HI fluctuations at large redshifts: I-visibility correlation

Abstract

We investigate the possibility of probing the large scale structure in the universe at large redshifts by studying fluctuations in the redshifted 1420 MHz emission from the neutral hydrogen (HI) at early epochs. The neutral hydrogen content of the universe is known from absorption studies for z 4.5. The HI distribution is expected to be inhomogeneous in the gravitational instability picture and this inhomogeneity leads to anisotropy in the redshifted HI emission. The best hope of detecting this anisotropy is by using a large low-frequency interferometric instrument like the Giant Meter-Wave Radio Telescope (GMRT). We calculate the visibility correlation function V ν (U)V ν ′ (U) at two frequencies ν and ν ′ of the redshifted HI emission for an interferometric observation. In particular we give numerical results for the two GMRT channels centered around ν = 325 MHz and ν = 610 MHz from density inhomogeneity and peculiar velocity of the HI distribution. The visibility correlation is ≃ 10 −10 -10 −9 Jy 2 . We calculate the signal-to-noise for detecting the correlation signal in the presence of system noise and show that the GMRT might detect the signal for integration times ≃ 100 hrs. We argue that the measurement of visibility correlation allows optimal use of the uncorrelated nature of the system noise across baselines and frequency channels.

Prospects of Detecting HI using Redshifted 21-cm Radiation at z∼3

“…The observables in the context of the HI intensity mapping are visibility correlations and these have been shown to be related to the redshift space power spectrum of HI fluctuations (Bharadwaj & Sethi, 2001;Chang et al, 2008) in the limit of a small field of view. A physically motivated scheme to populate dark matter halos with neutral Hydrogen has been used in the past for modelling the HI power spectrum (Bagla, Khandai, & Datta, 2010) and similar results for clustering are obtained in the halo model with a similar approach (Wyithe & Brown, 2010).…”

“…The observable signal here is the visibility correlation and it is related to an integral over the power spectrum and hence it has some contribution from power spectrum at other scales (Bharadwaj & Sethi, 2001;Bharadwaj & Ali, 2005).…”

“…A discussion of foreground removal is beyond the scope of this paper. S 2 (U n , ∆ν) from the HI signal directly probes the redshift space power spectrum P HI (k, z) (Bharadwaj & Sethi, 2001;Bharadwaj & Ali, 2005). The multi-frequency angular power spectrum can be calculated for the HI signal using equation (4).…”

“…Several upcoming radio telescopes can be used to detect neutral Hydrogen at high redshifts using the redshifted 21 cm radiation using intensity mapping (Bharadwaj & Sethi, 2001;Santos et al, 2015;Tingay et al, 2013). The focus of the present study is at redshift z 3.…”

Constraining neutrino mass using the large-scale H i distribution in the post-reionization epoch

“…The collective emission from the clouds, however form a diffuse background in all radio observations at observing frequencies less than 1420 MHz. Tomographic imaging using the low resolution intensity mapping of this diffuse background radiation is a potentially rich probe of cosmology (Bharadwaj & Sethi 2001;Chang et al 2008;Wyithe 2008;Bharadwaj et al 2009;Wyithe & Loeb 2009;Camera et al 2013;Bull et al 2015).…”