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' × 5' or 20.8 kpc × 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_H_2) and kinetic temperature (T_K). We determined n_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_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_H_2 and T_K, and found a positive correlation between SFE and n_H_2 with correlation coefficient for the least-squares power-law fit R² of 0.50. This suggests that molecular gas density governs the spatial variations in SFEs.