An Unbiased CO Survey toward the Northern Region of the Small Magellanic Cloud with the Atacama Compact Array. I. Overview: CO Cloud Distributions

Abstract

We have analyzed the data from a large-scale CO survey toward the northern region of the Small Magellanic Cloud (SMC) obtained with the Atacama Compact Array (ACA) stand-alone mode of ALMA. The primary aim of this study is to comprehensively understand the behavior of CO as an H2 tracer in a low-metallicity environment (Z ∼ 0.2 Z ⊙). The total number of mosaic fields is ∼8000, which results in a field coverage of 0.26 deg2 (∼2.9 ×105 pc2), corresponding to ∼10% of the area of the galaxy. The sensitive ∼2 pc resolution observations reveal the detailed structure of the molecular clouds previously detected in the single-dish NANTEN survey. We have detected a number of compact CO clouds within lower H2 column density (∼1020 cm−2) regions whose angular scale is similar to the ACA beam size. Most of the clouds in this survey also show peak brightness temperature as low as <1 K, which for optically thick CO emission implies an emission size much smaller than the beam size, leading to beam dilution. The comparison between an available estimation of the total molecular material traced by thermal dust emission and the present CO survey demonstrates that more than ∼90% of H2 gas cannot be traced by the low-J CO emission. Our processed data cubes and 2D images are publicly available.

An ALMA Study of the Massive Molecular Clump N159W-North in the Large Magellanic Cloud: A Possible Gas Flow Penetrating One of the Most Massive Protocluster Systems in the Local Group

“…Table 4 lists several massive clumps studied in the Local Group of galaxies (MW, LMC, and M33). Note that a large-scale survey of molecular gas has been conducted at the SMC, but such a massive clump has not been discovered yet (Tokuda et al 2021). The mass estimates based on the thermal dust emission and/or molecular line emission are subject to a factor of 2 or 3 uncertainties due to methodological differences and systematic errors arising from, e.g., dust properties and molecular abundance.…”

The First Detection of a Protostellar CO Outflow in the Small Magellanic Cloud with ALMA

“…Such environments are rarely seen in typical CO-detectable molecular clouds, but some local regions show stronger CO(3-2) and higher J emission than lower-J CO lines, e.g., at a cloud edge (Tachihara et al 2012) and protostellar outflows (e.g., Shimajiri et al 2008;Lefloch et al 2015). This behavior may be further enhanced in the SMC because CO lines trace denser and warmer gas in lowermetallicity environments (e.g., Muraoka et al 2017;Tokuda et al 2021) than in typical giant molecular clouds in the Galaxy (e.g., Nishimura et al 2015). We thus speculate that CO(3-2) worked favorably as an outflow tracer in the SMC.…”

“…These discoveries strongly demonstrated the universality of the outflow activities associated with (high-mass) protostars within the local universe. However, any high-resolution CO observations in the Small Magellanic Cloud (SMC), which is in the lowest metallicity condition (Z ∼ 0.2 Z e ) among the moleculargas-accessible Local Group of galaxies, have yet to discover molecular components of outflows from protostellar sources (Muraoka et al 2017;Jameson et al 2018;Neelamkodan et al 2021;Tokuda et al 2021). Because Ward et al (2017) reported shocked H 2 emission originating from outflows of massive YSOs in the SMC N81 and N88 A regions, the presence of outflows seems to be fairly likely.…”

The First Detection of a Protostellar CO Outflow in the Small Magellanic Cloud with ALMA

“…Such environments are rarely seen in typical CO detectable molecular clouds, but some local regions show stronger CO(3-2) and higher J emission than lower-J CO lines, e.g., at a cloud edge (Tachihara et al 2012) and protostellar outflows (e.g., Shimajiri et al 2008;Lefloch et al 2015). This behavior may be further enhanced in the SMC because CO lines trace denser and warmer gas in lower-metallicity environments (e.g., Muraoka et al 2017;Tokuda et al 2021) than in typical giant molecular clouds in the Galaxy (e.g., Nishimura et al 2015). We thus speculate that CO(3-2) worked favorably as an outflow tracer in the SMC.…”

“…These discoveries strongly demonstrated the universality of the outflow activities associated with (highmass) protostars within the local universe. However, any high-resolution CO observations in the Small Magellanic Cloud (SMC), which is in the lowest metallicity condition (Z ∼0.2 Z ⊙ ) among the molecular gas accessible Local Group of galaxies, have yet to discover molecular components of outflows from protostellar sources (Muraoka et al 2017;Jameson et al 2018;Neelamkodan et al 2021;Tokuda et al 2021). Because Ward et al (2017) reported shocked H 2 emission originated from outflows of massive YSOs in the SMC N81 and N88 A regions, the presence of outflow seems to be fairly likely.…”