The 107 GHz methanol transition is a dasar in G0.253+0.016

Abstract

We present observations of population anti-inversion in the $3_1-4_0{A}^{+}$ transition of CH${}_3$OH (methanol) at 107.013831 GHz toward the Galactic Center cloud G0.253+0.016 ("The Brick"). Anti-inversion of molecular level populations can result in absorption lines against the cosmic microwave background (CMB) in a phenomenon known as a "dasar." We model the physical conditions under which the 107 GHz methanol transition dases and determine that dasing occurs at densities below $10^6$ cm${}^{-3}$ and column densities between $10^{13}$ and $10^{16}$ cm${}^{-2}$. We also find that for this transition, dasing does not strongly depend on the gas kinetic temperature. We evaluate the potential of this tool for future deep galaxy surveys. We note that other works have already reported absorption in this transition (e.g., in NGC 253), but we provide the first definitive evidence that it is absorption against the CMB rather than against undetected continuum sources.

19 pages, 9 figures, 1 table. Accepted for publication in ApJ (August 14, 2023)

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