From H i to stars: H I depletion in starbursts and star-forming galaxies in the ALFALFA Hα survey

H i in galaxies traces the fuel for future star formation and reveals the effects of feedback on neutral gas. Using a statistically uniform, H i-selected sample of 565 galaxies from the Arecibo Legacy Fast ALFA (ALFALFA) Hα survey, we explore H i properties as a function of star formation activity. ALFALFA Hα provides R-band and Hα imaging for a volume-limited subset of the 21 cm ALFALFA survey. We identify eight starbursts based on Hα equivalent width and six with enhanced star formation relative to the main sequence. Both starbursts and non-starbursts have similar H i-to-stellar mass ratios ($M_{HI}$/$M_{\ast }$), which suggests that feedback is not depleting the starbursts' H i. Consequently, the starbursts do have shorter H i depletion times ($t_{dep}$), implying more efficient H i-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 $M_{\ast }$. Two starbursts appear to be pre-coalescence mergers; their elevated $M_{HI}$/$M_{\ast }$ suggest that H i-to-H₂ conversion is still ongoing at this stage. By comparing with the GASS sample, we find that $t_{dep}$ anticorrelates 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 $M_{HI}$/$M_{\ast }$ contrasts with the well-known correlation between $M_{HI}$/$M_{\ast }$ and color. We show that dust extinction can explain the H i–color trend, which may arise from the relationship between $M_{\ast }$, $M_{HI}$, and metallicity.