Gravitational instability and star formation in NGC 628
  The gas-stars instability criterion for infinitesimally thin disc was applied
to the galaxy NGC 628. Instead of using the azimuthally averaged profiles of
data the maps of the gas surface densities (THINGS, HERACLES), of the velocity
dispersions of stars (VENGA) and gas (THINGS) and of the surface brightness of
the galaxy (S$^4$G) were analyzed. All these maps were collected for the same
region with a noticeable star formation rate and were superimposed on each
other. Using the data on the rotation curve values of $Q_\mathrm{eff}$ were
calculated for each pixel in the image. The areas within the contours
$Q_\mathrm{eff}<3$ were compared with the ongoing star formation regions
($\Sigma_\mathrm{SFR}> 0.007 \, M_{\odot}$yr$^{-1}$kpc$^{-2}$) and showed a
good coincidence between them. The Romeo-Falstad disc instability diagnostics
taking into account the thickness of the stellar and gas layers does not change
the result. If the one-fluid instability criterion is used, the coincidence is
worse. The analysis was carried out for the area $r < $ 0.5$r_{25} $. Leroy et
al. (2008) using azimuthally averaged data obtained $Q_\mathrm{eff} \approx
3-4$ for this area of the disc, which makes it stable against non-axisymmetric
perturbations and gas dissipation, and does not predict the location of star
forming regions. Since in the galaxies the distribution of hydrogen and the
regions of star formation is often patchy, the relationship between
gravitational instability and star formation should be sought using data maps
rather than azimuthally averaged data.
