CO observations of a sample towards nearby galaxies

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 $1.44\pm 0.84\times 10^{20}{\mathrm{cm}}^{-2}(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}{)}^{-1}$ is slightly lower than that in the Milky Way.

arXiv comment17 pages, 4 figures, accepted for publication in RAA journal

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• Dense gas tracing the collisional past of Andromeda. An atypical inner region?