Molecular gas and dust in NGC 4550 - A galaxy with two counterrotating stellar disks

We report the detection of $1 \times 10^7$ $M_{\odot}$ of molecular gas in the central region of the S0/E7 galaxy NGC 4550, inferred from observations of CO(1-0) emission. Dust is detected in HST WFPC2 images and found to be asymmetrically distributed around the nucleus, only extending to a galactocentric distance of 7 '' (600 pc). The shape of the CO emission profile is consistent with a molecular gas distribution following the dust. The distribution of the dust and gas in the center could be the result of an $m=1$ instability, which is the fastest growing unstable mode in counterrotating stellar disks. On a global scale the molecular gas in NGC 4550 is stable against gravitational collapse but nevertheless star formation appears to be ongoing with normal star formation efficiency and gas consumption time scales. The stellar velocity dispersion in NGC 4550 resembles that of elliptical galaxies. It is therefore likely that a hot X-ray emitting plasma limits the lifetime of the molecular gas, that must arise from a recent ( ≪ 1 Gyr) accretion event.