Variation of Mid and Far-IR Luminosities among Early-Type Galaxies:
  Relation to Stellar Metallicity and Cold Dust
  The Hubble morphological sequence from early to late galaxies corresponds to
an increasing rate of specific star formation. The Hubble sequence also follows
a banana-shaped correlation between 24 and 70 micron luminosities, both
normalized with the K-band luminosity. We show that this correlation is
significantly tightened if galaxies with central AGN emission are removed, but
the cosmic scatter of elliptical galaxies in both 24 and 70 micron luminosities
remains significant along the correlation. We find that the 24 micron variation
among ellipticals correlates with stellar metallicity, reflecting emission from
hot dust in winds from asymptotic giant branch stars of varying metallicity.
Infrared surface brightness variations in elliptical galaxies indicate that the
K - 24 color profile is U-shaped for reasons that are unclear. In some
elliptical galaxies cold interstellar dust emitting at 70 and 160 microns may
arise from recent gas-rich mergers. However, we argue that most of the large
range of 70 micron luminosity in elliptical galaxies is due to dust transported
from galactic cores by feedback events in (currently IR-quiet) active galactic
nuclei. Cooler dusty gas naturally accumulates in the cores of elliptical
galaxies due to dust-cooled local stellar mass loss and may accrete onto the
central black hole, releasing energy. AGN-heated gas can transport dust in
cores 5-10 kpc out into the hot gas atmospheres where it radiates extended 70
micron emission but is eventually destroyed by sputtering. This, and some
modest star formation, defines a cycle of dust creation and destruction.
Elliptical galaxies evidently undergo large transient excursions in the banana
plot in times comparable to the sputtering time or AGN duty cycle, 10 Myrs.
Normally regarded as passive, elliptical galaxies are the most active galaxies
in the IR color-color correlation.
