Spatially resolved carbon and oxygen isotopic ratios in NGC 253 using
  optically thin tracers
  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 $\sim50$ 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$\sim21\pm6$, $^{16}$O/$^{18}$O$\sim130\pm40$, and
$^{18}$O/$^{17}$O$\sim4.5\pm0.8$ 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.
