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 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 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 relationship between M_HI/M_* and [OI]/Halpha,
suggesting that higher fractions of diffuse and/or shock-heated gas are more
prevalent in gas-rich galaxies.
