ASTROPHYSICS New Clues on the Nature of Extremely Red Galaxies

We present near-infrared VLT-UT1+ISAAC spectroscopy of a sample of 9 extremely red galaxies (ERGs) with R K > 5 and K < 19.0. Neither strong emission lines (Flim <1-5×10−16 erg s−1cm−2) nor contin- uum breaks are detected. From near-infrared spectropho- tometry, complemented with broad-band optical and near- IR photometry, we estimate "spectro-photometric" red- shifts to be in the range of 0.8 � < zsphot � < 1.8. We de- rive upper limits on the star formation rates in range of SFR < 6 30h−2 50 M⊙yr−1. Two of the observed ERGs are dusty starburst candidates because they require strong dust reddening to reproduce their global spectral energy distributions. The other ERGs are consistent with being dustless old passively evolved spheroidals at z � > 0.8. We discuss the general implications of our findings in relation with the problem of the formation of early type galaxies.