The Bluedisks project, a study of unusually HI-rich galaxies: I. HI Sizes and Morphology

TL;DR

This study maps and analyzes the HI properties of unusually HI-rich galaxies, revealing their larger, more clumpy HI disks with no signs of recent disturbances, suggesting gas accretion from hot halos rather than mergers.

Contribution

First detailed analysis of HI sizes and morphology in a large sample of HI-rich galaxies, comparing them to control galaxies to understand their gas accretion mechanisms.

Findings

HI-rich galaxies follow the same HI size-mass relation as normal spirals.

HI-rich galaxies have larger HI-to-optical size ratios and more clumpy HI disks.

No evidence of recent mergers; gas may be accreted from hot halos.

Abstract

We introduce the "Bluedisk" project, a large program at the Westerbork Synthesis Radio Telescope (WSRT) that has mapped the HI in a sample of 23 nearby galaxies with unusually high HI mass fractions, along with a similar-sized sample of control galaxies matched in stellar mass, size, inclination and redshift. This paper presents the sample selection, observational set-up, data reduction strategy, and a first analysis of the sizes and structural properties of the HI disks. We find that the HI-rich galaxies lie on the same HI mass versus HI size relation as normal spiral galaxies, extending it to total HI masses of $2 \times 10^{10} M_{\odot}$ and radii R1 of $\sim 100$ kpc (where R1 is defined as the radius where the HI column density reaches 1 $M_{\odot}$ pc$^{-2}$). HI-rich galaxies have significantly larger values of HI-to-optical size ratio at fixed stellar mass, concentration index, stellar and star formation rate surface density compared to the control sample. The disks of HI-rich galaxies are also significantly more clumpy (i.e. have higher HI Gini and $\Delta$Area coefficient) than those of normal spirals. There is no evidence that the disks of HI-rich galaxies are more disturbed: HI-rich galaxies exhibit no difference with respect to control samples in their distributions of HI asymmetry indices or optical/HI disk position angle differences. In fact, the center of the HI distribution corresponds more closely with the center of the optical light in the HI-rich galaxies than in the controls. All these results argue against a scenario in which new gas has been brought in by mergers. It is possible that they may be more consistent with cooling from a surrounding quasi-static halo of warm/hot gas.