test our
simulations for yielding in close detail the observed split of galaxies into a
red, early and a blue, late population. We find that the black hole accretion
activities likewise give rise to two source populations: early, bright quasars
and later, dimmer AGNs. We predict for their Eddington parameter --
the ratio of the current to the past black hole accretions -- a bimodal
distribution; the two branches sit now under (mainly
contributed by low-luminosity AGNs) and around . These
not only mark out the two populations of AGNs, but also will turn out to
correlate strongly with the red or blue color of their host galaxies.Comment: 7 pages, accepted for publication in the Astrophysical Journa
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
Cosmological Parameters from Velocities, CMB and Supernovae
We compare and combine likelihood functions of the cosmological parameters
Omega_m, h and sigma_8, from peculiar velocities, CMB and type Ia supernovae.
These three data sets directly probe the mass in the Universe, without the need
to relate the galaxy distribution to the underlying mass via a "biasing"
relation. We include the recent results from the CMB experiments BOOMERANG and
MAXIMA-1. Our analysis assumes a flat Lambda CDM cosmology with a
scale-invariant adiabatic initial power spectrum and baryonic fraction as
inferred from big-bang nucleosynthesis. We find that all three data sets agree
well, overlapping significantly at the 2 sigma level. This therefore justifies
a joint analysis, in which we find a joint best fit point and 95 per cent
confidence limits of Omega_m=0.28 (0.17,0.39), h=0.74 (0.64,0.86), and
sigma_8=1.17 (0.98,1.37). In terms of the natural parameter combinations for
these data sigma_8 Omega_m^0.6 = 0.54 (0.40,0.73), Omega_m h = 0.21
(0.16,0.27). Also for the best fit point, Q_rms-ps = 19.7 muK and the age of
the universe is 13.2 Gyr.Comment: 8 pages, 5 figures. Submitted to MNRA
A Local Hubble Bubble from SNe Ia?
We analyze the monopole in the peculiar velocities of 44 Type Ia supernovae
(SNe Ia) to test for a local void. The sample extends from 20 to 300 Mpc/h,
with distances, deduced from light-curve shapes, accurate to ~6%. Assuming
Omega_m=1 and Omega_lambda=0, the most significant deviation we find from the
Hubble law is an outwards flow of (6.6+/-2.2)% inside a sphere of radius 70
Mpc/h as would be produced by a void of ~20% underdensity surrounded by a dense
shell. This shell roughly coincides with the local Great Walls. Monte Carlo
analyses, using Gaussian errors or bootstrap resampling, show the probability
for chance occurrence of this result out of a pure Hubble flow to be ~2%. The
monopole could be contaminated by higher moments of the velocity field,
especially a quadrupole, which are not properly probed by the current limited
sky coverage. The void would be less significant if Omega_m is low and
Omega_lambda is high. It would be more significant if one outlier is removed
from the sample, or if the size of the void is constrained a-priori. This
putative void is not in significant conflict with any of the standard
cosmological scenarios. It suggests that the Hubble constant as determined
within 70 Mpc/h could be overestimated by ~6% and the local value of Omega may
be underestimated by ~20%. While the present evidence for a local void is
marginal in this data set, the analysis shows that the accumulation of SNe Ia
distances will soon provide useful constraints on elusive and important aspects
of regional cosmic dynamics.Comment: 21 pages, 3 figures. Slightly revised version. To appear in ApJ, 503,
Aug. 20, 199
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
Large Scale Power Spectrum from Peculiar Velocities Via Likelihood Analysis
The power spectrum (PS) of mass density fluctuations, independent of
`biasing', is estimated from the Mark III catalog of peculiar velocities using
Bayesian statistics. A parametric model is assumed for the PS, and the free
parameters are determined by maximizing the probability of the model given the
data. The method has been tested using detailed mock catalogs. It has been
applied to generalized CDM models with and without COBE normalization.
The robust result for all the models is a relatively high PS, with at . An
extrapolation to smaller scales using the different CDM models yields . The peak is weakly constrained to the range
. These results are consistent with a direct
computation of the PS (Kolatt & Dekel 1996). When compared to galaxy-density
surveys, the implied values for () are of order
unity to within 25%.
The parameters of the COBE-normalized, flat CDM model are confined by a 90%
likelihood contour of the sort , where
and for models with and without tensor
fluctuations respectively. For open CDM the powers are and (no tensor fluctuations). A -shape model free of COBE
normalization yields only a weak constraint: .Comment: 19 pages, 8 figures, 2 tables. Accepted for publication in The
Astrophysical Journa
Picosecond Nonlinear Relaxation of Photoinjected Carriers in a Single GaAs/AlGaAs Quantum Dot
Photoemission from a single self-organized GaAs/AlGaAs quantum dot (QD) is
temporally resolved with picosecond time resolution. The emission spectra
consisting of the multiexciton structures are observed to depend on the delay
time and the excitation intensity. Quantitative agreement is found between the
experimental data and the calculation based on a model which characterizes the
successive relaxation of multiexcitons. Through the analysis we can determine
the carrier relaxation time as a function of population of photoinjected
carriers. Enhancement of the intra-dot carrier relaxation is demonstrated to be
due to the carrier-carrier scattering inside a single QD.Comment: 4 pages, 4 figures, to be published in Phys. Rev. B, Rapid
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