CHIMPS2: ${}^{13}$CO $J=3\to 2$ emission in the Central Molecular Zone

Abstract

We present the initial data for the ($J=3\to 2$) transition of ${}^{13}$CO obtained from the Central Molecular Zone (CMZ) of the Milky Way as part of the CO Heterodyne Inner Milky Way Plane Survey 2 (CHIMPS2). Covering $359^{\circ }\le l\le 1^{\circ }$ and $|b|\le 0.5^{\circ }$ with an angular resolution of 19 arcsec, velocity resolution of 1 km s${}^{-1}$, and rms $T_A^{\ast }=0.59$ K at these resolutions, our observations unveil the complex structure of the CMZ molecular gas in improved detail. Complemented by the ${}^{12}$CO CHIMPS2 data, we estimate a median optical depth of ${\tau }_{13}=0.087$. The preliminary analysis yields a median ${}^{13}$CO column density range equal to $N{(}^{13}\text{CO})=2$--$5\times 10^{18}$ cm${}^{-2}$, median H${}_2$ column density equal to $N({\text{H}}_2)=4\times 10^{22}$ cm${}^{-2}$ to $1\times 10^{23}$ cm${}^{-2}$. We derive $N({\text{H}}_2)$-based total mass estimates of $M({\text{H}}_2)=2$--$6\times 10^7{M}_{\odot }$, in agreement with previous studies. We analyze the relationship between the integrated intensity of ${}^{13}$CO and the surface density of compact sources identified by Herschel Hi-GAL, and find that younger Hi-GAL sources detected at 500 $\mu$m but not at 70 $\mu$m follow the dense gas of the CMZ more closely than those that are bright at 70 $\mu$m. The latter, actively star-forming sources, appear to be more associated with material in the foreground spiral arms.

13 pages, 9 figures

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