Giant molecular association in spiral arms of M 31: I. Evidence for dense gas formation via spiral shock associated with density waves?

We present observations of (CO)-C-12 (J = 1-0), (CO)-C-13 (J = 1-0), and (CO)-C-12 (J = 3-2) emissions toward a Giant Molecular Association (GMA) in the southern spiral arm of M 31 using the NRO 45 m and the ASTE 10 m telescopes. Observed regions are 3' x 4' (0.6 kpc x 0.8 kpc) with an angular resolution of 16"-17" for (CO)-C-12 (1-0) and (CO)-C-13 (1-0), and 1.'2 x 1.'4 with 23" for (CO)-C-12 (3-2). The GMA has a size of a few 100pc and a mass of 5.6x 10(6) M-circle dot. The (CO)-C-12 (1-0) to (CO)-C-13 (1-0) integrated intensity ratio (R-12/13) and the (CO)-C-12 (3-2) to (CO)-C-12 (1-0) ratio (R3-2/1-0), averaged over the entire region of the GMA, are similar to 10 and 0.3, respectively. These line ratios suggest gas densities of (3-6) x 10(2) cm(-3) at a temperature of 15-25 K, which are similar to, or slightly larger than, those of GMCs in the Galactic disk. We found a radial gradient of R-12/13 within the GMA, ranging from 6 at the center to 14 at the edges. The distribution of R-12/13 shows a smooth structure with an overall density gradient. The GMA consists of two velocity components, blue (similar to -505 km s(-1)) and red (>= -490 km s(-1)). In both the (CO)-C-12 (1-0) and (CO)-C-13 (1-0) profiles, the blue component shows a strong peak intensity and a narrow velocity width, while the red is weaker and wider. The R-12/13 value of the red component is 5 and that of the blue is 16, indicating that the red component has a higher gas density. Moreover, we suggest that the red component is "post-shock" dense gas decelerated by shock due to the density wave.