From HI to Stars: HI Depletion in Starbursts and Star-Forming Galaxies in the ALFALFA H-alpha Survey

TL;DR

This study investigates how neutral hydrogen (HI) gas relates to star formation in a large galaxy sample, revealing that HI depletion efficiency varies with galaxy type and structure, and is influenced by feedback, mergers, and gas distribution.

Contribution

It provides new insights into HI depletion times and their dependence on galaxy morphology, star formation activity, and structural properties in a statistically uniform, HI-selected galaxy sample.

Findings

Starbursts have shorter HI depletion times, indicating more efficient HI-to-H2 conversion.

HI to stellar mass ratios are similar in starbursts and non-starbursts, suggesting feedback does not deplete HI.

HI depletion time correlates with stellar surface density in disks but not in spheroids.

Abstract

HI in galaxies traces the fuel for future star formation and reveals the effects of feedback on neutral gas. Using a statistically uniform, HI-selected sample of 565 galaxies from the ALFALFA H-alpha survey, we explore HI properties as a function of star formation activity. ALFALFA H-alpha provides R-band and H-alpha imaging for a volume-limited subset of the 21-cm ALFALFA survey. We identify eight starbursts based on H-alpha equivalent width and six with enhanced star formation relative to the main sequence. Both starbursts and non-starbursts have similar HI to stellar mass ratios (MHI/M*), which suggests that feedback is not depleting the starbursts' HI. Consequently, the starbursts do have shorter HI depletion times (t_dep), implying more efficient HI-to-H2 conversion. While major mergers likely drive this enhanced efficiency in some starbursts, the lowest mass starbursts may experience periodic bursts, consistent with enhanced scatter in t_dep at low M*. Two starbursts appear to be pre-coalescence mergers; their elevated MHI/M* suggest that HI-to-H2 conversion is still ongoing at this stage. By comparing with the GASS sample, we find that t_dep anti-correlates with stellar surface density for disks, while spheroids show no such trend. Among early-type galaxies, t_dep does not correlate with bulge-to-disk ratio; instead, the gas distribution may determine the star formation efficiency. Finally, the weak connection between galaxies' specific star formation rates and MHI/M* contrasts with the well-known correlation between MHI/M* and color. We show that dust extinction can explain the HI-color trend, which may arise from the relationship between M*, MHI, and metallicity.