HI-MaNGA: Tracing the physics of the neutral and ionized ISM with the second data release

We present the second data release for the HI-MaNGA programme of HI follow-up observations for the SDSS-IV MaNGA survey. This release contains HI measurements from 2135 Green Bank Telescope (GBT) observations and an updated crossmatch of the SDSS-IV MaNGA sample with the ALFALFA survey. We combine these data with MaNGA spectroscopic measurements to examine relationships between HI-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 Halpha, implying a loose connection between the instantaneous star formation rate and the HI reservoir, although the link between M_HI/M_* and star formation strengthens when averaged even over only moderate timescales (~30 Myrs). Galaxies with elevated HI depletion times have enhanced [OI]/Halpha and depressed Halpha surface brightness, consistent with more HI 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 [OI]/Halpha and Halpha 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 positive relationship between M_HI/M_* and [OI]/Halpha, independent of metallicity, which may suggest that higher fractions of diffuse and/or shock heated gas are more prevalent in gas-rich galaxies.