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Molecular gas plays a critical role in explaining the quiescence of starformation (SF) in massive isolated spiral galaxies, which could be a result ofeither the low molecular gas content and/or the low SF efficiency. We presentIRAM 30m observations of the CO lines in the Sombrero galaxy (NGC~4594), themost massive spiral at $d\lesssim30\rm~Mpc$. We detect at least one of thethree CO lines covered by our observations in all 13 observed positions locatedat the galactic nucleus and along a $\sim25\rm~kpc$-diameter dusty ring. Thetotal extrapolated molecular gas mass of the galaxy is $M_{\rmH_2}\approx4\times10^{8}\rm~M_\odot$. The measured maximum CO gas rotationvelocity of $\approx379\rm~km~s^{-1}$ suggests that NGC~4594 locates in a darkmatter halo with a mass $M_{\rm200}\gtrsim10^{13}\rm~M_\odot$. Comparing toother galaxy samples, NGC~4594 is extremely gas poor and SF inactive, but theSF efficiency is apparently not inconsistent with that predicted by theKennicutt-Schmidt law, so there is no evidence of enhanced SF quenching in thisextremely massive spiral with a huge bulge. We also calculate the predicted gassupply rate from various sources to replenish the cold gas consumed in SF, andfind that the galaxy must experienced a starburst stage at high redshift, thenthe leftover or recycled gas provides SF fuels to maintain the gradual growthof the galactic disk at a gentle rate.

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