| Abstract: |
The molecular gas content of early-type galaxies is constrained and discussed in relation to their evolution. The observations provide a much greater understanding of the gas cycle in local early-type galaxies, and argue for the continuing importance of (minor) mergers and cold gas accretion. First, at least 23% of local early-types are shown to possess a substantial amount of molecular gas, the necessary ingredient for star formation, independent of mass and environment. Second, a variety of molecular gas morphologies is revealed. The kinematics of the molecular gas and stars are often misaligned, implying an external gas origin in at least half of the systems in the field, while external gas accretion is shut down in clusters. Third, many objects appear to be in the process of forming regular kpc-size decoupled disks, and a star formation sequence can be sketched by piecing together multi-wavelength information. Fourth, early-type galaxies do not seem to systematically obey all our usual prejudices regarding star formation (e.g. Kennicutt-Schmidt law and far infrared-radio continuum correlation), suggesting a greater diversity of star formation processes than observed in disk galaxies. Last, a first step toward constraining the physical properties of the molecular gas in early-type galaxies is taken, by modeling the line ratios of density- and opacity-sensitive molecules in a few objects. |
