Probing the Pre-Reionization Epoch with Molecular Hydrogen Intensity Mapping

Molecular hydrogen is now understood to be the main coolant of the primordial gas clouds leading to the formation of the very first stars and galaxies. The line emissions associated with molecular hydrogen should then be a good tracer of the matter distribution at the onset of reionization of the universe. Here we propose intensity mapping of H2 line emission in rest-frame mid-infrared wavelengths to map out the spatial distribution of gas at redshifts z > 10. We calculate the expected mean intensity and clustering power spectrum for several H2 lines. We find that the 0-0S(3) rotational line at a rest wavelength of 9.66 microns is the brightest line over the redshift range of 10 to 30 with an intensity of about 5 to 10 Jy/sr at z~15. To reduce astrophysical and instrumental systematics, we propose the cross-correlation between multiple lines of the H2 rotational and vibrational line emission spectrum. Our estimates of the intensity can be used as a guidance in planning instruments for future mid-IR spectroscopy missions such as SPICA.