Molecular gas and star formation in early-type galaxies
  We present new mm interferometric and optical integral-field unit (IFU)
observations and construct a sample of 12 E and S0 galaxies with molecular gas
which have both CO and optical maps. The galaxies contain 2 x 10^7 to 5 x 10^9
M\odot of molecular gas distributed primarily in central discs or rings (radii
0.5 to 4 kpc). The molecular gas distributions are always coincident with
distributions of optically-obscuring dust that reveal tightly-wound spiral
structures in many cases. The ionised gas always approximately corotates with
the molecular gas, evidencing a link between these two gas components, yet star
formation is not always the domi- nant ionisation source. The galaxies with
less molecular gas tend to have [O III]/H{\beta} emission-line ratios at high
values not expected for star formation. Most E/S0s with molecular gas have
young or intermediate age stellar populations based on optical colours,
ultraviolet colours and absorption linestrengths. The few that appear purely
old lie close to the limit where such populations would be undetectable based
on the mass fractions of expected young to observed old stars. The 8{\mu}m
polycyclic aromatic hydrocarbon (PAH) and 24{\mu}m emission yield similar star
formation rate estimates of E/S0s, but the total infrared overpredicts the rate
due to a contribution to dust heating from older stars. The radio-far infrared
relation also has much more scatter than for other star-forming galaxies.
However, despite these biases and additional scatter, the derived star
formation rates locate the E/S0 galaxies within the large range of the
Schmidt-Kennicutt and constant efficiency star formation laws. Thus the star
formation process in E/S0s is not overwhelmingly different than in other
star-forming galaxies, although one of the more reliable tracers (24{\mu}m)
points to a possible lower star-formation efficiency at a given gas surface
density.
