Speaker
Description
The number density of extragalactic 21-cm radio sources as a function of their spectral line-widths -- the HI width function (HI WF) -- is in principle a sensitive tracer of the dark matter halo mass function (HMF). The Λ cold dark matter model predicts that the HMF should be identical everywhere provided it is sampled in sufficiently large volumes, implying that the same should be true of the HI WF. The ALFALFA 21-cm survey measured the HI WF in two separate, northern (‘spring') and southern (‘fall') Galactic fields and found a systematically higher number density of sources in the spring field. Taken at face value, this is in tension with theoretical predictions. Using the Sibelius-DARK N-body simulation and the semi-analytical galaxy formation model GALFORM to create a mock ALFALFA survey, we find that the offset in number density likely has two origins: the sensitivity of the survey is different in the two survey fields, which has not been correctly accounted for in previous measurements; and the limited ability of the $1/V_\mathrm{eff}$ algorithm used for completeness corrections to mitigate biases arising from spatial clustering in the galaxy distribution. The latter bias is primarily driven by a foreground overdensity in the spring field within a distance of 30 Mpc, but more distant structure also plays a role. We provide an updated measurement of the ALFALFA HI WF (and HI MF) correcting for the variations in survey sensitivity. Only when systematic effects such as these are understood and corrected for can the HI WF fulfil its potential as a test of cosmological models.
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