Deep Andromeda JCMT-SCUBA2 Observations. The Submillimeter Maps and Giant Molecular Clouds
Authors
Sihan Jiao
Jingwen Wu
Hauyu Baobab Liu
Chao-Wei Tsai
Yuxin Lin
Di Li
Zhi-Yu Zhang
Yu Cheng
Linjing Feng
Henrik Beuther
Junzhi Wang
Lihwai Lin
Jakob den Brok
Ludan Zhang
Fengwei Xu
Fanyi Meng
Zongnan Li
Ryan P. Keenan
Si-Yue Yu
Niankun Yu
Zheng Zheng
Junhao Liu
Yuxiang Liu
Hao Ruan
Fangyuan Deng
Yuanzhen Xiong
Abstract
We have carried out unprecedentedly deep, nearly confusion-limited JCMT-SCUBA2 mapping observations on the nearest spiral galaxy, M31 (Andromeda). The 850 $μ$m image with a $\sim$50 pc resolution yields a comprehensive catalog of 383 giant molecular clouds (GMCs) that are associated with the spiral arms. In addition, it unveiled a population of 189 compact inter-arm GMCs in M31, which are mostly unresolved or marginally resolved. The masses of all these GMCs are in the range of 2$\times$10$^4$ -- 6$\times$10$^6$ $M_{\odot}$; the sizes are in the range of 30--130 pc. They follow a mass-size correlation, $M$ $\propto$ $R_{c}$$^{2.5}$. The inter-arm GMCs are systematically less massive, more diffuse, colder, and have lower star-forming efficiency (SFE) than on-arm GMCs. Moreover, within individual spatially resolved on-arm and off-arm M31 GMCs, the SFE is considerably lower than the SFE in molecular clouds in main sequence and green valley galaxies. Follow-up investigations on M31 GMCs may provide clues for how star formation may be quenched in galactic environments. Finally, we reconstrained the dust opacity spectral index $β$ in the M31 galaxy by combining our new JCMT observations with archival Herschel and Planck data and found that the radial variation of $β$ may not be as large as was proposed by previous studies.
