Infra-Red Emission from Cold Gas Dusty Disks in Massive Ellipticals
  What is the expected infrared output of elliptical galaxies? Here we report
the latest findings obtained in this high time resolution (~10 years) and high
spatial resolution (2.5 parsec at center) study. We add a set of grain physics
to the MACER code, including (a) dust grains made in passive stellar evolution;
(b) dust grain growth due to collision and sticking; (c) grain destruction due
to thermal sputtering; (d) dust cooling of hot gas via inelastic collisions;
and (e) radiation pressure on dust grains. The code improvements enable us to
analyze the effects of dust on metal depletion and AGN obscuration, and also to
assess the infrared output of the modeled galaxies. We simulate a
representative massive elliptical galaxy of a central stellar velocity
dispersion ~ 260 km/s and modest rotation. We find that: (1) the circumnuclear
disk (of a size <~ 1 kpc) is dusty in its outer region where most of the metals
are in dust grains, while in the inner disk most of the dust grains are
destroyed by the AGN irradiation; (2) the dusty disk is optically thick to both
the starlight within the disk and the radiation from the central AGN. Thus the
AGN will be obscured behind the disk, and the latter is of a covering factor ~
0.2; (3) the dust infrared emission is mainly due to the AGN irradiation. The
median infrared luminosity is ~ 2e44 erg/s, and it can reach >~ 1e46 erg/s
during outbursts; (4) the duty cycles of the AGN activities, star formation,
and the dust infrared luminosity roughly match observations, e.g., in most of
its lifetime, the simulated galaxy is a stereotypical "quiescent" elliptical
galaxy with L_{IR} ~ 1e11*L_{solar}, while the star formation rate can exceed
250 M_{solar}/yr during central outbursts.
