The redshifted 1420 MHz emission from the HI in unresolved damped Lyman-\alpha clouds at high z will appear as a background radiation in low frequency radio observations. This holds the possibility of a new tool for studying the universe at high-z, using the mean brightness temperature to probe the HI content and its fluctuations to probe the power  spectrum.

Existing estimates of the HI density at z~3 imply a mean brightness temperature of 1 mK  at 320 Mhz. The cross-correlation between the temperature fluctuations across different frequencies and sight lines is predicted to vary from 10^{-7} K^2 to 10^{-8} K^2 over intervals corresponding to spatial scales from 10 Mpc to 40 Mpc for some of the currently favoured cosmological models. We discuss a strategy based on the very distinct spectral properties of the foregrounds as against the HI emission, possibly allowing the removal of the foregrounds from the observed maps.

astro-ph/0402262
Title: HI Fluctuations at Large Redshifts: III - Simulating the Signal Expected at GMRT
Authors: Somnath Bharadwaj, Pennathur Sridharan Srikant
Submitted to JApA

astro-ph/0307303
Title: HI Fluctuations at Large Redshifts: II - the Signal Expected for GMRT
Authors: Somnath Bharadwaj  and  Sanjay K. Pande
Journal-ref: 2003, JApA,  24, 23-35

astro-ph/0203269
Title: HI Fluctuations at Large Redshifts: I--Visibility correlation
Authors: Somnath Bharadwaj and Shiv K. Sethi
Journal Reference: 2001,  J.ApA. 22, 293

 astro-ph/0003200
Title: Using HI to probe large scale structures at z ~ 3
Authors: Somnath Bharadwaj, Biman B. Nath  and Shiv K. Sethi
Journal Reference: 2001, JApA, 22, 21

References to Related Papers

astro-ph/0209216
Title: Nonlinear clustering during the cosmic Dark Ages and its effect on the 21-cm background from minihalos
Authors: Ilian T. Iliev (1), Evan Scannapieco (1), Hugo Martel (2), Paul R. Shapiro (2) ((1) Arcetri, (2) University of Texas at Austin)

Ongoing Research

                    We are currently investigating the possibility of detecting the fluctuations in the radio background arising from the HI at high redshifts. Most large radio telescopes arrays use  the principle of Fourier synthesis to make radio images. The primary quantity measured in such observations are the visibilities i.e. correlations between the electric fields measured by different telescops in the array.  The Fourier transform of the visibilities gives the radio image. We are presently considering the possibility of detecting the contribution from fluctuations in the HI distribution directly in the visibilites.

                       Each visibility is sensitive to a particular Fourier mode in the plane of the sky. Thus, the visibilities directly probe the Power Spectrum of the fluctuations in the HI distribution at high redshifts. Analytic estimates of this signal are presented in  astro-ph/0203269.     The contribution from the HI fluctuations is expected to be around 10 to 30 micro Jy in the visibility measured at each baseline <.3 Kilo-lambda at frequency ~320 MHz. Also, the contributions of the HI fluctuations to the visibilities measured by different baselines are uncorrelated - this is a consequence of the assumption that the density fluctuations are a Gaussian Random field. Correlations are expected for the same antenna-separation across different  frequencies. The question is whether these correlations can be detected?

The various hurdles in the detection of this signal are:

1. Continuum signal from our own Galaxy
2. Continuum signal from extragalactic radio sources.
3. System noise
3. Poisson noise arising from the fact that the HI is not continuously distributed, but it is in discrete clouds.

                We have still to uderstand the contribution from these factors, and devise methods for discriminating the HI signal from the continuum signal and noise.

                  Among these the System noise is, in principle,  the  simplest to handle. The contribution from system noise to the measurement of visibilities at different frequency channels and antenna pairs are expected to be uncorrelated.

                We are currently simulating low frequency radio observations with the aim of determining whether it will be possible to detect the HI signal using the Giant Meterwave Radio Telescope GMRT.

                How are we doing this ?

       1. Generate a clustered distribution of HI clouds with the total HI consistent with Omega_HI from observations of DLA absorption lines, and teh dsitribution of the HI consistent with the distribution of the column densities.

           This is done by first randomly generating the cloud positions, and then using the Zel'dovich approximation to incorporate the clustering pattern and the peculiar velocities, using any desired power spectrum for the dark matter fluctuations.

        2. Generate distribution of continuum radio sources.

        3. Calculate the GMRT baselines for a typical observations.

        4. Calculate the visibilities for each baseline and each frequency channel for the discrete  sources generated in 1. and 2.

        5. The contribution of the galactic foregrounds has to be trated as from a continuous, fluctuatting  medium. This can be directly generated in the space of baselines.

        6. Incorporate the effect of System noise in each visibility.