First detection of a deuterated molecule in a starburst environment within NGC 253

Abstract

Deuterium was primarily created during the Big Bang nucleosynthesis. This fact, alongside its fractionation reactions resulting in enhanced abundances of deuterated molecules, means that deuterium abundances can be used to better understand many processes within the interstellar medium as well as its history. Previously, observations of deuterated molecules have been limited to the Galaxy, the Magellanic Clouds, and (with respect to HD) to quasar absorption spectra. We present the first robust detection of a deuterated molecule in a starburst environment and, apart from HD, the first one detected outside the Local Group. As such, we could constrain the deuterium fractionation as observed by DCN. We observed the central molecular zone (CMZ) of the nearby starburst galaxy NGC 253 covering multiple giant molecular clouds (GMCs) with cloud scale observations ($ 30$ pc) using the Atacama Large Millimeter/submillimeter Array. Via the MADCUBA package, we were able to perform local thermodynamic equilibrium analysis in order to obtain deuterium fractionation estimates. We detected DCN in the nuclear region of the starburst galaxy NGC 253 and estimated the deuterium fractionation (D/H ratio) of DCN within the GMCs of the CMZ of NGC 253. We found a range of $5 $ to $10 $, which is relatively similar to the values observed in warm galactic star-forming regions. We also determined an upper limit of D/H of $8 $ from DCO within one region, closer to the cosmic value of D/H. Our observations of deuterated molecules within NGC 253 appear to be consistent with previous galactic studies of star-forming regions. This implies that warmer gas temperatures increase the abundance of DCN relative to other deuterated species. This study also further expands the regions, particularly in the extragalactic domain, in which deuterated species have been detected.