Origins of the north-south asymmetry in the ALFALFA HI velocity width function

TL;DR

This study investigates the north-south asymmetry in the ALFALFA HI velocity width function, attributing it to survey systematics and demonstrating how corrected data can better test cosmological models.

Contribution

The paper identifies survey sensitivity and clustering biases as sources of asymmetry and shows how correcting these allows for more accurate cosmological testing.

Findings

Survey sensitivity differs between northern and southern fields.

Clustering biases affect completeness corrections.

Correcting systematics aligns observations with cosmological predictions.

Abstract

The number density of extragalactic 21-cm radio sources as a function of their spectral line-widths -- the HI width function (HIWF) -- is a tracer of the dark matter halo mass function. The ALFALFA 21-cm survey measured the HIWF in northern and southern Galactic fields finding a systematically higher number density in the north; an asymmetry which is in tension with $\Lambda$ cold dark matter models which predicts the HIWF should be identical everywhere if sampled in sufficiently large volumes. We use the Sibelius-DARK N-body simulation and semi-analytical galaxy formation model GALFORM to create mock ALFALFA surveys to investigate survey systematics. We find the asymmetry has two origins: the sensitivity of the survey is different in the two fields, and the algorithm used for completeness corrections does not fully account for biases arising from spatial galaxy clustering. Once survey systematics are corrected, cosmological models can be tested against the HIWF.