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SUMMARY:The north-south asymmetry of the ALFALFA HI velocity width functio
 n
DTSTART;VALUE=DATE-TIME:20230323T142600Z
DTEND;VALUE=DATE-TIME:20230323T142800Z
DTSTAMP;VALUE=DATE-TIME:20241114T092451Z
UID:indico-contribution-235@meetings.aip.de
DESCRIPTION:Speakers: Richard Brooks (University College London)\nThe numb
 er density of extragalactic 21-cm radio sources as a function of their spe
 ctral line-widths -- the HI width function (HI WF) -- is in principle a se
 nsitive tracer of the dark matter halo mass function (HMF). The Λ cold da
 rk matter model predicts that the HMF should be identical everywhere provi
 ded it is sampled in sufficiently large volumes\, implying that the same s
 hould be true of the HI WF. The ALFALFA 21-cm survey measured the HI WF in
  two separate\, northern (‘spring') and southern (‘fall') Galactic fie
 lds and found a systematically higher number density of sources in the spr
 ing field. Taken at face value\, this is in tension with theoretical predi
 ctions. Using the Sibelius-DARK N-body simulation and the semi-analytical 
 galaxy formation model GALFORM to create a mock ALFALFA survey\, we find t
 hat the offset in number density likely has two origins: the sensitivity o
 f the survey is different in the two survey fields\, which has not been co
 rrectly accounted for in previous measurements\; and the limited ability o
 f the $1/V_\\mathrm{eff}$ algorithm used for completeness corrections to m
 itigate biases arising from spatial clustering in the galaxy distribution.
  The latter bias is primarily driven by a foreground overdensity in the sp
 ring field within a distance of 30 Mpc\, but more distant structure also p
 lays a role. We provide an updated measurement of the ALFALFA HI WF (and H
 I MF) correcting for the variations in survey sensitivity. Only when syste
 matic effects such as these are understood and corrected for can the HI WF
  fulfil its potential as a test of cosmological models.\n\nhttps://iaus379
 .aip.de/event/20/contributions/235/
LOCATION:Haus H\, Telegrafenberg
URL:https://iaus379.aip.de/event/20/contributions/235/
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