In the past two years, Keck and HST deep observations have allowed to find a population of star-forming galaxies at z>2 (Steidel et al. 1996, ApJ 462,L17; Hu & Cowie 1998, ApJ in press), up to discovering the most distant galaxy currently known (z=5.34, Dey et al. 1998, ApJ, 498, L93). Together with the spectroscopic surveys of galaxies at z<2 (Lilly et al. 1995, ApJ, 455, 108; Cowie et al. 1996, AJ, 112, 832), these results have been used to trace back the history of the global star formation of the Universe (Madau et al. 1996, MNRAS, 283,1388). However, these galaxies have been selected in the optical according to criteria (Ly-break continuum and Ly emission) that can be severely affected by the presence of strong dust reddening, and thus leading to a biased and partial view of the galaxy evolution at high redshifts. The existence of dusty star-forming galaxies (i.e., where the UV continuum from OB stars is heavily absorbed by dust and re-emitted in far-IR) is expected in several scenarios of galaxy formation and evolution (Blain & Longair 1993,MNRAS,264,509; Franceschini et al. 1994,427,140; Mazzei & De Zotti 1996, MNRAS 279,535; Zepf & Silk 1996,ApJ, 466,114), and it is suggested by the recent discovery of a cosmic FIR-mm background (Puget et al. 1996, A&A, 308, L5). The combination of optical and near-IR imaging has found a field population of enigmatic extremely red galaxies (Hu & Ridgway 1994, AJ 107, 1303; Cowie et al. 1996, AJ, 112, 839) with typical colors 6-8 or (or even redder) and . Their faintness does not allow to obtain spectra with the 4m telescopes, and only the Keck 10m telescope provided so far the only redshift available (z=1.44, Graham & Dey 1996, ApJ, 471, 720). Although their properties indicate that these galaxies are at z>1, their nature is presently unknown, and their colors are consistent with two main scenarios : (1) they are dusty and star-forming galaxies where the extreme colors are the result of a severe reddening of the UV continuum; (2) they are old galaxies at z>1, the colors being so red because of lack of star formation and because of the strong K-correction effect.
Recent observations with the JCMT 15m (+SCUBA) and IRAM 30m telescopes have detected strong dust thermal continuum emission and high FIR luminosity from a galaxy with I-K=6.5 (I-J=3.8) located at z=1.4(Cimatti, Andreani, Röttgering & Tilanus 1998, Nature, 392, 895). The FIR luminosity of this galaxy implies a star formation rate up to several hundreds Myr-1. This discovery suggests that at least a fraction of the very red galaxies are dusty star-forming systems, and it shows that it is possible to unveil them with deep optical+near-IR imaging coupled with sub-mm continuum observations.