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

Probing large scale filaments with HI and 3^3HeII

We explore the observability of the neutral hydrogen (HI) and the singly-ionized isotope helium-3 ($^3$HeII) in the intergalactic medium (IGM) from the Epoch of Reionization down to the local Universe. The hyperfine transition of $^3$HeII, which is not as well known as the HI transition, has energy splitting corresponding to 8 cm. It also has a larger spontaneous decay rate than that of neutral hydrogen, whereas its primordial abundance is much smaller. Although both species are mostly ionized in the IGM, the balance between ionization and recombination in moderately high density regions renders them abundant enough to be observed. We estimate the emission signal of both hyperfine transitions from large scale filamentary structures and discuss the prospects for observing them with current and future radio telescopes. We conclude that HI in filaments is possibly observable even with current telescopes after 100 hours of observation. On the other hand, $^3$HeII is only detectable with future telescopes, such as SKA, after the same amount of time.Comment: 21 pages, 13 figures, 2 tables, accepted to MNRA

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

Filament Hunting: Integrated HI 21cm Emission From Filaments Inferred by Galaxy Surveys

Large scale filaments, with lengths that can reach tens of Mpc, are the most prominent features in the cosmic web. These filaments have only been observed indirectly through the positions of galaxies in large galaxy surveys or through absorption features in the spectra of high redshift sources. In this study we propose to go one step further and directly detect intergalactic medium filaments through their emission in the HI 21cm line. We make use of high resolution cosmological simulations to estimate the intensity of this emission in low redshift filaments and use it to make predictions for the direct detectability of specific filaments previously inferred from galaxy surveys, in particular the Sloan Digital Sky Survey. Given the expected signal of these filaments our study shows that HI emission from large filaments can be observed by current and next generation radio telescopes. We estimate that gas in filaments of length $l \gtrsim$ 15 $h^{-1}$Mpc with relatively small inclinations to the line of sight ($\lesssim 10^\circ$) can be observed in $\sim40-100$ hours with telescopes such as GMRT or EVLA, potentially providing large improvements over our knowledge of the astrophysical properties of these filaments. Due to their large field of view and sufficiently long integration times, upcoming HI surveys with the Apertif and ASKAP instruments will be able to detect large filaments independently of their orientation and curvature. Furthermore, our estimates indicate that a more powerful future radio telescope like SKA-2 can be used to map most of these filaments, which will allow them to be used as a strong cosmological probe.Comment: 16 pages, 11 figures, Accepted for publication in MNRA