Molecular gas and star formation in early-type galaxies

Despite pervasive residual star formation, early-type galaxies are
generally considered "red and dead", composed exclusively of old stars
with little gas and no star formation. Here, the molecular gas content
of early-type galaxies is constrained and discussed in relation to
their evolution, providing a much greater understanding of the gas
cycle in these objects and arguing for the continuing importance of
(minor) mergers and cold gas accretion. Early-types also provide a new
laboratory to study star formation, revealing unusual star-formation
scaling relations.

First, nearly a quarter 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. Despite this, the kinematics
of the molecular is often regular, allowing kinematic work ranging
from Tully-Fisher studies to black hole mass measurements. 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. 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
temperature-, density- and opacity-sensitive molecules in a few
objects.