New clues on the nature of extremely red galaxies

We present near-infrared VLT-UT1+ISAAC spec- troscopy of a sample of 9 extremely red galaxies (ERGs) with R K> 5 and K< 19:0. Neither strong emission lines (Flim <1-510 16 erg s 1 cm 2 ) nor continuum breaks are detected. From near-infrared spectrophotometry, comple- mented with broad-band optical and near-IR photometry, we estimate "spectro-photometric" redshifts to be in the range of 0:8 . zsphot . 1:8. We derive upper limits on the star forma- tion rates in range of SFR < 6-30h 2 50 Myr 1 . Two of the observed ERGs are dusty starburst candidates because they re- quire 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 dis- cuss the general implications of our findings in relation with the problem of the formation of early type galaxies.