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

Abstract

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