CO-CHANGES I: IRAM 30m CO Observations of Molecular Gas in the Sombrero Galaxy

Abstract

Molecular gas plays a critical role in explaining the quiescence of star formation (SF) in massive isolated spiral galaxies, which could be a result of either the low molecular gas content and/or the low SF efficiency. We present IRAM 30m observations of the CO lines in the Sombrero galaxy (NGC~4594), the most massive spiral at $d\lesssim 30\mathrm{Mpc}$. We detect at least one of the three CO lines covered by our observations in all 13 observed positions located at the galactic nucleus and along a $\sim 25\mathrm{kpc}$-diameter dusty ring. The total extrapolated molecular gas mass of the galaxy is $M_{{\mathrm{H}}_2}\approx 4\times 10^8{\mathrm{M}}_{\odot }$. The measured maximum CO gas rotation velocity of $\approx 379\mathrm{km}{\mathrm{s}}^{-1}$ suggests that NGC~4594 locates in a dark matter halo with a mass $M_{200}\gtrsim 10^{13}{\mathrm{M}}_{\odot }$. Comparing to other galaxy samples, NGC~4594 is extremely gas poor and SF inactive, but the SF efficiency is apparently not inconsistent with that predicted by the Kennicutt-Schmidt law, so there is no evidence of enhanced SF quenching in this extremely massive spiral with a huge bulge. We also calculate the predicted gas supply rate from various sources to replenish the cold gas consumed in SF, and find that the galaxy must experienced a starburst stage at high redshift, then the leftover or recycled gas provides SF fuels to maintain the gradual growth of the galactic disk at a gentle rate.

21 pages, 13 figures, accepted for publication in MNRAS