Giant Molecular Association in Spiral Arms of M 31: Evidence for Dense Gas Formation via Spiral Shock Associated with Density Waves?

Abstract

We present observations of CO (J = 1–0), CO (J = 1–0), and CO (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 30 40 (0.6 kpc 0.8 kpc) with an angular resolution of 1600–1700 for CO (1–0) and CO (1–0), and 1:02 1:04 with 2300 for CO (3–2). The GMA has a size of a few 100 pc and a mass of 5.6 10Mˇ. The CO (1–0) to CO (1–0) integrated intensity ratio (R12=13) and the CO (3–2) to CO (1–0) ratio (R3 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) 10 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 R12=13 within the GMA, ranging from 6 at the center to 14 at the edges. The distribution of R12=13 shows a smooth structure with an overall density gradient. The GMA consists of two velocity components, blue ( 505 km s ) and red ( 490 km s ). In both the CO (1–0) and CO (1–0) profiles, the blue component shows a strong peak intensity and a narrow velocity width, while the red is weaker and wider. The R12=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.