There are two major sources of submm radiation from galaxies: thermal continuum emission from dust grains, the solid phase of the interstellar medium (ISM), and line emission from atomic and molecular transitions in the interstellar gas. The ladder of carbon monoxide (CO) rotational transitions, spaced every 115 GHz, is the most important source of molecular line emission, but there is a rich zoo of other emitting molecules in the denser phases of the ISM. Submm surveys for distant galaxies have so far been made using cameras that detect only continuum dust emission, and so this will be the main focus of this review. However, the search for line emission is already important, and its study will become increasingly significant. The spectral resolution provided by line observations reveals much more about the physical and chemical conditions in the ISM, for studies of kinematics, metallicity and excitation conditions. Molecular lines can also be used to obtain a very accurate spectroscopic redshift for the ISM in high-redshift galaxies with prior optical redshifts (for example Frayer et al., 1998). Searches for redshifts at cm and (sub)mm wavelengths using CO lines will be possible using future telescopes.
The best studied regions of the Universe in the submm waveband are Giant Molecular Clouds (GMCs) in the Milky Way, in which ongoing star formation is taking place (Hollenbach and Tielens, 1997). GMCs are perhaps very low-luminosity archetypes for distant dusty galaxies, although these galaxies have far-IR luminosities that are up to 4 orders of magnitude greater than that of the whole Milky Way.
Detailed, resolved submm-wave images and spectra only
exist for low-redshift galaxies (for example
Regan et al., 2001;
Sakamoto et al., 1999),
and it is often necessary to use them as
templates to interpret the properties of more distant galaxies.
A very important
class of well-studied galaxies similar in luminosity, and perhaps in
physical properties, to high-redshift submm galaxies are the
ultraluminous IR galaxies (ULIRGs) discovered in the
InfraRed Astronomy Satellite (IRAS) all-sky
survey in the mid 1980's (see the review by
Sanders and Mirabel,
1996).
ULIRGs are usually defined as having a
bolometric luminosity, integrated over all wavelengths at which dust
emission dominates the SED (from about 1 mm-8 µm), in excess of
1012
L
.
5
They are amongst the most luminous of all galaxies, but
number less than 0.1% of galaxies in the
local Universe. Due to their selection by IRAS, they
are typically at relatively low redshifts, less than
about 0.3. The first IRAS-detected high-redshift ULIRG
was identified by
Rowan-Robinson et
al. (1991)
at z = 2.3. The current record redshift for a galaxy detected by
IRAS is z = 3.9 for APM 08279+5255
(Irwin et al., 1998).
Both these
galaxies appear to be extremely luminous; however, their luminosities are
boosted by at least a factor of ten due to gravitational lensing
by foreground galaxies. A compilation of the properties of some
of the most extreme ULIRGs is given by
Rowan-Robinson (2000).
The IR spectral energy distributions (SEDs) of some low-redshift ULIRGs
and a compilation of results for the more sparsely sampled SEDs of
high-redshift dusty galaxies are illustrated in
Fig. 2.
 |
Figure 2. Various observed restframe spectral energy distributions (SEDs) of galaxies from the radio to the near-IR wavebands. Two examples of the most luminous low-redshift galaxies detected by IRAS are included (I). Five very luminous high-redshift galaxies that have been, or could have been, detected directly in deep submm surveys (S), three high-redshift galaxies serendipitously magnified and made easier to study by the gravitational lensing effect of foreground galaxies and also detected by IRAS (L), and five high-redshift AGNs detected in optical or radio surveys (H) are also shown. In addition, three template SEDs are shown. One includes the properties of CO and atomic fine-structure emission lines in the (sub)mm waveband at wavelengths from 100 to 3000 µm (Blain et al., 2000b), one includes polycyclic aromatic hydrocarbon (PAH) molecular emission features at wavelengths ~ 10 µm in the mid-IR waveband (Guiderdoni et al., 1998), and one is normalized to the typical SED of a sample of low-redshift IRAS galaxies (Dunne et al., 2000). For further information on far-IR SEDs see Dale et al. (2001). With the exception of the high-redshift AGNs and the lensed galaxies, the templates tend to provide a reasonable description of the SED at wavelengths around and longer than its peak, the regime probed by submm surveys. Less luminous galaxies like the Milky Way have dust spectra that peak at a wavelength about a factor of 2 longer than these templates (Reach et al., 1995). |