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Register a new userEvolution of the cold gas fraction and the star formation history: Prospects with current and future radio facilities
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S. J. Curran
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It has recently been shown that the abundance of cold neutral gas may follow a similar evolution as the star formation history. This is physically motivated, since stars form out of this component of the neutral gas and if the case, would resolve the longstanding issue that there is a clear disparity between the total abundance of neutral gas and star forming activity over the history of the Universe. Radio-band 21-cm absorption traces the cold gas and comparison with the Lyman-alpha absorption, which traces all of the gas, provides a measure of the cold gas fraction or the spin temperature. The recent study has shown that the spin temperature (degenerate with the ratio of the absorber/emitter extent) appears to be anti-correlated with the star formation density, undergoing a similar steep evolution as the star formation rate over redshifts of 0 < z < 3, whereas the total neutral hydrogen exhibits little evolution. Above z > 3, where the SFR shows a steep decline with redshift, there is insufficient 21-cm data to determine whether the spin temperature continues to follow the SFR. Knowing this is paramount in ascertaining whether the cold neutral gas does trace the star formation over the Universes history. We explore the feasibility of resolving this with 21-cm observations of the largest contemporary sample of reliable damped Lyman-alpha absorption systems and conclude that, while todays largest radio interferometers can reach the required sensitivity at z < 3.5, the Square Kilometre Array is required to probe to higher redshifts.
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There is a well known disparity between the evolution the star formation rate density, {psi}*, and the abundance of neutral hydrogen (HI), the raw material for star formation. Recently, however, we have shown that {psi}* may be correlated with the fraction of cool atomic gas, as traced through the 21-cm absorption of HI. This is expected since star formation requires cold (T ~ 10 K) gas and so this could address the issue of why the star formation rate density does not trace the bulk atomic gas. The data are, however, limited to redshifts of z < 2, where both {psi}* and the cold gas fraction exhibit a similar steep climb from the present day (z = 0), and so it is unknown whether the cold gas fraction follows the same decline as {psi}* at higher redshift. In order to address this, we have used unpublished archival observations of 21-cm absorption in high redshift damped Lyman-{alpha} absorption systems to increase the sample at z > 2. The data suggest that the cold gas fraction does exhibit a decrease, although this is significantly steeper than {psi}* at z ~ 3. This is, however, degenerate with the extents of the absorbing galaxy and the background continuum emission and upon removing these, via canonical evolution models, we find the mean spin temperature of the gas to be <T> ~ 3000 K, compared to the ~2000 K expected from the fit at z < 2. These temperatures are consistent with the observed high neutral hydrogen column densities, which require T < 4000 K in order for the gas not to be highly ionised.
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