Science in the Thermal
Infrared
The thermal infrared spectrum from 2.2 - 5.6 microns, detectable with
current InSb arrays, provides a unique window on
the Universe. The most dynamic phases of stellar evolution,
the birth and death of stars, are the most difficult to study
observationally due to the
large quantities of obscuring dust. Since optical and ultraviolet
radiation is trapped, we can best study the very youngest and oldest
stars via the re-emission of their energy at thermal infrared
wavelengths. Consequently, multiwavelength imaging of thermal
continuum emission can reveal the temperature and density structures
of evolved stars, embedded protostars, protoplanetary disks, and
circumstellar dust near newly formed stars. The 2.2 - 5.6 micron continuum,
therefore, holds important clues about the birth of stars and planets,
the death of stars, and the evolution of stars and galaxies.
In particular, at large redshifts, visible starlight is shifted into
this band. Consequently, this spectral regime holds a key diagnostic
of the stellar content, luminosity, mass, and evolution of high
redshift galaxies.
In addition, the 2.2 - 5.6 micron band contains a number of
astrophysically important spectral lines (see Table 1), most notably:
(1) the 3.3 micron PAH feature that
traces very small dust grains exposed to UV fields;
(2) several solid-phase
absorption features of various ices, such as the 3.0 micron water ice
feature;
(3) Brackett alpha, a tracer of ionized gas around hot OB stars that suffers
very little extinction; and
(4) the Q-branch molecular hydrogen lines, near 2.4 microns, are a unique, yet
heretofore untapped, diagnostic of dense molecular gas.
An example of the beautiful and unprecedented detail of PDRs, as
revealed by the 3.3 micron PAH feature. This image of the
star-forming region NGC 6334 was taken with the SPIREX/Abu system at
the South Pole last season. NGC 6334 contains several embedded H II
regions, revealed by their 4.05 micron Br-alpha emission (right panel).
The intricate PDR structure, however, is delineated by the 3.3 micron
PAH emission (left panel). The wealth of detail over such a large
field shows the power of wide-field imaging in the thermal infrared
from the South Pole.
HKL image of 30 Doradus region taken with Abu/SPIREX
The red objects are embedded stars that are invisible at
shorter wavelengths.
Model spectra as a function of redshift, z, for a maximally old $L^*$
galaxy in which all stars formed in an instantaneous burst at z =
infinity, and evolve passively thereafter, in an H_0 = 50, q_0 =
0.1 cosmology. The flux is normalized to an absolute K magnitude of
--25.1 today. Also shown are the senstivities for SIRTF/IRAC (1 sigma
in 500 sec), Abu/SPIREX (first season; 1 sigma in 10 hours),
and AIRO (1 sigma in 10 hours).
Both SIRTF and AIRO can detect high-z galaxies, but
AIRO has better resolution and larger fields of view.
From Fazio et al (1998)
Synthetic flux distribution of a brown dwarf model with T_eff = 900
K, log g = 5.0, and assumed solar metallicity. The spectrum
of a 900 K blackbody is shown for comparison. Figure from
Fazio et al (1998) adapted from Allard et al (1996).
jackson@bu.edu