2,221 research outputs found
Objective Classification of Galaxy Spectra using the Information Bottleneck Method
A new method for classification of galaxy spectra is presented, based on a
recently introduced information theoretical principle, the `Information
Bottleneck'. For any desired number of classes, galaxies are classified such
that the information content about the spectra is maximally preserved. The
result is classes of galaxies with similar spectra, where the similarity is
determined via a measure of information. We apply our method to approximately
6000 galaxy spectra from the ongoing 2dF redshift survey, and a mock-2dF
catalogue produced by a Cold Dark Matter-based semi-analytic model of galaxy
formation. We find a good match between the mean spectra of the classes found
in the data and in the models. For the mock catalogue, we find that the classes
produced by our algorithm form an intuitively sensible sequence in terms of
physical properties such as colour, star formation activity, morphology, and
internal velocity dispersion. We also show the correlation of the classes with
the projections resulting from a Principal Component Analysis.Comment: submitted to MNRAS, 17 pages, Latex, with 14 figures embedde
The Cosmological Mean Density and its Local Variations Probed by Peculiar Velocities
Peculiar velocities thoughout the region of the local supercluster are
reconstructed by two different orbit-retracing methods. The requirement of the
optimal correlation between the radial components of reconstructed velocities
and the observed peculiar velocities derived from our extensive new catalog of
distances puts stringent constraints on the values of the cosmological
parameters. Our constraints intersect those from studies of microwave
background fluctuations and statistical properties of galaxy clustering: the
ensemble of constraints are consistent with Omega_m=0.22\pm 0.02. While motions
throughout the Local Supercluster provide a measure of the mean ratio of mass
to light, there can be large local fluctuations. Our reconstruction of the
infall velocities in the immediate vicinity of the Virgo Cluster shows that
there is a mass-to-light anomaly of a factor of 3 to 6 between groups in the
general field environment and the heavily populated Virgo Cluster.Comment: 4 pages, 2 figures, version to appear in Astrophysical Journal
Letter
Acoustic peaks and dips in the CMB power spectrum: observational data and cosmological constraints
The locations and amplitudes of three acoustic peaks and two dips in the last
releases of the Boomerang, MAXIMA and DASI measurements of the cosmic microwave
background (CMB) anisotropy power spectra as well as their statistical
confidence levels are determined in a model-independent way. It is shown that
the Boomerang-2001 data (Netterfield et al. 2001) fixes the location and
amplitude of the first acoustic peak at more than 3\sigma confidence level. The
next two peaks and dips are determined at a confidence level above 1\sigma but
below 2\sigma. The locations and amplitudes of the first three peaks and two
dips are 212+/-17, 5426+/-1218\mu K^2, 544+/-56, 2266+/-607\mu K^2, 843+/-35,
2077+/-876\mu K^2, 413+/-50, 1960+/-503\mu K^2, 746+/-89, 1605+/-650\mu K^2
respectively (1\sigma errors include statistical and systematic errors). The
MAXIMA and DASI experiments give similar values for the extrema which they
determine. The determined cosmological parameters from the CMB acoustic extrema
data show good agreement with other determinations, especially with the baryon
content as deduced from standard nucleosynthesis constraints. These data
supplemented by the constraints from direct measurements of some cosmological
parameters and data on large scale structure lead to a best-fit model which
agrees with practically all the used experimental data within 1\sigma. The
best-fit parameters are: \Omega_{\Lambda}=0.64^{+0.14}_{-0.27}, \Omega_{m}=
0.36^{+0.21}_{-0.11}, \Omega_b=0.047^{+0.093}_{-0.024},
n_s=1.0^{+0.59}_{-0.17}, h=0.65^{+0.35}_{-0.27} and \tau_c=0.15^{+0.95}_{-0.15}
(plus/minus values show 1\sigma upper/lower limits obtained by marginalization
over all other model parameters). The best-fit values of \Omega_{\nu} and T/S
are close to zero, their 1\sigma upper limits are 0.17 and 1.7 respectively.Comment: 34 pages, 10 figures; accepted by ApJ; some corrections in the text
are made and a few references are adde
Initial Conditions for Large Cosmological Simulations
This technical paper describes a software package that was designed to
produce initial conditions for large cosmological simulations in the context of
the Horizon collaboration. These tools generalize E. Bertschinger's Grafic1
software to distributed parallel architectures and offer a flexible alternative
to the Grafic2 software for ``zoom'' initial conditions, at the price of large
cumulated cpu and memory usage. The codes have been validated up to resolutions
of 4096^3 and were used to generate the initial conditions of large
hydrodynamical and dark matter simulations. They also provide means to generate
constrained realisations for the purpose of generating initial conditions
compatible with, e.g. the local group, or the SDSS catalog.Comment: 12 pages, 11 figures, submitted to ApJ
The X-ray Cluster Dipole
We estimate the dipole of the whole sky X-ray flux-limited sample of
Abell/ACO clusters (XBACs) and compare it to the optical Abell/ACO cluster
dipole. The X-ray cluster dipole is well aligned () with the
CMB dipole, while it follows closely the radial profile of its optical cluster
counterpart although its amplitude is per cent lower. In view of
the fact that the the XBACs sample is not affected by the volume incompleteness
and the projection effects that are known to exist at some level in the optical
parent Abell/ACO cluster catalogue, our present results confirm the previous
optical cluster dipole analysis that there are significant contributions to the
Local Group motion from large distances (Mpc). In order to
assess the expected contribution to the X-ray cluster dipole from a purely
X-ray selected sample we compare the dipoles of the XBACs and the Brightest
Cluster Sample (Ebeling et al. 1997a) in their overlap region. The resulting
dipoles are in mutual good aggreement with an indication that the XBACs sample
slightly underestimates the full X-ray dipole (by per cent) while the
Virgo cluster contributes about 10 - 15 per cent to the overall X-ray cluster
dipole. Using linear perturbation theory to relate the X-ray cluster dipole to
the Local group peculiar velocity we estimate the density parameter to be
.Comment: 16 pages, latex, + 4 ps figures, submitted to Ap
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