The velocity function of gas-rich galaxies

TL;DR

This paper measures the distribution of rotational velocities of gas-rich galaxies using HIPASS data, revealing a flat velocity function below 100 km/s and discrepancies with LCDM predictions at low velocities.

Contribution

First direct measurement of the velocity function of late-type galaxies from the same survey data, extending to lower velocities than previous indirect methods.

Findings

Velocity function is flat below 100 km/s with a slope of about -1.0.

Good agreement with previous measurements at higher velocities.

LCDM models over-predict galaxy counts below 100 km/s, especially at 30 km/s.

Abstract

We measure the distribution function of rotational velocities phi(V_c) of late-type galaxies from the HIPASS galaxy catalogue. Previous measurements of the late-type velocity function are indirect, derived by converting the galaxy luminosity function using the relation between galaxy luminosity and rotation velocity (the Tully-Fisher relation). The advantage of HIPASS is that space densities and velocity widths are both derived from the same survey data. We find good agreement with earlier inferred measurements of phi(V_c), but we are able to define the space density of objects with V_c as low as 30 km/s. The measured velocity function is `flat' (power-law slope alpha ~ -1.0) below V_c = 100 km/s. We compare our results with predictions based on LCDM simulations and find good agreement for rotational velocities in excess of 100 km/s, but at lower velocities current models over-predict the space density of objects. At V_c=30 km/s this discrepancy is approximately a factor 20.