The Mopra Southern Galactic Plane CO Survey

Abstract

<jats:title>Abstract</jats:title><jats:p>We present the first results from a new carbon monoxide (CO) survey of the southern Galactic plane being conducted with the Mopra radio telescope in Australia. The <jats:sup>12</jats:sup>CO, <jats:sup>13</jats:sup>CO, and C<jats:sup>18</jats:sup>O <jats:italic>J</jats:italic> = 1–0 lines are being mapped over the <jats:inline-formula><jats:alternatives><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" mimetype="image" xlink:type="simple" xlink:href="S1323358013000222_inline1" /><jats:tex-math>$l = 305^{\circ }\text{--} 345^{\circ }, b = \pm 0.5^{\circ }$</jats:tex-math></jats:alternatives></jats:inline-formula> portion of the fourth quadrant of the Galaxy, at 35 arcsec spatial and 0.1 km s<jats:sup>−1</jats:sup> spectral resolution. The survey is being undertaken with two principal science objectives: (i) to determine where and how molecular clouds are forming in the Galaxy and (ii) to probe the connection between molecular clouds and the ‘missing’ gas inferred from gamma-ray observations. We describe the motivation for the survey, the instrumentation and observing techniques being applied, and the data reduction and analysis methodology. In this paper, we present the data from the first degree surveyed, <jats:inline-formula><jats:alternatives><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" mimetype="image" xlink:type="simple" xlink:href="S1323358013000222_inline2" /><jats:tex-math>$l = 323^{\circ } \text{--} 324^{\circ }, b = \pm 0.5^{\circ }$</jats:tex-math></jats:alternatives></jats:inline-formula>. We compare the data to the previous CO survey of this region and present metrics quantifying the performance being achieved; the rms sensitivity per 0.1 km s<jats:sup>−1</jats:sup> velocity channel is ~1.5 K for <jats:inline-formula><jats:alternatives><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" mimetype="image" xlink:type="simple" xlink:href="S1323358013000222_inline3" /><jats:tex-math>${\rm ^{12}CO}$</jats:tex-math></jats:alternatives></jats:inline-formula> and ~0.7 K for the other lines. We also present some results from the region surveyed, including line fluxes, column densities, molecular masses, <jats:inline-formula><jats:alternatives><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" mimetype="image" xlink:type="simple" xlink:href="S1323358013000222_inline4" /><jats:tex-math>${\rm ^{12}CO/^{13}CO}$</jats:tex-math></jats:alternatives></jats:inline-formula> line ratios, and <jats:inline-formula><jats:alternatives><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" mimetype="image" xlink:type="simple" xlink:href="S1323358013000222_inline5" /><jats:tex-math>${\rm ^{12}CO}$</jats:tex-math></jats:alternatives></jats:inline-formula> optical depths. We also examine how these quantities vary as a function of distance from the Sun when averaged over the 1 square degree survey area. Approximately 2 × 10<jats:sup>6</jats:sup>M<jats:sub>⊙</jats:sub> of molecular gas is found along the G323 sightline, with an average H<jats:sub>2</jats:sub> number density of <jats:inline-formula><jats:alternatives><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" mimetype="image" xlink:type="simple" xlink:href="S1323358013000222_inline6" /><jats:tex-math>$n_{\text{H}_2} \sim 1$</jats:tex-math></jats:alternatives></jats:inline-formula> cm<jats:sup>−3</jats:sup> within the Solar circle. The CO data cubes will be made publicly available as they are published.</jats:p>