ALMA Observations of Giant Molecular Clouds in M33 III: Spatially Resolved Features of the Star-Formation Inactive Million-solar-mass Cloud

2 Mar 2021  ·  Hiroshi Kondo, Kazuki Tokuda, Kazuyuki Muraoka, Atsushi Nishimura, Shinji Fujita, Tomoka Tosaki, Sarolta Zahorecz, Rie E. Miura, Masato I. N. Kobayashi, Sachiko Onodera, Kazufumi Torii, Nario Kuno, Hidetoshi Sano, Toshikazu Onishi, Kazuya Saigo, Yasuo Fukui, Akiko Kawamura, Kisetsu Tsuge, Kengo Tachihara ·

We present $^{12}$CO ($J$ = 2-1), $^{13}$CO ($J$ = 2-1), and C$^{18}$O ($J$ = 2-1) observations toward GMC-8, one of the most massive giant molecular clouds (GMCs) in M33 using ALMA with an angular resolution of 0".44 $\times$ 0".27 ($\sim$2 pc $\times$ 1pc). The earlier studies revealed that its high-mass star formation is inactive in spite of a sufficient molecular reservoir with the total mass of $\sim$10$^{6}$ $M_{\odot}$... The high-angular resolution data enable us to resolve this peculiar source down to a molecular clump scale. One of the GMC's remarkable features is that a round-shaped gas structure (the "Main cloud" ) extends over $\sim$50 pc scale, which is quite different from the other two active star-forming GMCs dominated by remarkable filaments/shells obtained by our series of studies in M33. The fraction of the relatively dense gas traced by the $^{13}$CO data with respect to the total molecular mass is only $\sim$2 %, suggesting that their spatial structure and the density are not well developed to reach an active star formation. The CO velocity analysis shows that the GMC is composed of a single component as a whole, but we found some local velocity fluctuations in the Main cloud and extra blueshifted components at the outer regions. Comparing the CO with previously published large-scale H I data, we suggest that an external atomic gas flow supplied a sufficient amount of material to grow the GMC up to $\sim$10$^6$ $M_{\odot}$. read more

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Astrophysics of Galaxies