H_I-MaNGA: tracing the physics of the neutral and ionized ISM with the second data release

We present the second data release for the H_I-MaNGA programme of H_I follow-up observations for the SDSS-IV MaNGA survey. This release contains measurements for 3669 unique galaxies, combining 2108 Green Bank Telescope observations with an updated crossmatch of the MaNGA sample with the ALFALFA survey. We combine these data with MaNGA spectroscopic measurements to examine relationships between H_I-to-stellar mass ratio (⁠M_HI/M∗⁠) and average ISM/star formation properties probed by optical emission lines. M_HI/M∗ is very weakly correlated with the equivalent width of H α, implying a loose connection between the instantaneous star formation rate and the H_I reservoir, although the link between M_HI/M∗ and star formation strengthens when averaged even over only moderate timescales (∼30 Myrs). Galaxies with elevated H_I depletion times have enhanced [O_I]/Hα and depressed Hα surface brightness, consistent with more H_I residing in a diffuse and/or shock heated phase which is less capable of condensing into molecular clouds. Of all optical lines, M_HI/M∗ correlates most strongly with oxygen equivalent width, EW(O), which is likely a result of the existing correlation between M_HI/M∗ and gas-phase metallicity. Residuals in the M_HI/M∗−EW(O) relation are again correlated with [O_I]/H α and H α surface brightness, suggesting they are also driven by variations in the fraction of diffuse and/or shock-heated gas. We recover the strong anti-correlation between M_HI/M∗ and gas-phase metallicity seen in previous studies. We also find a relationship between M_HI/M∗ and [O_I]6302/H α, suggesting that higher fractions of diffuse and/or shock-heated gas are more prevalent in gas-rich galaxies.