Extreme CO Isotopic Abundances in the ULIRG IRAS 13120-5453: An Extremely Young Starburst or Top-heavy Initial Mass Function

We present ALMA 12CO (J = 1-0, 3-2 and 6-5), 13CO (J = 1-0), and C18O (J = 1-0) observations of the local ultraluminous infrared galaxy (ULIRG) IRAS 13120-5453. The morphologies of the three isotopic species differ, as 13CO shows a hole in emission toward the center. We measure integrated brightness temperature line ratios of 12CO/13CO ≥ 60 (exceeding 200) and 13CO/C18O ≤ 1 in the central region. Assuming optical thin emission, C18O is more …