3.3. AGN or Starbursts
For a long time, the highest redshift objects known were quasars, although now, with higher sensitivities, the situation is reversed (e.g. Hu et al. 1998). Surveys have therefore been done in the millimeter continuum and lines for high-z quasars (McMahon et al. 1994; Isaak et al. 1994; Omont et al. 1996a, 1996b; Guilloteau et al. 1999). Results have shown that the far-infrared luminosities of many of them are due to dust heated by a starburst, although the AGN activity is simultaneously present. But since the manifestations of the two nuclear activities (starburst or AGN) are in many cases similar, and are most of the time associated, it has become a controversial question to disentangle the two interpretations. Ultra-luminous starbursts take place over the few central 100pc (Solomon et al. 1997), and even pure AGN activities can be mimicked by radio supernovae (Boyle & Terlevich 1998). Genzel et al. (1998) from ISO mid-infrared diagrams concluded that the ultra-luminous galaxies are powered at 30% from AGN and 70% from star formation; a similar conclusion is suggested by Cooray & Haiman (1999) for submm catalogued sources. McMahon et al. (1999) however suggest that a much more significant fraction of submm sources, between 15 to 100%, could be AGN-powered. Note that distinction criteria are hard to find (cf. Stein 1995), and even symbiotic starburst-black hole models are likely (Williams et al. 1999). Discriminating between the two possibilities can have important consequences on the star formation history of the Universe.