The detection of the Epoch of Reionization (EoR) in the redshifted 21-cm line is a challenging task. Here we formulate the detection of the EoR signal using the drift scan strategy. This method potentially has better instrumental stability as compare
d to the case where a single patch of sky is tracked. We demonstrate that the correlation time between measured visibilities could extend up to 1-2 hr for an interferometer array such as the Murchison Widefield Array (MWA), which has a wide primary beam. We estimate the EoR power based on cross-correlation of visibilities across time and show that the drift scan strategy is capable of the detection of the EoR signal with comparable/better signal-to-noise as compared to the tracking case. We also estimate the visibility correlation for a set of bright point sources and argue that the statistical inhomogeneity of bright point sources might allow their separation from the EoR signal.
We have observed the DEEP2 galaxies using the Giant Meterwave Radio Telescope in the frequency band of 610 MHz. There are $simeq 400$ galaxies in the redshift range $1.24 < z < 1.36$ and within the field of view $simeq 44$, of the GMRT dishes. We h
ave coadded the HI 21 cm-line emissions at the locations of these DEEP2 galaxies. We apply stacking on three different data cubes: primary beam uncorrected, primary beam corrected (uniform weighing ) and primary beam corrected (optimal weighing). We obtain a peak signal strength in the range $8hbox{--}25 , rm mu$Jy/beam for a velocity width in the range $270hbox{--} 810 , rm km , sec^{-1}$. The error on the signal, computed by bootstrapping, lies in the range $2.5hbox{--}6 , rm mu$Jy/beam, implying a 2.5--4.7-$sigma$ detection of the signal at $z simeq 1.3$. We compare our results with N-body simulations of the signal at $zsimeq 1$ and find reasonable agreement. We also discuss the impact of residual continuum and systematics.
We use a large N-body simulation to examine the detectability of HI in emission at redshift z ~ 1, and the constraints imposed by current observations on the neutral hydrogen mass function of galaxies at this epoch. We consider three different models
for populating dark matter halos with HI, designed to encompass uncertainties at this redshift. These models are consistent with recent observations of the detection of HI in emission at z ~ 0.8. Whilst detection of 21 cm emission from individual halos requires extremely long integrations with existing radio interferometers, such as the Giant Meter Radio Telescope (GMRT), we show that the stacked 21 cm signal from a large number of halos can be easily detected. However, the stacking procedure requires accurate redshifts of galaxies. We show that radio observations of the field of the DEEP2 spectroscopic galaxy redshift survey should allow detection of the HI mass function at the 5-12 sigma level in the mass range 10^(11.4) M_sun/h < M_halo < 10^(12.5)M_sun/h, with a moderate amount of observation time. Assuming a larger noise level that corresponds to an upper bound for the expected noise for the GMRT, the detection significance for the HI mass function is still at the 1.7-3 sigma level. We find that optically undetected satellite galaxies enhance the HI emission profile of the parent halo, leading to broader wings as well as a higher peak signal in the stacked profile of a large number of halos. We show that it is in principle possible to discern the contribution of undetected satellites to the total HI signal, even though cosmic variance limitation make this challenging for some of our models.
The implication of primordial magnetic-field-induced structure formation for the HI signal from the epoch of reionization is studied. Using semi-analytic models, we compute both the density and ionization inhomogeneities in this scenario. We show tha
t: (a) The global HI signal can only be seen in emission, unlike in the standard $Lambda$CDM models, (b) the density perturbations induced by primordial fields, leave distinctive signatures of the magnetic field Jeans length on the HI two-point correlation function, (c) the length scale of ionization inhomogeneities is $la 1 rm Mpc$. We find that the peak expected signal (two-point correlation function) is $simeq 10^{-4} rm K^2$ in the range of scales $0.5hbox{-}3 rm Mpc$ for magnetic field strength in the range $5 times 10^{-10} hbox{-}3 times 10^{-9} rm G$. We also discuss the detectability of the HI signal. The angular resolution of the on-going and planned radio interferometers allows one to probe only the largest magnetic field strengths that we consider. They have the sensitivity to detect the magnetic field-induced features. We show that thefuture SKA has both the angular resolution and the sensitivity to detect the magnetic field-induced signal in the entire range of magnetic field values we consider, in an integration time of one week.
