CHIMPS2: 13CO J=3→2 emission in the Central Molecular Zone

We present the initial data for the (J = 3 → 2) transition of ¹³CO 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^∘ ≤ l ≤ 1^∘ and |b| ≤ 0.5^∘ with an angular resolution of 19 arcsec, velocity resolution of 1 km s⁻¹, and rms TA^* = 0.59 K at these resolutions, our observations unveil the complex structure of the CMZ molecular gas in improved detail. Complemented by the ¹²CO CHIMPS2 data, we estimate a median optical depth of τ₁₃ = 0.087. The preliminary analysis yields a median ¹³CO column density range equal to N(¹³CO)= 2--5 × 10¹⁸ cm⁻², median H₂ column density equal to N(H₂)= 4 × 10²² cm⁻² to 1 × 10²³ cm⁻². We derive N(H₂)-based total mass estimates of M(H₂)= 2--6 × 10⁷\, M_, in agreement with previous studies. We analyze the relationship between the integrated intensity of ¹³CO and the surface density of compact sources identified by Herschel Hi-GAL, and find that younger Hi-GAL sources detected at 500 μm but not at 70 μm follow the dense gas of the CMZ more closely than those that are bright at 70 μm. The latter, actively star-forming sources, appear to be more associated with material in the foreground spiral arms.