Spatially resolved carbon and oxygen isotopic ratios in NGC 253 using optically thin tracers

Abstract

To trace chemical evolution in a prototypical starburst environment, we spatially resolve the carbon and oxygen isotope ratios across the central molecular zone (full size ~$\sim 600$ pc) in the nearby starburst galaxy NGC 253. We imaged the emission of the optically thin isotopologues ${}^{13}$CO, C${}^{18}$O, C${}^{17}$O, ${}^{13}$C${}^{18}$O at a spatial resolution $\sim 50$ pc, comparable to the typical size of giant molecular associations. Optical depth effects and contamination of ${}^{13}$C${}^{18}$O by C${}_4$H is discussed and accounted for to derive column densities. This is the first extragalactic detection of the double isotopologue ${}^{13}$C${}^{18}$O. Derived isotopic ratios ${}^{12}$C/${}^{13}$C$\sim 21\pm 6$, ${}^{16}$O/${}^{18}$O$\sim 130\pm 40$, and ${}^{18}$O/${}^{17}$O$\sim 4.5\pm 0.8$ differ from the generally adopted values in the nuclei of galaxies. The molecular clouds in the central region of NGC 253 show similar rare isotope enrichment to those within the central molecular zone of the Milky way. This enrichment is attributed to stellar nucleosynthesis. Measured isotopic ratios suggest an enhancement of ${}^{18}$O as compared to our Galactic center, which we attribute to an extra ${}^{18}$O injection from massive stars. Our observations show evidence for mixing of distinct gas components with different degrees of processing. We observe an extra molecular component of highly processed gas on top of the already proposed less processed gas being transported to the central region of NGC 253. Such multicomponent nature and optical depth effects may hinder the use of isotopic ratios based on spatially unresolved line to infer the star formation history and/or initial stellar mass function properties in the nuclei of galaxies.

14 pages, 9 figures, Accepted for publication in A&A

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Cited by in corpus (8)

• ALMA Observations of Multiple-CO and C Lines Toward the Active Galactic Nucleus of NGC 7469: X-Ray-dominated Region Caught in the Act
• Tracing Interstellar Heating: An ALCHEMI Measurement of the HCN Isomers in NGC 253
• Extreme CO Isotopologue Line Ratios in ULIRGS: Evidence for a top-heavy IMF
• The Molecular ISM in the Super Star Clusters of the Starburst NGC 253
• A chemical study of carbon fractionation in external galaxies
• The Evolution of NGC 7465 as Revealed by its Molecular Gas Properties
• VALES VI: ISM enrichment in star-forming galaxies up to z$\sim$0.2 using ${}^{12}$CO(1-0), ${}^{13}$CO(1-0) and C${}^{18}$O(1-0) line luminosity ratios
• When did the initial mass function become bottom-heavy?