4.3. Dusty Sub-mm (IR) Galaxies
In recent years it has become evident that a modest number of fairly high redshift galaxies (z 2) are most readily recognized as unusual at far-IR or sub-mm wavelengths. Our Earth's atmosphere is a substantial barrier to sub-mm research on galaxies likely to be both very dusty and also have a rapid pace of star formation. That recipe augurs for reddened high IR emission, which we can most readily discover at sub-mm wavelengths. It follows that the most-secure continuum detections are at a wavelength of 850 µm (where receivers are fairly efficient, and our atmosphere fairly transparent). The detection system of choice at the moment is called SCUBA, for Submillimeter Common User Bolometer Array. This camera, utilized on the JCMT (James Clerk Maxwell Telescope), has enabled, for the first time, deep and relatively unbiased surveys which may identify the distant dusty galaxies (and/or AGN).
It is important to clarify which galaxies (or AGN) radiate so profusely at IR and sub-mm wavelengths. They may be largely responsible for the Far-IR extragalactic background. With the presently available redshifts for securely identified IR/sub-mm galaxies, their integrated energy density may be quite comparable to the integrated optical (emitted UV galaxy light measured in the HDFs) energy (e.g., Genzel et al. 2003).
To deal with this global question, and also to understand the limit of SFR in a huge star-burst, reddened or not, the crying need is a reliable set of redshifts.
Blain et al. (1999) comment on the reason why many galaxies detected in the sub-mm spectral window are likely to be at high redshift. This is because the long wavelength side of the canonical sub-mm source spectrum has a very steep slope (cf. Blain et al. 2002). The steepness of the long-wave side leads to substantial negative K-corrections. That is, the observer's band (850 µm) benefits from a larger redshift moving the emitted and then redshifted peak distribution into that atmospheric window. This effect compensates for the usual geometric dimming of increasing luminosity-distance at higher z. Thus comparing the sub-mm (850 µm) flux with the VLA radio flux (say, near 1.4 GHz) can yield approximate redshifts without an optical spectrum. But they are not individually robust.
Another method of deriving a more precise redshift for a sub-mm galaxy detection is to make good use of the fact that dusty systems occasionally also show strong molecular lines of CO in emission. The transitions in CO are (3-2) or (4-3) for the redshift domain of z ~ 2.6-2.8 (Frayer et al. 1998). But only a small minority have yielded CO molecular redshifts to date.
Very recently Chapman et al. (2003) have succeeded in obtaining good numbers of optical spectroscopic redshifts for sub-mm galaxies and AGN with precise radio positions. 16 redshifts of quality were obtained; probably one is a quasar. A few others may have some weaker AGN signal. The median redshift for the galaxies is z = 2.4, with a maximum redshift of z = 3.699. Thus one must extrapolate the 850 µm fluxes down to 1-2mJy in anticipation of future achievements in the z 5 domain for sub-mm galaxies. That will surely require new hardware.
One sort of instrument planned for the near future is the APEX antenna (the Atacama [Chile] Pathfinder Experiment). It is a planned 12-m diameter sub-mm telescope at a high, dry site in northern Chile.
Surveys with the APEX should go deeper than the present SCUBA system. And that will be just a taste of what is to come with ALMA (the Atacama Large Millimeter Array). ALMA will be the mm/sub-mm counterpart of the VLT with 64 times the collecting area of APEX! It should make possible IR galaxy detections 100 times fainter than we now do with SCUBA and with good spatial acuity. This great array should lead to many redshifts with molecular CO lines and the [CII]158 µm line.
We end this section with an astrophysical speculation: with the Chapman et al. (2003) data we suggest a relatively high space density of very luminous and distant sub-mm (z > 2) galaxies. They may be 1000 times the density of similarly IR-luminous local star-bursts found here and now. Hence the detailed study of a few of the powerful IR galaxies will tell us much about young galaxy SF and dust interactions.