The velocity width function of galaxies from the 40% ALFALFA survey: shedding light on the cold dark matter overabundance problem

TL;DR

This study uses the ALFALFA survey to measure the distribution of galaxy velocity widths, revealing a significant discrepancy with cold dark matter predictions and exploring potential solutions like warm dark matter or measurement biases.

Contribution

It provides the largest HI-selected galaxy sample to date and compares observed velocity width distributions with CDM simulations, highlighting the overabundance problem and proposing new tests for dark matter models.

Findings

Significant discrepancy between observed and predicted galaxy counts at low velocity widths.

Potential solutions include warm dark matter scenarios and measurement limitations of HI disk extents.

Statistical relationship between HI rotational velocity and host halo mass inferred for low-mass galaxies.

Abstract

The ongoing Arecibo Legacy Fast ALFA (ALFALFA) survey is a wide-area, extragalactic HI-line survey conducted at the Arecibo Observatory. Sources have so far been extracted over ~3,000 sq.deg of sky (40% of its final area), resulting in the largest HI-selected sample to date. We measure the space density of HI-bearing galaxies as a function of their observed velocity width (uncorrected for inclination) down to w = 20 km/s, a factor of 2 lower than the previous generation HI Parkes All-Sky Survey. We confirm previous results that indicate a substantial discrepancy between the observational distribution and the theoretical one expected in a cold dark matter (CDM) universe, at low widths. In particular, a comparison with synthetic galaxy samples populating state-of-the-art CDM simulations imply a factor of ~8 difference in the abundance of galaxies with w = 50 km/s (increasing to a factor of ~100 when extrapolated to the ALFALFA limit of w = 20 km/s). We furthermore identify possible solutions, including a keV warm dark matter scenario and the fact that HI disks in low mass galaxies are usually not extended enough to probe the full amplitude of the galactic rotation curve. In this latter case, we can statistically infer the relationship between the measured HI rotational velocity of a galaxy and the mass of its host CDM halo. Observational verification of the presented relationship at low velocities would provide an important test of the validity of the established dark matter model.