A Three-dimensional Decomposition of the Infrared Emission from Dust in the Milky Way
T. J. Sodroski, N. Odegard, R. G. Arendt, E. Dwek, J. L. Weiland, M. G. Hauser, T. Kelsall;
ApJ, 1997, 480, 173
ABSTRACT:We have constructed a three-dimensional model of the Galactic large-scale infrared
emission from dust associated with the molecular (H2), neutral atomic (H I), and
extended low-density (ne ~ 1--100 cm-3) ionized (H II) gas phases of the
interstellar medium.
The model incorporates a three-dimensional map of the
molecular and neutral atomic hydrogen gas distributions, derived from
available 12CO and H I surveys by using the radial velocity information in the
spectral lines as a distance indicator, and available 5 and 19 GHz radio
continuum surveys to trace the column density of ionized gas.
We use the model to
decompose the COBE 5 Diffuse Infrared Background Experiment (DIRBE) 12--240 mu m
observations of the Galactic plane region (|b| <= 5 deg), from which the zodiacal
light and stellar emission have been subtracted, into distinct emission
components associated with each gas phase within selected ranges of
Galactocentric distance.
An interstellar dust model is fitted to the resulting
infrared spectra to derive the following quantities within each
Galactocentric distance interval: (1) the abundance and equilibrium temperature of
the large dust grain component within each gas phase; (2) estimates of the
abundance of very small (<200 A) transiently heated dust grains and polycyclic
aromatic hydrocarbon (PAH) molecules; and (3) constraints on various model
parameters, such as the energy density of the ambient interstellar radiation
field, which heats the dust within the H I gas phase.
Our results show steep
negative Galactocentric gradients in the equilibrium temperature of the large
dust grain component within the H I, H2, and H II gas phases, the Galaxy's
ambient interstellar radiation field, and the dust-to-gas mass ratio for each
gas phase.
The intensity of the ambient interstellar radiation field
increases by a factor of ~3 between the solar circle (8.5 kpc) and the molecular
ring at a Galactocentric distance of ~5 kpc.
The dust abundance gradient of
(-0.05 +/- 0.03) dex kpc-1 is equivalent, within the uncertainties, to the
metallicity gradient in the Galactic disk.
The derived emission spectra are
consistent with a model in which very small transiently heated dust grains and PAHs
are abundant and the dominant contributors to the mid-infrared (5 mu m <
lambda < 40 mu m) luminosity from a Galactocentric distance of 2 kpc out to a
Galactocentric distance of at least 12 kpc, and indicate that the relative abundance of
the PAHs is significantly higher in the outer region of the Galactic disk
than inside the solar circle.
We combine the results of our decomposition
algorithm with the results of a study of optical extinction at high Galactic
latitude to derive the radial distribution of optical opacity in the Galactic
disk and find that our Galaxy would be effectively transparent [AB(total
Galaxy) < 0.2 mag] to an external observer viewing it at a low inclination (i < 30
deg).
All of the Galactic infrared emission observed by the DIRBE can be
accounted for by dust associated with gas that is detected by current radio
surveys, refuting the recent suggestion that a large fraction of the
dynamically inferred hidden mass in spiral galaxies may be due to unseen gas and
stars in the disk of the galaxies.
KEYWORDS: ism: dust, extinction, infrared: ism: continuum, ism: molecules, ism: abundances, galaxy: structure, cosmology: diffuse radiation
PERSOKEY:fir, h_i, h+, h2, milky way, ,
CODE: sodroski97