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The ALFALFA HI velocity width function

Oman, Kyle A.

The ALFALFA HI velocity width function Thumbnail



We make the most precise determination to date of the number density of extragalactic 21-cm radio sources as a function of their spectral line widths – the H I velocity width function (H I WF) – based on 21 827 sources from the final 7000deg2 data release of the Arecibo Legacy Fast ALFA (ALFALFA) survey. The number density of sources as a function of their neutral hydrogen masses – the H I mass function (H I MF) – has previously been reported to have a significantly different low-mass slope and ‘knee mass’ in the two sky regions surveyed during ALFALFA. In contrast with this, we find that the shape of the H I WF in the same two sky regions is remarkably similar, consistent with being identical within the confidence intervals implied by the data (but the overall normalization differs). The spatial uniformity of the H I WF implies that it is likely a stable tracer of the mass function of dark matter haloes, in spite of the environmental processes to which the measured variation in the H I MF are attributed, at least for galaxies containing enough neutral hydrogen to be detected. This insensitivity of the H I WF to galaxy formation and evolution can be exploited to turn it into a powerful constraint on cosmological models as future surveys yield increasingly precise measurements. We also report on the possible influence of a previously overlooked systematic error affecting the H I WF, which may plausibly see its low-velocity slope steepen by ∼40 per cent in analyses of future, deeper surveys. Finally, we provide an updated estimate of the ALFALFA completeness limit.


Oman, K. A. (2022). The ALFALFA HI velocity width function. Monthly Notices of the Royal Astronomical Society, 509(3), 3268-3284.

Journal Article Type Article
Acceptance Date Oct 27, 2021
Online Publication Date Nov 1, 2021
Publication Date 2022-01
Deposit Date Jan 17, 2022
Publicly Available Date Jan 17, 2022
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 509
Issue 3
Pages 3268-3284
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Copyright Statement
© 2021 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society.<br /> This is an Open Access article distributed under the terms of the Creative Commons Attribution License (, which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.

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