827 research outputs found
Is the Redshift Clustering of Long-Duration Gamma-Ray Bursts Significant?
The 26 long-duration gamma-ray bursts (GRBs) with known redshifts form a
distinct cosmological set, selected differently than other cosmological probes
such as quasars and galaxies. Since the progenitors are now believed to be
connected with active star-formation and since burst emission penetrates dust,
one hope is that with a uniformly-selected sample, the large-scale redshift
distribution of GRBs can help constrain the star-formation history of the
Universe. However, we show that strong observational biases in ground-based
redshift discovery hamper a clean determination of the large-scale GRB rate and
hence the connection of GRBs to the star formation history. We then focus on
the properties of the small-scale (clustering) distribution of GRB redshifts.
When corrected for heliocentric motion relative to the local Hubble flow, the
observed redshifts appear to show a propensity for clustering: 8 of 26 GRBs
occurred within a recession velocity difference of 1000 km/s of another GRB.
That is, 4 pairs of GRBs occurred within 30 h_65^-1 Myr in cosmic time, despite
being causally separated on the sky. We investigate the significance of this
clustering. Comparison of the numbers of close redshift pairs expected from the
simulation with that observed shows no significant small-scale clustering
excess in the present sample; however, the four close pairs occur only in about
twenty percent of the simulated datasets (the precise significance of the
clustering is dependent upon the modeled biases). We conclude with some
impetuses and suggestions for future precise GRB redshift measurements.Comment: Published in the Astronomical Journal, June 2003: see
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2003AJ....125.2865
Cluster versus POTENT Density and Velocity Fields: Cluster Biasing and Omega
The density and velocity fields as extracted from the Abell/ACO clusters are
compared to the corresponding fields recovered by the POTENT method from the
Mark~III peculiar velocities of galaxies. In order to minimize non-linear
effects and to deal with ill-sampled regions we smooth both fields using a
Gaussian window with radii ranging between 12 - 20\hmpc. The density and
velocity fields within 70\hmpc exhibit similarities, qualitatively consistent
with gravitational instability theory and a linear biasing relation between
clusters and mass. The random and systematic errors are evaluated with the help
of mock catalogs. Quantitative comparisons within a volume containing
independent samples yield
\betac\equiv\Omega^{0.6}/b_c=0.22\pm0.08, where is the cluster biasing
parameter at 15\hmpc. If , as indicated by the cluster
correlation function, our result is consistent with .Comment: 18 pages, latex, 2 ps figures 6 gif figures. Accepted for
pubblications in MNRA
The dwarf low surface brightness population in different environments of the Local Universe
The nature of the dwarf galaxy population as a function of location in the
cluster and within different environments is investigated. We have previously
described the results of a search for low surface brightness objects in data
drawn from an East-West strip of the Virgo cluster (Sabatini et al., 2003) and
have compared this to a large area strip outside of the cluster (Roberts et
al., 2004). In this talk I compare the East-West data (sampling sub-cluster A
and outward) to new data along a North-South cluster strip that samples a
different region (part of sub-cluster A, and the N,M clouds) and with data
obtained for the Ursa Major cluster and fields around the spiral galaxy M101.
The sample of dwarf galaxies in different environments is obtained from uniform
datasets that reach central surface brightness values of ~26 B mag/arcsec^2 and
an apparent B magnitude of 21 (M_B=-10 for a Virgo Cluster distance of 16 Mpc).
We discuss and interpret our results on the properties and distribution of
dwarf low surface brightness galaxies in the context of variuos physical
processes that are thought to act on galaxies as they form and evolve.Comment: 10 pages, 3 figures, to appear in "Dark Galaxies and Lost Baryons",
IAU244 conference proceeding
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
Nonlinear Peculiar-Velocity Analysis and PCA
We allow for nonlinear effects in the likelihood analysis of peculiar
velocities, and obtain ~35%-lower values for the cosmological density parameter
and for the amplitude of mass-density fluctuations. The power spectrum in the
linear regime is assumed to be of the flat LCDM model (h=0.65, n=1) with only
Om_m free. Since the likelihood is driven by the nonlinear regime, we "break"
the power spectrum at k_b=0.2 h/Mpc and fit a two-parameter power-law at k>k_b.
This allows for an unbiased fit in the linear regime. Tests using improved mock
catalogs demonstrate a reduced bias and a better fit. We find for the Mark III
and SFI data Om_m=0.35+-0.09$ with sigma_8*Om_m^0.6=0.55+-0.10 (90% errors).
When allowing deviations from \lcdm, we find an indication for a wiggle in the
power spectrum in the form of an excess near k~0.05 and a deficiency at k~0.1
h/Mpc --- a "cold flow" which may be related to a feature indicated from
redshift surveys and the second peak in the CMB anisotropy. A chi^2 test
applied to principal modes demonstrates that the nonlinear procedure improves
the goodness of fit. The Principal Component Analysis (PCA) helps identifying
spatial features of the data and fine-tuning the theoretical and error models.
We address the potential for optimal data compression using PCA.Comment: 15 pages, LaTex, in Mining the Sky, July 31 - August 4, 2000,
Garching, German
Cosmological Density and Power Spectrum from Peculiar Velocities: Nonlinear Corrections and PCA
We allow for nonlinear effects in the likelihood analysis of galaxy peculiar
velocities, and obtain ~35%-lower values for the cosmological density parameter
Om and the amplitude of mass-density fluctuations. The power spectrum in the
linear regime is assumed to be a flat LCDM model (h=0.65, n=1, COBE) with only
Om as a free parameter. Since the likelihood is driven by the nonlinear regime,
we "break" the power spectrum at k_b=0.2 h/Mpc and fit a power law at k>k_b.
This allows for independent matching of the nonlinear behavior and an unbiased
fit in the linear regime. The analysis assumes Gaussian fluctuations and
errors, and a linear relation between velocity and density. Tests using proper
mock catalogs demonstrate a reduced bias and a better fit. We find for the
Mark3 and SFI data Om_m=0.32+-0.06 and 0.37+-0.09 respectively, with
sigma_8*Om^0.6 = 0.49+-0.06 and 0.63+-0.08, in agreement with constraints from
other data. The quoted 90% errors include cosmic variance. The improvement in
likelihood due to the nonlinear correction is very significant for Mark3 and
moderately so for SFI. When allowing deviations from LCDM, we find an
indication for a wiggle in the power spectrum: an excess near k=0.05 and a
deficiency at k=0.1 (cold flow). This may be related to the wiggle seen in the
power spectrum from redshift surveys and the second peak in the CMB anisotropy.
A chi^2 test applied to modes of a Principal Component Analysis (PCA) shows
that the nonlinear procedure improves the goodness of fit and reduces a spatial
gradient of concern in the linear analysis. The PCA allows addressing spatial
features of the data and fine-tuning the theoretical and error models. It shows
that the models used are appropriate for the cosmological parameter estimation
performed. We address the potential for optimal data compression using PCA.Comment: 18 pages, LaTex, uses emulateapj.sty, ApJ in press (August 10, 2001),
improvements to text and figures, updated reference
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
Molecular Hydrogen and Global Star Formation Relations in Galaxies
(ABRIDGED) We use hydrodynamical simulations of disk galaxies to study
relations between star formation and properties of the molecular interstellar
medium (ISM). We implement a model for the ISM that includes low-temperature
(T<10^4K) cooling, directly ties the star formation rate to the molecular gas
density, and accounts for the destruction of H2 by an interstellar radiation
field from young stars. We demonstrate that the ISM and star formation model
simultaneously produces a spatially-resolved molecular-gas surface density
Schmidt-Kennicutt relation of the form Sigma_SFR \propto Sigma_Hmol^n_mol with
n_mol~1.4 independent of galaxy mass, and a total gas surface density -- star
formation rate relation Sigma_SFR \propto Sigma_gas^n_tot with a power-law
index that steepens from n_tot~2 for large galaxies to n_tot>~4 for small dwarf
galaxies. We show that deviations from the disk-averaged Sigma_SFR \propto
Sigma_gas^1.4 correlation determined by Kennicutt (1998) owe primarily to
spatial trends in the molecular fraction f_H2 and may explain observed
deviations from the global Schmidt-Kennicutt relation.Comment: Version accepted by ApJ, high-res version available at
http://kicp.uchicago.edu/~brant/astro-ph/molecular_ism/rk2007.pd
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