CO observations of a sample towards nearby galaxies

TL;DR

This study observed multiple CO isotopologues in 58 nearby spiral galaxies, revealing typical line ratios, correlations between luminosities, and an average X-factor slightly below the Milky Way's, enhancing understanding of molecular gas properties in galaxies.

Contribution

First simultaneous observations of $^{12}$CO, $^{13}$CO, and C$^{18}$O in a large galaxy sample, providing new insights into molecular gas ratios and conversion factors.

Findings

Most galaxies show $^{12}$CO/$^{13}$CO ratios between 5 and 13.

Strong correlations exist between $^{12}$CO and $^{13}$CO luminosities.

The average X-factor is slightly lower than in the Milky Way.

Abstract

We have simultaneously observed $^{12}$CO, $^{13}$CO, and C$^{18}$O (J=1$-$0) rotational transitions in the centers of a sample of 58 nearby spiral galaxies using the 13.7-m millimeter-wave telescope of the Purple Mountain Observatory. Forty-two galaxies were detected in $^{13}$CO emission, but there was a null detection for C$^{18}$O emission with a sigma upper limit of 2 mK. Central beam ratios, \R, of $^{12}$CO and $^{13}$CO range mostly from 5 to 13, with an average value of 8.1$\pm$4.2, which is slightly lower than previous estimates for normal galaxies. Clear correlations are found between $^{12}$CO and $^{13}$CO luminosities. An average $X$-factor of $1.44\pm0.84\times10^{20}\ \mathrm{cm}^{-2} (\mathrm{K} \mathrm{km} \mathrm{s}^{-1})^{-1}$ is slightly lower than that in the Milky Way.