131 research outputs found
Probing the Epoch of Reionization with Low Frequency Arrays
The Epoch of Reionization (EoR) is the epoch in which hydrogen in the
Universe reionize after the "Dark Ages". This is the second of two major phase
transitions that hydrogen in the Universe underwent, the first phase being the
recombination era in which hydrogen became neutral at redshift about 1100. The
EoR, occurs around z of 10 and is probably caused by the first radiation
emitting astrophysical sources, hence it is crucial to our understanding of
when and how the Universe "decided" to start forming astrophysical objects and
how that influenced subsequent structure formation in the Universe. As such,
the EoR is related to many fundamental questions in cosmology, galaxy
formation, quasars and very metal poor stars; all are foremost research issues
in modern astrophysics. The redshifted 21 cm hyperfine line is widely
considered as the most promising probe for studying the EoR in detail. In the
near future a number of low frequency radio telescopes (LOFAR, MWA, GMRT and
SKA) will be able to observe the 21 cm radiation arriving from the high
redshift Universe. In this paper I present our current picture of the
ionization process, review the 21 cm line physics and discuss the challenges
that the current generation experiments are expected to face. Finally, I
discuss the potential of SKA in exploring the EoR and the Universe's Dark Ages.Comment: 9 pages and 9 figures. To be published in SKADS Conference 2009
"Widefield Science and Technology for the SKA", eds. S.A. Torchinsky, A. van
Ardenne, T. van den Brink-Havinga, A. van Es, A.J. Faulkne
Goodness-of-Fit Analysis of Radial Velocities Surveys
Using eigenmode expansion of the Mark-3 and SFI surveys of cosmological
radial velocities a goodness-of-fit analysis is applied on a mode-by-mode
basis. This differential analysis complements theBayesian maximum likelihood
analysis that finds the most probable model given the data. Analyzing the
surveys with their corresponding most likely models from the CMB-like family of
models, as well as with the currently popular Lambda-CDM model, reveals a
systematic inconsistency of the data with these `best' models. There is a
systematic trend of the cumulative chi^2 to increase with the mode number
(where the modes are sorted by decreasing order of the eigenvalues). This
corresponds to a decrease of the chi^2 with the variance associated with a
mode, and hence with its effective scale. It follows that the differential
analysis finds that on small (large) scales the global analysis of all the
modes `puts' less (more) power than actually required by the data. This
observed trend might indicate one of the followings: a. The theoretical model
(i.e. power spectrum) or the error model (or both) have an excess of power on
large scales; b. Velocity bias; c. The velocity data suffers from still
uncorrected systematic errors.Comment: 12 pages including 2 figures. Accepted for publication in the Ap.J.
Letter
On the definition of superclusters
To obtain a physically well-motivated definition of superclusters, we
proposed in our previous work to select superclusters with an overdensity
criterion that selects only those objects that will collapse in the future,
including those that are at a turn-around in the present epoch. In this paper
we present numerical values for these criteria for a range of standard
cosmological models. We express these criteria in terms of a density ratio or,
alternatively, as an infall velocity and show that these two criteria give
almost identical results. To better illustrate the implications of this
definition, we applied our criteria to some prominent structures in the local
Universe, the Local supercluster, Shapley supercluster, and the recently
reported Laniakea supercluster to understand their future evolution. We find
that for the Local and Shapley superclusters, only the central regions will
collapse in the future, while Laniakea does not constitute a significant
overdensity and will disperse in the future. Finally, we suggest that those
superclusters that will survive the accelerating cosmic expansion and collapse
in the future be called "superstes-clusters", where "superstes" means survivor
in Latin, to distinguish them from traditional superclusters.Comment: Accepted for publication as Letter in A&A, 6 page
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