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We have analyzed the data from a large-scale CO survey toward the northernregion of the Small Magellanic Cloud (SMC) obtained with the Atacama CompactArray (ACA) stand-alone mode of ALMA. The primary aim of this study is tocomprehensively understand the behavior of CO as an H$_2$ tracer in alow-metallicity environment ($Z\sim0.2~Z_{\odot}$). The total number of mosaicfields is $\sim$8000, which results in a field coverage of 0.26$~$degree$^{2}$($\sim$2.9 $\times$10$^{5}$$~$pc$^2$), corresponding to $\sim$10$\%$ area ofthe galaxy. The sensitive $\sim$2$~$pc resolution observations reveal thedetailed structure of the molecular clouds previously detected in thesingle-dish NANTEN survey. We have detected a number of compact CO cloudswithin lower H$_2$ column density ($\sim$10$^{20}$$~$cm$^{-2}$) regions whoseangular scale is similar to the ACA beam size. Most of the clouds in thissurvey also show peak brightness temperature as low as $<$1$~$K, which foroptically thick CO emission implies an emission size much smaller than the beamsize, leading to beam dilution. The comparison between an available estimationof the total molecular material traced by thermal dust emission and the presentCO survey demonstrates that more than $\sim$90$\%$ H$_2$ gas cannot be tracedby the low-$J$ CO emission. Our processed data cubes and 2-D images arepublicly available.

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