2007PASJ...59...33T

begin{HTML}We present observations of ¹²CO (J = 1-0), ¹³CO (J = 1-0), and ¹²CO (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 ¹²CO (1-0) and ¹³CO (1-0), and 1.2' x 1.4' with 23'' for ¹²CO (3-2). The GMA has a size of a few 100 pc and a mass of 5.6x10⁶ M_☉. The ¹²CO (1-0) to ¹³CO (1-0) integrated intensity ratio (R_12/13) and the ¹²CO (3-2) to ¹²CO (1-0) ratio (R_3-2/1-0), averaged over the entire region of the GMA, are ∼ 10 and 0.3, respectively. These line ratios suggest gas densities of (3-6) x 10² 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 (∼ -505 km s^-1) and red (>=-490 km s^-1). In both the ¹²CO (1-0) and ¹³CO (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.