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تسجيل مستخدم جديدALMA Observations of Giant Molecular Clouds in M33 III: Spatially Resolved Features of the Star-Formation Inactive Million-solar-mass Cloud
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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 GMCs 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$ $M_{odot}$.
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