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

Abstract

Abstract We present observations of $^{12}$CO ($J$ = 1–0), $^{13}$CO ($J$ = 1–0), and $^{12}$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 3$^\prime \times$ 4$^\prime$ (0.6 kpc $\times$ 0.8 kpc) with an angular resolution of 16$^{\prime\prime}$–17$^{\prime\prime}$ for $^{12}$CO (1–0) and $^{13}$CO (1–0), and 1.$^\prime$2 $\times$ 1.$^\prime$4 with 23$^{\prime\prime}$ for $^{12}$CO (3–2). The GMA has a size of a few 100 pc and a mass of 5.6$\times 10^6 M_\odot$. The $^{12}$CO (1–0) to $^{13}$CO (1–0) integrated intensity ratio ($R_{12/13}$) and the $^{12}$CO (3–2) to $^{12}$CO (1–0) ratio ($R_{3-2/1-0}$), averaged over the entire region of the GMA, are $\sim 10$ and 0.3, respectively. These line ratios suggest gas densities of (3–6) $\times 10^2$ 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 ($\sim-505$ km s$^{-1}$) and red ($\geq-490$ km s$^{-1}$). In both the $^{12}$CO (1–0) and $^{13}$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 $R_{12/13}$ 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.