The Origin of Dust in Early-Type Galaxies and Implications for Accretion
  onto Supermassive Black Holes
  We have conducted an archival Spitzer study of 38 early-type galaxies (ETGs)
in order to determine the origin of the dust in approximately half of this
population. Our sample galaxies generally have good wavelength coverage from
3.6um to 160um, as well as visible-wavelength HST images. We use the Spitzer
data to estimate dust masses, or establish upper limits, and find that all of
the ETGs with dust lanes in the HST data are detected in all of the Spitzer
bands and have dust masses of ~10^{5-6.5} Msun, while galaxies without dust
lanes are not detected at 70um and 160um and typically have <10^5 Msun of dust.
The apparently dust-free galaxies do have 24um emission that scales with the
shorter wavelength flux, yet substantially exceeds the expectations of
photospheric emission by approximately a factor of three. We conclude this
emission is dominated by hot, circumstellar dust around evolved stars that does
not survive to form a substantial interstellar component. The order of
magnitude variations in dust masses between galaxies with similar stellar
populations rules out a subtantial contribution from continual, internal
production in spite of the clear evidence for circumstellar dust. We
demonstrate that the interstellar dust is not due to purely external accretion,
unless the product of the merger rate of dusty satellites and the dust lifetime
is at least an order of magnitude higher than expected. We propose that dust in
ETGs is seeded by external accretion, yet the accreted dust is maintained by
continued growth in externally-accreted cold gas beyond the nominal lifetime of
individual grains. The several Gyr depletion time of the cold gas is long
enough to reconcile the fraction of dusty ETGs with the merger rate of gas-rich
satellites. As the majority of dusty ETGs are also low-luminosity AGN and
likely fueled by this cold gas, their lifetime should similarly be several Gyr.
