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Register a new userThe HI Mass Density in Galactic Halos, Winds, and Cold Accretion as Traced by MgII Absorption
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Authors
G. G. Kacprzak
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It is well established that MgII absorption lines detected in background quasar spectra arise from gas structures associated with foreground galaxies. The degree to which galaxy evolution is driven by the gas cycling through halos is highly uncertain because their gas mass density is poorly constrained. Fitting the MgII equivalent width (W) distribution with a Schechter function and applying the N(HI)-W correlation of Menard & Chelouche, we computed Omega(HI)_MgII ~ Omega(HI)_halo =(1.41 +0.75 -0.44)x10^-4 for 0.4<z<1.4. We exclude DLAs from our calculations so that Omega(HI)_halo comprises accreting and/or outflowing halo gas not locked up in cold neutral clouds. We deduce the cosmic HI gas mass density fraction in galactic halos traced by MgII absorption is Omega(HI)_halo/Omega(HI)_DLA=15% and Omega(HI)_halo/Omega_b=0.3%. Citing several lines of evidence, we propose infall/accretion material is sampled by small W whereas outflow/winds are sampled by large W, and find Omega(HI)_infall is consistent with Omega(HI)_outflow for bifurcation at W=1.23^{+0.15}_{-0.28}AA; cold accretion would then comprise no more than ~7% of of the total HI mass density. We discuss evidence that (1) the total HI mass cycling through halos remains fairly constant with cosmic time and that the accretion of HI gas sustains galaxy winds, and (2) evolution in the cosmic star formation rate depends primarily on the rate at which cool HI gas cycles through halos.
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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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