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 abundant than 13CO in several regions. The abundances within the central 500 pc are consistent with the enrichment of the interstellar medium via a young starburst (<7 Myr), a top-heavy initial mass function, or a combination of both.