HI Selected Galaxies in the Sloan Digital Sky Survey II: The Colors of Gas-Rich Galaxies

TL;DR

This study investigates the colors of HI-selected galaxies, revealing that they are predominantly bluer due to recent star formation bursts, with emission lines significantly affecting their observed colors and low-mass galaxies showing more color variation.

Contribution

It demonstrates that HI selection biases the galaxy sample towards bluer, star-forming galaxies and highlights the importance of emission lines in modeling galaxy colors.

Findings

HI selection results in bluer galaxy colors due to recent star formation.

Emission lines significantly influence the colors, especially r-i, of the bluest galaxies.

Low-mass galaxies exhibit increased color dispersion, possibly due to different star formation processes.

Abstract

We utilize color information for an HI-selected sample of 195 galaxies to explore the star formation histories and physical conditions that produce the observed colors. We show that the HI selection creates a significant offset towards bluer colors that can be explained by enhanced recent bursts of star formation. There is also no obvious color bimodality, because the HI selection restricts the sample to bluer, actively star forming systems, diminishing the importance of the red sequence. Rising star formation rates are still required to explain the colors of galaxies bluer than g-r < 0.3. We also demonstrate that the colors of the bluest galaxies in our sample are dominated by emission lines and that stellar population synthesis models alone (without emission lines) are not adequate for reproducing many of the galaxy colors. These emission lines produce large changes in the r-i colors but leave the g-r color largely unchanged. In addition, we find an increase in the dispersion of galaxy colors at low masses that may be the result of a change in the star formation process in low-mass galaxies.