New Clues on the Nature of Extremely Red Galaxies

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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 continuum breaks are detected. From near-infrared spectrophotometry, complemented 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 formation rates in range of SFR<6-30h₅0⁻² Mₛolar 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.