Cross-correlation of the HI 21-cm Signal and Lyman-alpha Forest: A Probe Of Cosmology

Abstract

Separating the cosmological redshifted 21-cm signal from foregrounds is a major challenge. We present the cross-correlation of the redshifted 21-cm emission from neutral hydrogen (HI) in the post-reionization era with the Ly-alpha forest as a new probe of the large scale matter distribution in the redshift range z=2 to 3 without the problem of foreground contamination. Though the 21-cm and the Ly-alpha forest signals originate from different astrophysical systems, they are both expected to trace the underlying dark matter distribution on large scales. The multi-frequency angular cross-correlation power spectrum estimator is found to be unaffected by the discrete quasar sampling, which only affects the noise in the estimate. We consider a hypothetical redshifted 21-cm observation in a single field of view 1.3 degrees (FWHM) centered at z=2.2 where the binned 21-cm angular power spectrum can be measured at an SNR of 3 sigma or better across the range 500 < l < 4000 . Keeping the parameters of the 21-cm observation fixed, we have estimated the SNR for the cross-correlation signal varying the quasar angular number density n of the Ly-alpha forest survey. Assuming that the spectra have SNR ~5 in pixels of length 44 km/s, we find that a 5 sigma detection of the cross-correlation signal is possible at 600 < l < 2000 with n=4 deg^{-2}. This value of n is well within the reach of upcoming Ly-alpha forest surveys. The cross-correlation signal will be a new, independent probe of the astrophysics of the diffuse IGM, the growth of structure and the expansion history of the Universe.

Revised paper, accepted to MNRAS

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• Modelling the post-reionization neutral Hydrogen (\HI) bias
• Constraining large scale HI bias using redshifted 21-cm signal from the post-reionization epoch
• The cross-correlation between 21cm intensity mapping maps and the Lyman-alpha forest in the post-reionization era
• On using large scale correlation of the Ly-$\alpha$ forest and redshifted 21-cm signal to probe HI distribution during the post reionization era
• Primordial Non-Gaussianity in the Forest: 3D Bispectrum of Ly-alpha Flux Spectra Along Multiple Lines of Sight
• Constraining dark energy using the cross correlations of weak lensing with post-reionization probes of neutral hydrogen
• Predictions for BAO distance estimates from the cross-correlation of the Lyman-alpha forest and redshifted 21-cm emission
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• The redshifted HI 21 cm signal from the post-reionization epoch: Cross-correlations with other cosmological probes
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• Probing dark energy using anisotropies in the clustering of post-EoR HI distribution