A panchromatic spatially resolved analysis of nearby galaxies -- II. The
  main sequence - gas relation at sub-kpc scale in grand-design spirals
  In the second work of this series, we analyse the connection between the
availability of gas and the position of a region with respect to the spatially
resolved main sequence (MS) relation. Following the procedure presented in
Paper I we obtain 500pc scales estimates of stellar mass and star formation
rate surface densities ($\Sigma_{\star}$ and $\Sigma_{\rm{SFR}}$). Our sample
consists of five face-on, grand design spiral galaxies located on the MS.
Thanks to HI 21cm and $^{12}$CO(2-1) maps, we connect the gas surface densities
and gas fractions to the observed star formation properties of each region. We
find that the spatially resolved MS ($\sigma=0.23$ dex) is the combination of
two relations: the Kennicutt-Schmidt law ($\sigma=0.19$ dex) and the molecular
gas MS (MGMS, $\sigma=0.22$ dex); $\Sigma_{\star}$, $\Sigma_{\rm{SFR}}$ and the
surface density of the molecular gas, $\Sigma_{\rm{H_2}}$, define a 3D relation
as proposed by \citet{2019ApJ...884L..33L}. We find that $\Sigma_{\rm{H_2}}$
steadily increases along the MS relation, varies little towards higher
$\Sigma_{\rm{SFR}}$ at fixed stellar surface densities (not enough to sustain
the change in SFR), and it is almost constant perpendicular to the relation.
The surface density of neutral gas ($\Sigma_{\rm{HI}}$) is constant along the
MS, and increases in its upper envelop. $\Sigma_{\rm{SFR}}$ can be expressed as
a function of $\Sigma_{\star}$ and $\Sigma_{\rm{HI}}$, following the Equation:
$\log\Sigma_{\rm{SFR}}$ = 0.97$\log\Sigma_{\star}$ + 1.99$\log\Sigma_{\rm{HI}}$
- 11.11. Finally, we show that f$_{\rm{gas}}$ increases significantly towards
the starburst region in the $\log\Sigma_{\star}$ - $\log\Sigma_{\rm{SFR}}$
plane, accompanied by a slight increase in SFE.
