| Abstract | Strong CO emission in the J = 1-0 and the J = 2-1 lines is reported from the early-type galaxy NGC 404. The distribution and kinematics of the molecular clouds are determined from the ^12^CO, J = 1-0 line. ^13^CO,J = 1-0 emission is also detected, for the first time in an early-type galaxy. The CO emission is associated with a prominent dust lane in the central region of NGC 404, which might be part of an annular structure around the nucleus, reminiscent of structures seen in both H I and Hα in other early-type galaxies. The kinematical data indicate rotational motion of the gas around the center. The total molecular gas mass is determined in three different ways: from the ratio of the H_2_ column density to the integrated CO intensity, from a virial mass estimate and from the far-IR emissivity. There is a number of arguments indicating that NGC 404 is most likely situated outside the Local Group at a distance of 10 Mpc, rather than at 1.5 Mpc, as indicated by its systemic velocity. NGC 404 should therefore be classified as an S 0 rather than as a dwarf elliptical galaxy. At a distance of 10 Mpc, the molecular gas mass is about 7 10^7^ M_sun_, less than 10% of the H I mass and about 0.1 % of the total mass determined from the optical luminosity. The average number density of H_2_ is estimated to be of the order of 10^2^ -10^3^ cm-3. Convolving the CO, J = 2-1 measurements to an angular resolution corresponding to the J = 1-0 data, the line intensity ratio between the J = 2-1 and J = 1-0 transitions becomes 0.5. The ratio of far-IR to blue luminosity is smaller than for normal spiral galaxies, indicating that the current star formation rate (SFR), estimated to be 0.2 M_sun_ yr^-1^, is lower than the SFR averaged over a longer period. The current SFR may even be small relative to the rate at which stellar material is returned back to the interstellar medium. We estimate the star formation efficiency of NGC 404 to be similar to that of the Milky Way and a sample of field and weakly interacting spiral galaxies, indicating that spiral arm density waves may not be an important parameter in regulating the star formation efficiency in disk galaxies Furthermore, the presence of a large and massive molecular cloud complex in an S 0 galaxy such as NGC 404 implies that spiral density waves are not necessary for the formation of massive GMC complexes. |
