5,957 research outputs found
Surface brightness measurements for APM galaxies
This paper considers some simple surface brightness (SB) estimates for
galaxies in the Automated Plate Measuring Machine (APM) catalogue in order to
derive homogeneous SB data for a very large sample of faint galaxies. The
isophotal magnitude and area are used to estimate the central surface
brightness and total magnitude based on the assumption of an exponential SB
profile. The surface brightness measurements are corrected for field effects on
each UK Schmidt plate and the zero-point of each plate is adjusted to give a
uniform sample of SB and total magnitude estimates over the whole survey.
Results are obtained for 2.4 million galaxies with blue photographic magnitudes
brighter than b_J = 20.5 covering 4300 deg^2 in the region of the south
galactic cap. Almost all galaxies in our sample have central surface brightness
in the range 20 to 24 b_J mag per arcsec^2. The SB measurements we obtain are
compared to previous SB measurements and we find an acceptable level of error
of +/- 0.2 b_J mag per arcsec^2. The distribution of SB profiles is considered
for different galaxy morphologies for the bright APM galaxies. We find that
early-type galaxies have more centrally concentrated profiles.Comment: 14 pages, 17 figures, accepted for publication in MNRA
Testing for Non-Gaussianity in the Wilkinson Microwave Anisotropy Probe Data: Minkowski Functionals and the Length of the Skeleton
The three Minkowski functionals and the recently defined length of the
skeleton are estimated for the co-added first-year Wilkinson Microwave
Anisotropy Probe (WMAP) data and compared with 5000 Monte Carlo simulations,
based on Gaussian fluctuations with the a-priori best-fit running-index power
spectrum and WMAP-like beam and noise properties. Several power
spectrum-dependent quantities, such as the number of stationary points, the
total length of the skeleton, and a spectral parameter, gamma, are also
estimated. While the area and length Minkowski functionals and the length of
the skeleton show no evidence for departures from the Gaussian hypothesis, the
northern hemisphere genus has a chi^2 that is large at the 95% level for all
scales. For the particular smoothing scale of 3.40 degrees FWHM it is larger
than that found in 99.5% of the simulations. In addition, the WMAP genus for
negative thresholds in the northern hemisphere has an amplitude that is larger
than in the simulations with a significance of more than 3 sigma. On the
smallest angular scales considered, the number of extrema in the WMAP data is
high at the 3 sigma level. However, this can probably be attributed to the
effect of point sources. Finally, the spectral parameter gamma is high at the
99% level in the northern Galactic hemisphere, while perfectly acceptable in
the southern hemisphere. The results provide strong evidence for the presence
of both non-Gaussian behavior and an unexpected power asymmetry between the
northern and southern hemispheres in the WMAP data.Comment: 17 pages, 10 figures, accepted for publication in Ap
Topology of Neutral Hydrogen Within the Small Magellanic Cloud
In this paper, genus statistics have been applied to an HI column density map
of the Small Magellanic Cloud in order to study its topology. To learn how
topology changes with the scale of the system, we provide the study of topology
for column density maps at varying resolution. To evaluate the statistical
error of the genus we randomly reassign the phases of the Fourier modes while
keeping the amplitudes. We find, that at the smallest scales studied () the genus shift is in all regions negative,
implying a clump topology. At the larger scales () the topology shift is detected to be negative in 4 cases and positive
(``swiss cheese'' topology) in 2 cases. In 4 regions there is no statistically
significant topology shift at large scales
A New Estimate of the Hubble Time with Improved Modeling of Gravitational Lenses
This paper examines free-form modeling of gravitational lenses using Bayesian
ensembles of pixelated mass maps. The priors and algorithms from previous work
are clarified and significant technical improvements are made. Lens
reconstruction and Hubble Time recovery are tested using mock data from simple
analytic models and recent galaxy-formation simulations. Finally, using
published data, the Hubble Time is inferred through the simultaneous
reconstruction of eleven time-delay lenses. The result is
H_0^{-1}=13.7^{+1.8}_{-1.0} Gyr.Comment: 24 pages, 9 figures. Accepted to Ap
Cosmological Parameter Determination in Free-Form Strong Gravitational Lens Modeling
We develop a novel statistical strong lensing approach to probe the
cosmological parameters by exploiting multiple redshift image systems behind
galaxies or galaxy clusters. The method relies on free-form mass inversion of
strong lenses and does not need any additional information other than
gravitational lensing. Since in free-form lensing the solution space is a
high-dimensional convex polytope, we consider Bayesian model comparison
analysis to infer the cosmological parameters. The volume of the solution space
is taken as a tracer of the probability of the underlying cosmological
assumption. In contrast to parametric mass inversions, our method accounts for
the mass-sheet degeneracy, which implies a degeneracy between the steepness of
the profile and the cosmological parameters. Parametric models typically break
this degeneracy, introducing hidden priors to the analysis that contaminate the
inference of the parameters. We test our method with synthetic lenses, showing
that it is able to infer the assumed cosmological parameters. Applied to the
CLASH clusters, the method might be competitive with other probes.Comment: 11 pages, 5 figures. Accepted for publication in MNRA
Two-Dimensional Topology of the 2dF Galaxy Redshift Survey
We study the topology of the publicly available data released by the 2dFGRS.
The 2dFGRS data contains over 100,000 galaxy redshifts with a magnitude limit
of b_J=19.45 and is the largest such survey to date. The data lie over a wide
range of right ascension (75 degree strips) but only within a narrow range of
declination (10 degree and 15 degree strips). This allows measurements of the
two-dimensional genus to be made.
The NGP displays a slight meatball shift topology, whereas the SGP displays a
bubble like topology. The current SGP data also have a slightly higher genus
amplitude. In both cases, a slight excess of overdense regions are found over
underdense regions. We assess the significance of these features using mock
catalogs drawn from the Virgo Consortium's Hubble Volume LCDM z=0 simulation.
We find that differences between the NGP and SGP genus curves are only
significant at the 1 sigma level. The average genus curve of the 2dFGRS agrees
well with that extracted from the LCDM mock catalogs.
We compare the amplitude of the 2dFGRS genus curve to the amplitude of a
Gaussian random field with the same power spectrum as the 2dFGRS and find,
contradictory to results for the 3D genus of other samples, that the amplitude
of the GRF genus curve is slightly lower than that of the 2dFGRS. This could be
due to a a feature in the current data set or the 2D genus may not be as
sensitive as the 3D genus to non-linear clustering due to the averaging over
the thickness of the slice in 2D. (Abridged)Comment: Submitted to ApJ A version with Figure 1 in higher resolution can be
obtained from http://www.physics.drexel.edu/~hoyle
Nonlinear stochastic biasing from the formation epoch distribution of dark halos
We propose a physical model for nonlinear stochastic biasing of one-point
statistics resulting from the formation epoch distribution of dark halos. In
contrast to previous works on the basis of extensive numerical simulations, our
model provides for the first time an analytic expression for the joint
probability function. Specifically we derive the joint probability function of
halo and mass density contrasts from the extended Press-Schechter theory. Since
this function is derived in the framework of the standard gravitational
instability theory assuming the random-Gaussianity of the primordial density
field alone, we expect that the basic features of the nonlinear and stochastic
biasing predicted from our model are fairly generic. As representative
examples, we compute the various biasing parameters in cold dark matter models
as a function of a redshift and a smoothing length. Our major findings are (1)
the biasing of the variance evolves strongly as redshift while its
scale-dependence is generally weak and a simple linear biasing model provides a
reasonable approximation roughly at R\simgt 2(1+z)\himpc, and (2) the
stochasticity exhibits moderate scale-dependence especially on R\simlt
20\himpc, but is almost independent of . Comparison with the previous
numerical simulations shows good agreement with the above behavior, indicating
that the nonlinear and stochastic nature of the halo biasing is essentially
understood by taking account of the distribution of the halo mass and the
formation epoch.Comment: 34 pages, 11 figures, ApJ (2000) in pres
Getting the Measure of the Flatness Problem
The problem of estimating cosmological parameters such as from noisy
or incomplete data is an example of an inverse problem and, as such, generally
requires a probablistic approach. We adopt the Bayesian interpretation of
probability for such problems and stress the connection between probability and
information which this approach makes explicit.
This connection is important even when information is ``minimal'' or, in
other words, when we need to argue from a state of maximum ignorance. We use
the transformation group method of Jaynes to assign minimally--informative
prior probability measure for cosmological parameters in the simple example of
a dust Friedman model, showing that the usual statements of the cosmological
flatness problem are based on an inappropriate choice of prior. We further
demonstrate that, in the framework of a classical cosmological model, there is
no flatness problem.Comment: 11 pages, submitted to Classical and Quantum Gravity, Tex source
file, no figur
A Counts-in-Cells Analysis of Lyman-break Galaxies at z~3
We have measured the counts-in-cells fluctuations of 268 Lyman-break galaxies
with spectroscopic redshifts in six 9 arcmin by 9 arcmin fields at z~3. The
variance of galaxy counts in cubes of comoving side length 7.7, 11.9, 11.4
h^{-1} Mpc is \sigma_{gal}^2 ~ 1.3\pm0.4 for \Omega_M=1, 0.2 open, 0.3 flat,
implying a bias on these scales of \sigma_{gal} / \sigma_{mass} = 6.0\pm1.1,
1.9\pm0.4, 4.0\pm0.7. The bias and abundance of Lyman-break galaxies are
surprisingly consistent with a simple model of structure formation which
assumes only that galaxies form within dark matter halos, that Lyman-break
galaxies' rest-UV luminosities are tightly correlated with their dark masses,
and that matter fluctuations are Gaussian and have a linear power-spectrum
shape at z~3 similar to that determined locally (\Gamma~0.2). This conclusion
is largely independent of cosmology or spectral normalization \sigma_8. A
measurement of the masses of Lyman-break galaxies would in principle
distinguish between different cosmological scenarios.Comment: Accepted for publication in ApJ, 16 pages including 4 figure
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