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

TL;DR

This large-scale CO survey of the Small Magellanic Cloud's northern region using ALMA's ACA provides detailed insights into molecular cloud structures and highlights the limitations of low-J CO emission in tracing H$_2$ in low-metallicity environments.

Contribution

The study offers the first comprehensive high-resolution CO mapping of the SMC's northern region, revealing the prevalence of small, faint clouds and the significant fraction of H$_2$ gas undetectable by CO.

Findings

Most CO clouds have low brightness temperatures (<1K).

Over 90% of H$_2$ gas is not traced by low-J CO emission.

Detected molecular clouds are smaller and fainter than previously known.

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 H$_2$ tracer in a low-metallicity environment ($Z\sim0.2~Z_{\odot}$). The total number of mosaic fields is $\sim$8000, which results in a field coverage of 0.26$~$degree$^{2}$ ($\sim$2.9 $\times$10$^{5}$$~$pc$^2$), corresponding to $\sim$10$\%$ area of the galaxy. The sensitive $\sim$2$~$pc resolution observations reveal the detailed structure of the molecular clouds previously detected in the single-dish NANTEN survey. We have detected a number of compact CO clouds within lower H$_2$ column density ($\sim$10$^{20}$$~$cm$^{-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 $\sim$90$\%$ H$_2$ gas cannot be traced by the low-$J$ CO emission. Our processed data cubes and 2-D images are publicly available.