Spitzer Observations of Passive and Star Forming Early-type Galaxies: an
  Infrared Color-Color Sequence
  We describe the infrared properties of a large sample of early type galaxies,
comparing data from the Spitzer archive with Ks-band emission from 2MASS. While
most representations of this data result in correlations with large scatter, we
find a remarkably tight relation among colors formed by ratios of luminosities
in Spitzer-MIPS (24, 70 and 160 um) bands and the Ks-band. Remarkably, this
correlation among E and S0 galaxies follows that of nearby normal galaxies of
all morphological types. In particular, the tight infrared color-color
correlation for S0 galaxies alone follows that of the entire Hubble sequence of
normal galaxies, roughly in order of galaxy type from ellipticals to spirals to
irregulars. The specific star formation rate of S0 galaxies estimated from the
24um luminosity increases with decreasing Ks-band luminosity (or stellar mass)
from essentially zero, as with most massive ellipticals, to rates typical of
irregular galaxies. Moreover, the luminosities of the many infrared-luminous S0
galaxies can significantly exceed those of the most luminous (presumably
post-merger) E galaxies. Star formation rates in the most infrared-luminous S0
galaxies approach 1-10 solar masses per year. Consistently with this picture we
find that while most early-type galaxies populate an infrared red sequence,
about 24% of the objects (mostly S0s) are in an infrared blue cloud together
with late type galaxies. For those early-type galaxies also observed at radio
frequencies we find that the far-infrared luminosities correlate with the mass
of neutral and molecular hydrogen, but the scatter is large. This scatter
suggests that the star formation may be intermittent or that similar S0
galaxies with cold gaseous disks of nearly equal mass can have varying radial
column density distributions that alter the local and global SF rates.
