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The north–south asymmetry of the ALFALFA H i velocity width function

Brooks, Richard A N; Oman, Kyle A; Frenk, Carlos S

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Richard A N Brooks

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Dr Kyle Oman
Assistant Professor - Royal Society Dorothy Hodgkin Fellow


The number density of extragalactic 21-cm radio sources as a function of their spectral line widths - the HI width function (HIWF) - is a sensitive tracer of the dark matter halo mass function (HMF). The Lambda 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 Arecibo Legacy Fast ALFA (ALFALFA) 21-cm survey measured the HI WF in northern and southern Galactic fields and found a systematically higher number density in the north. At face value, this is in tension with theoretical predictions. We use 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 has two origins: the sensitivity of the survey is different in the two fields, which has not been correctly accounted for in previous measurements; and the 1/Veff algorithm used for completeness corrections does not fully account for biases arising from spatial clustering in the galaxy distribution. The latter is primarily driven by a foreground overdensity in the northern field within 30 Mpc, but more distant structure also plays a role. We provide updated measurements of the ALFALFA HI WF (and HI mass function) correcting for the variations in survey sensitivity. Only when systematic effects such as these are understood and corrected for can cosmological models be tested against the HI WF.


Brooks, R. A. N., Oman, K. A., & Frenk, C. S. (2023). The north–south asymmetry of the ALFALFA H i velocity width function. Monthly Notices of the Royal Astronomical Society, 522(3), 4043-4058.

Journal Article Type Article
Acceptance Date Apr 18, 2023
Online Publication Date Apr 17, 2023
Publication Date May 2, 2023
Deposit Date Oct 31, 2023
Publicly Available Date Nov 2, 2023
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 522
Issue 3
Pages 4043-4058
Public URL


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