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Register a new userThe spatial distribution of neutral hydrogen as traced by low HI mass galaxies
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Han-Seek Kim
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The formation and evolution of galaxies with low neutral atomic hydrogen (HI) masses, M$_{rm HI}$$<$10$^{8}h^{-2}$M$_{odot}$, are affected by host dark matter halo mass and photoionisation feedback from the UV background after the end of reionization. We study how the physical processes governing the formation of galaxies with low HI mass are imprinted on the distribution of neutral hydrogen in the Universe using the hierarchical galaxy formation model, GALFORM. We calculate the effect on the correlation function of changing the HI mass detection threshold at redshifts $0 le z le 0.5$. We parameterize the clustering as $xi(r)=(r/r_{0})^{-gamma}$ and we find that including galaxies with M$_{rm HI}$$<$10$^{8}h^{-2}$M$_{odot}$ increases the clustering amplitude $r_{0}$ and slope $gamma$ compared to samples of higher HI masses. This is due to these galaxies with low HI masses typically being hosted by haloes with masses greater than 10$^{12}{h}^{-1}$M$_{odot}$, and is in contrast to optically selected surveys for which the inclusion of faint, blue galaxies lowers the clustering amplitude. We show the HI mass function for different host dark matter halo masses and galaxy types (central or satellite) to interpret the values of $r_{0}$ and $gamma$ of the clustering of HI-selected galaxies. We also predict the contribution of low HI mass galaxies to the 21cm intensity mapping signal. We calculate that a dark matter halo mass resolution better than $sim$10$^{10}{h}^{-1}$M$_{odot}$ at redshifts higher than 0.5 is required in order to predict converged 21cm brightness temperature fluctuations.
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The spatial distribution of neutral hydrogen (HI) in the Universe contains a wealth of cosmological information. The 21 cm emission line can be used to map the HI up to very high redshift and therefore reveal us something about the evolution of the large scale structures in the Universe. However little is known about the abundance and clustering properties of the HI over cosmic time. Motivated by this, we build an analytic framework where the relevant parameters that govern how the HI is distributed among dark matter halos can be fixed using observations. At the same time we provide tools to study the column density distribution function of the HI absorbers together with their clustering properties. Our formalism is the first one able to account for all observations at a single redshift, $z = 2.3$. The linear bias of the HI and the mean number density of HI sources, two main ingredients in the calculation of the signal-to-noise ratio of a cosmological survey, are then discussed in detail, also extrapolating the results to low and high redshift. We find that HI bias is relatively higher than the value reported in similar studies, but the shot noise level is always sub dominant, making the HI Power Spectrum always a high signal-to-noise measurements up to $zsimeq5$ in the limit of no instrumental noise and foreground contamination.
We have studied the small scale distribution of atomic hydrogen (HI) using 21-cm absorption spectra against multiple-component background radio continuum sources from the 21-SPONGE survey and the Millennium Arecibo Absorption Line Survey. We have found $>5sigma$ optical depth variations at a level of $sim0.03-0.5$ between 13 out of 14 adjacent sightlines separated by a few arcseconds to a few arcminutes, suggesting the presence of neutral structures on spatial scales from a few to thousands of AU (which we refer to as tiny scale atomic structure, TSAS). The optical depth variations are strongest in directions where the HI column density and the fraction of HI in the cold neutral medium (CNM) are highest, which tend to be at low Galactic latitudes. By measuring changes in the properties of Gaussian components fitted to the absorption spectra, we find that changes in both the peak optical depth and the linewidth of TSAS absorption features contribute to the observed optical depth variations, while changes in the central velocity do not appear to strongly impact the observed variations. Both thermal and turbulent motions contribute appreciably to the linewidths, but the turbulence does not appear strong enough to confine overpressured TSAS. In a majority of cases, the TSAS column densities are sufficiently high that these structures can radiatively cool fast enough to maintain thermal equilibrium with their surroundings, even if they are overpressured. We also find that a majority of TSAS is associated with the CNM. For TSAS in the direction of the Taurus molecular cloud and the local Leo cold cloud, we estimate densities over an order of magnitude higher than typical CNM densities.
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