New clues of the nature of extremely red galaxies

We present near-infrared VLT-UT1+ISAAC spectroscopy of a sample of 9 extrem ely red galaxies (ERGs) with R - K > 5 and K < 19.0. Neither strong emissio n lines (F-lim < 1-5 x 10(-16) erg s(-1) cm(-2)) nor continuum breaks are d etected. From near-infrared spectrophotometry, complemented with broad-band optical and near-IR photometry, we estimate "spectro-photometric" redshift s to be in the range of 0.8 less than or similar to z(sphot) less than or s imilar to 1.8. We derive upper limits on the star formation rates in range of SFR < 6-30h(50)(-2)M.yr(-1). Two of the observed ERGs are dusty starburs t candidates because they require strong dust reddening to reproduce their global spectral energy distributions. The other ERGs are consistent with be ing dustless old passively evolved spheroidals at z greater than or similar to 0.8. We discuss the general implications of our findings in relation wi th the problem of the formation of early type galaxies.