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HI in galaxies traces the fuel for future star formation and reveals theeffects of feedback on neutral gas. Using a statistically uniform, HI-selectedsample of 565 galaxies from the ALFALFA H-alpha survey, we explore HIproperties as a function of star formation activity. ALFALFA H-alpha providesR-band and H-alpha imaging for a volume-limited subset of the 21-cm ALFALFAsurvey. We identify eight starbursts based on H-alpha equivalent width and sixwith enhanced star formation relative to the main sequence. Both starbursts andnon-starbursts have similar HI to stellar mass ratios (MHI/M*), which suggeststhat feedback is not depleting the starbursts' HI. Consequently, the starburstsdo have shorter HI depletion times (t_dep), implying more efficient HI-to-H2conversion. While major mergers likely drive this enhanced efficiency in somestarbursts, the lowest mass starbursts may experience periodic bursts,consistent with enhanced scatter in t_dep at low M*. Two starbursts appear tobe pre-coalescence mergers; their elevated MHI/M* suggest that HI-to-H2conversion 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 notcorrelate with bulge-to-disk ratio; instead, the gas distribution may determinethe star formation efficiency. Finally, the weak connection between galaxies'specific star formation rates and MHI/M* contrasts with the well-knowncorrelation between MHI/M* and color. We show that dust extinction can explainthe HI-color trend, which may arise from the relationship between M*, MHI, andmetallicity.

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