CO Multi-line Imaging of Nearby Galaxies (COMING): I. Physical
  properties of molecular gas in the barred spiral galaxy NGC 2903
  We present simultaneous mappings of J=1-0 emission of 12CO, 13CO, and C18O
molecules toward the whole disk (8' x 5' or 20.8 kpc x 13.0 kpc) of the nearby
barred spiral galaxy NGC 2903 with the Nobeyama Radio Observatory 45-m
telescope at an effective angular resolution of 20" (or 870 pc). We detected
12CO(J=1-0) emission over the disk of NGC 2903. In addition, significant
13CO(J=1-0) emission was found at the center and bar-ends, whereas we could not
detect any significant C18O(J=1-0) emission. In order to improve the
signal-to-noise ratio of CO emission and to obtain accurate line ratios of
12CO(J=2-1)/12CO(J=1-0) ($R_{2-1/1-0}$) and 13CO(J=1-0)/12CO(J=1-0)
($R_{13/12}$), we performed the stacking analysis for our 12CO(J=1-0),
13CO(J=1-0), and archival 12CO(J=2-1) spectra with velocity-axis alignment in
nine representative regions of NGC 2903. We successfully obtained the stacked
spectra of the three CO lines, and could measure averaged $R_{2-1/1-0}$ and
$R_{13/12}$ with high significance for all the regions. We found that both
$R_{2-1/1-0}$ and $R_{13/12}$ differ according to the regions, which reflects
the difference in the physical properties of molecular gas; i.e., density
($n_{\rm H_2}$) and kinetic temperature ($T_K$). We determined $n_{\rm H_2}$
and $T_K$ using $R_{2-1/1-0}$ and $R_{13/12}$ based on the large velocity
gradient approximation. The derived $n_{\rm H_2}$ ranges from ~ 1000 cm$^{-3}$
(in the bar, bar-ends, and spiral arms) to 3700 cm$^{-3}$ (at the center) and
the derived $T_K$ ranges from 10 K (in the bar and spiral arms) to 30 K (at the
center). We examined the dependence of star formation efficiencies (SFEs) on
$n_{\rm H_2}$ and $T_K$, and found the positive correlation between SFE and
$n_{\rm H_2}$ with the correlation coefficient for the least-square power-law
fit $R^2$ of 0.50. This suggests that molecular gas density governs the spatial
variations in SFEs.
