The micro-structure of the intergalactic medium I: the 21cm signature from dynamical minihaloes

Abstract

A unified description is provided for the 21cm signatures arising from minihaloes against a bright background radio source and against the Cosmic Microwave Background (CMB), within the context of a dynamical collapsing cosmological spherical halo model. The equivalent width distribution of the resulting 21cm forest is computed for LCDM cosmologies, along with the brightness temperature differential relative to the CMB. The effects of an ambient Ly-alpha radiation field and heating of the IGM on the signatures are included. It is shown that the dynamical effects of heating substantially suppress absorption features in the 21cm forest with observed equivalent widths exceeding 0.15 kHz. It is demonstrated how measurements of excess fluctuations beyond detector noise could make a statistical detection of the weaker absorption features against a bright background radio source. It is also shown that a Ly-alpha radiation field only a few percent the thermalisation rate is sufficient to render the minihalo signal against the CMB negligible compared with the signal from the diffuse IGM component. The 21cm signals are found to be very sensitive to the amount of small scale power in the primordial density fluctuation spectrum. The effects of gas cooling via radiative atomic and molecular processes and of star formation on setting the maximum mass of the minihaloes giving rise to a 21cm signal are included, with allowance made for the suppression of molecular hydrogen formation by an ambient UV radiation field.

Substantial revision: paper restructured; hydrodynamical model with heating included; discussion of statistical detection of absorption fluctuations from 21cm forest added. 34 pages; 34 figures. Definitive version published in Monthly Notices

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