First detection of CO isotopologues in a high-redshift main-sequence galaxy: evidence of a top-heavy stellar initial mass function

TL;DR

This study presents the first detection of CO isotopologues in a high-redshift main-sequence galaxy, providing evidence that such galaxies have a top-heavy stellar initial mass function, similar to starburst galaxies.

Contribution

It provides the first measurement of CO isotopologues in a high-redshift main-sequence galaxy, revealing a top-heavy IMF at early cosmic times.

Findings

Detected $^{13}$CO and C$^{18}$O in a high-redshift galaxy.

Found a low $^{13}$C/$^{18}$O ratio indicating a top-heavy IMF.

Chemical models support a more top-heavy IMF than the Milky Way.

Abstract

Recent observations and theories have presented a strong challenge to the universality of the stellar initial mass function (IMF) in extreme environments. A notable example has been found for starburst conditions, where evidence favours a top-heavy IMF, i.e. there is a bias toward massive stars compared to the IMF that is responsible for the stellar mass function and elemental abundances observed in the Milky Way. Local starburst galaxies have star-formation rates similar to those in high-redshift main-sequence galaxies, which appear to dominate the stellar mass budget at early epochs. However, the IMF of high-redshift main-sequence galaxies is yet to be probed. Since $^{13}$CO and C$^{18}$O isotopologues are sensitive to the IMF, we have observed these lines towards four strongly-lensed high-redshift main-sequence galaxies using the Atacama Large Millimeter/sub-millimeter Array. Of our four targets, SDSS J0901+1814, at $z \approx 2.26$, is seen clearly in $^{13}$CO and C$^{18}$O, the first detection of CO isotopologues in the high-redshift main-sequence galaxy population. The observed $^{13}$C/$^{18}$O ratio, $2.4 \pm 0.8$, is significantly lower than that of local main-sequence galaxies. We estimate the isotope ratio, oxygen abundance and stellar mass using a series of chemical evolution models with varying star-formation histories and IMFs. All models favour an IMF that is more top-heavy than that of the Milky Way. Thus, as with starburst galaxies, main-sequence galaxies in the high-redshift Universe have a greater fraction of massive stars than a Milky-Way IMF would imply.