Submm-selected galaxies are an important component of the Universe, but are typically very faint in other wavebands, and so difficult to study. This immediately implies that the submm population does not overlap significantly with other types of high-redshift galaxies, although it may be possible to infer their properties if detailed information about these other classes is available (Adelberger and Steidel, 2000). The likely lack of overlap is reinforced by the relatively low surface density of submm galaxies as compared with the faintest optically-selected galaxies. Nevertheless, a detailed understanding of the process of galaxy formation demands that the relationship of the submm galaxies to other populations of high-redshift galaxies is determined.
4.1. Optically-selected Lyman-break galaxies (LBGs)
The LBGs (Steidel et al., 1999) are sufficiently numerous to have a well-defined luminosity function (Adelberger and Steidel, 2000). The effects of dust extinction on the inferred luminosity of a small subset of LBGs have been estimated reliably from near-IR observations of H emission: corrections by factors of 4-7 are indicated (Pettini et al., 1998, 2001; Goldader et al. 2002). At present, it is difficult to confirm this degree of extinction directly, as attempts to detect LBGs using submm-wave instruments have not so far been successful: see Section 2.8. Observations suggest that a typical LBG has a 850-µm flux density of order 0.1 mJy, well below the confusion limit at the resolution of existing submm-wave images.
Adelberger and Steidel (2000) have discussed the various selection effects associated with submm, optical and faint radio selection of high-redshift galaxy samples. They assumed that the relation between the slope of the UV SED of a galaxy and the fraction of its luminosity emitted in the far-IR waveband that is observed for low-redshift IUE starbursts with luminosities less than about 1011 L (Meurer et al., 1999) holds at greater redshifts and luminosities. A common, smooth luminosity function can then account for the properties of LBGs and submm galaxies. A priori, there must be an underlying multi-waveband luminosity function of all high-redshift galaxies from which both classes of galaxies are drawn. However, while observations of some submm galaxies (a key example being SMM J14011+0252; Ivison et al., 2000a, 2001; Fig. 18) seem to support this interpretation at first sight, it is clear that only the J2 region of this galaxy would be identified as a LBG, while the submm emission is concentrated nearer to J1. Further discussion can be found in Goldader et al. (2002). Because of the apparent diversity of optical-submm properties of submm galaxies (Ivison et al., 2000a; Smail et al., 2002), this simple transformation is unlikely to hold. Hence, a fraction of submm galaxies will probably never be detected in rest-frame UV continuum surveys because of their extreme faintness. The confirmed ERO submm galaxies (Smail et al., 1999; Gear et al., 2000; Lutz et al., 2001) are clear examples of such a population.