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

Abstract

We present ALMA (CO)-C-12 (J = 1-0, 3-2 and 6-5), (CO)-C-13(J = 1-0), and (CO)-O-18 (J = 1-0) observations of the local ultraluminous infrared galaxy (ULIRG) IRAS. 13120-5453. The morphologies of the three isotopic species differ, as (CO)-C-13 shows a hole in emission toward the center. We measure integrated brightness temperature line ratios of (CO)-C-12/(CO)-C-13 >= 60 (exceeding 200) and (CO)-C-13/(CO)-O-18 <= 1 in the central region. Assuming optical thin emission, (CO)-O-18 is more abundant than (CO)-C-13 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.