Galaxy Properties at the Faint End of the HI Mass Function

TL;DR

This study examines the properties of extremely low-mass, gas-rich dwarf galaxies using Hubble and radio observations, revealing their distances, star formation, and gas characteristics, and developing new methods to measure their HI rotation.

Contribution

It provides detailed analysis of 30 faint dwarf galaxies, including new distance measurements and a novel methodology for assessing their HI rotation, advancing understanding of galaxy evolution at low masses.

Findings

TRGB distances can be significantly larger than flow model estimates.

Most galaxies are in under-dense regions, with some in voids.

Star formation properties follow trends seen in higher mass systems.

Abstract

The Survey of HI in Extremely Low-mass Dwarfs (SHIELD) includes a volumetrically complete sample of 82 gas-rich dwarfs with M_HI~<10^7.2 Msun selected from the ALFALFA survey. We are obtaining extensive follow-up observations of the SHIELD galaxies to study their gas, stellar, and chemical content, and to better understand galaxy evolution at the faint end of the HI mass function. Here, we investigate the properties of 30 SHIELD galaxies using Hubble Space Telescope imaging of their resolved stars and Westerbork Synthesis Radio Telescope observations of their neutral hydrogen. We measure tip of the red giant branch (TRGB) distances, star formation activity, and gas properties. The TRGB distances are up to 4x greater than estimates from flow models, highlighting the importance of velocity-independent distance indicators in the nearby universe. The SHIELD galaxies are in under-dense regions, with 23% located in voids; one galaxy appears paired with a more massive dwarf. We quantify galaxy properties at low masses including stellar and HI masses, SFRs, sSFRs, SFEs, birthrate parameters, and gas fractions. The lowest mass systems lie below the mass thresholds where stellar mass assembly is predicted to be impacted by reionization. Even so, we find the star formation properties follow the same trends as higher mass gas-rich systems, albeit with a different normalization. The HI disks are small (<r><0.7 kpc) making it difficult to measure the HI rotation using standard techniques; we develop a new methodology and report the velocity extent, and its associated spatial extent, with robust uncertainties.