ALFALFA Hα Reveals How Galaxies Use Their H i Fuel

• Published in: Proceedings of the International Astronomical Union 2015-08, Vol.11 (S315), Article E37
• Main Authors: Jaskot, Anne, Oey, Sally, Salzer, John, Van Sistine, Angie, Bell, Eric, Haynes, Martha
• Format: Article
• Language: English
• ISSN: 1743-9213; 1743-9221
• DOI: 10.1017/S1743921316007985

Atomic hydrogen traces the raw material from which molecular clouds and stars form. With 565 galaxies from the ALFALFA Hα survey, a statistically complete subset of the ALFALFA survey, we examine the processes that affect galaxies' abilities to access and consume their H i gas. On galaxy-wide scales, H i gas fractions correlate only weakly with instantaneous specific star formation rates (sSFRs) but tightly with galaxy color. We show that a connection between dust and H i content, arising from the fundamental mass-metallicity-H i relation, leads to this tight color correlation. We find that disk galaxies follow a relation between stellar surface density and H i depletion time, consistent with a scenario in which higher mid-plane pressure leads to more efficient molecular cloud formation from H i . In contrast, spheroids show no such trend. Starbursts, identified by Hα equivalent width, do not show enhanced H i gas fractions relative to similar mass non-starburst galaxies. The starbursts' shorter H i depletion times indicate more efficient consumption of H i , and galaxy interactions drive this enhanced star formation efficiency in several starbursts. Interestingly, the most disturbed starbursts show greater enhancements in H i gas fraction, which may indicate an excess of H i at early merger stages. At low galaxy stellar masses, the triggering mechanism for starbursts is less clear; the high scatter in efficiency and sSFR among low-mass galaxies may result from periodic bursts. We find no evidence for depleted H i reservoirs in starbursts, which suggests that galaxies may maintain sufficient H i to fuel multiple starburst episodes.