6. LOCAL TEMPLATES OF HIGH-z MERGERS

Q4: Are ULIGs local templates of the high luminosity tail of mergers at z = 1-4 ?

[Note: We have interpreted this question as asking whether there is evidence that low-z ULIGs are templates of high-z ULIGs, leaving aside the larger question of how high-z ULIGs may be related to optically-selected high-z objects (e.g. Lyman-break galaxies).]

There is now substantial evidence to suggest that the space density of ULIGs evolves steeply with cosmic lookback time, and that ULIGs were much more common at redshifts z ~ 1-4. In the mid- and far-infrared, the deepest surveys carried out by IRAS (e.g. Hacking & Houck 1987; Lonsdale & Hacking 1989; Gregorich et al. 1995; Kim & Sanders 1998), and more recently the deep surveys with ISO (e.g. Taniguchi et al. 1997; Kawara et al. 1998; Aussel et al. 1999; Puget et al. 1999) are consistent with number density evolution as steep as (1 + z)⁵ out to z ~ 1. Within the past year, submillimeter surveys with SCUBA on the JCMT (Smail, Ivison, & Blain 1997; Hughes et al. 1998; Barger et al. 1998; Eales et al. 1999) have revealed what appears to be a substantial population of high-z ULIGs (i.e. z ~ 1-4) , that are plausibly the high-z extension of the low-z ULIGs detected by IRAS.

It is still too early to tell whether all of the high-z ULIGs detected by ISO and SCUBA have properties similar to local ULIGs. However, studies of the few high-z objects whose redshifts have been identified show that these sources may indeed resemble their lower redshift counterparts more closely than might at first have been assumed. The two best studied sources from the SCUBA sample of Smail et al. (1998) are illustrative. SMM J02399-0136 at z ~ 2.8, with L_ir 10¹³ L, is morphologically compact with an optical classification as a narrow-line ``type-2" AGN (Ivison et al. 1998; Ivison, these proceedings), and contains ~ 10^10.5 M of molecular gas (Frayer et al. 1998). SMM J14011+0252 at z ~ 2.6 with L_ir ~ 10^12.3 L (Barger et al. 1999), is a strongly interacting/merger pair, with an H II-like optical spectrum and ~ 10^10.7 M of molecular gas (Frayer et al. 1999). These two sources fit into the pattern exhibited by ULIGs in the local Universe. In particular their molecular gas masses, optical luminosities, and optical morphologies are very similar to what is observed for local ULIGs [see Sanders & Mirabel (1996) for a review of local ULIGs].

A4: YES, There is evidence that local ULIGs have properties (gas content, morphology, etc. ) similar to the identified infrared luminous sources at z = 1-4, but there is no reason to believe that all high-z, high-Lir sources are as homogeneous as the low-z ULIGs appear to be.