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3.2. Extremely red objects, EROs

To search for primeval galaxies, already Elston et al. (1988) had identified extremely red objects that are conspicuous only in the near-infrared, and have R - K colors larger than 5. The ERO class could include several categories of objects: essentially old stellar populations at high redshift, or high-z dust-reddened starbursts (Hu & Ridgway 1994; Cowie et al. 1996). Maybe 10% of the submm sources could be EROs (Smail et al. 1999). A proto-typical ERO at z = 1.44 (Dey et al. 1999) has been detected in submm continuum (Cimatti et al. 1998), and has been found to be an ultraluminous starburst shrouded by dust, with a star formation rate of 200-500 Msun per yr. The surface density of such EROs at K < 20 and color R - K > 6 is about 500 per square degree, and for R - K > 7, about 50 per square degree (which is comparable to the number of QSO with B < 21.5, Andreani et al. 1999). At least for the few objects known, the AGN cannot be the source of the huge luminosity, given the FIR/radio ratio.