Deep GMOS spectroscopy of extremely red galaxies in GOODS-South: ellipticals, mergers and red spirals at 1 < z < 2

We have performed a deep (35.5-h exposure) spectroscopic survey of extremely red (I - K > 4) galaxies (ERGs) on the Great Observatories Origins Deep Survey (GOODS)-South Field, using the Gemini multi-object spectrograph on the 8-m Gemini South Telescope. We present here spectra and redshifts for 16 ERGs at 0.87 < z < 2.02, to a limit of K_s ~= 20.2. In the spectra of 10 of these galaxies we find emission lines of [OII] 3727Å with fluxes corresponding to a mean star formation rate (uncorrected for dust) of 1.6M_solaryr^-1. For three ERGs we detect no emission lines and the remaining three lie at z > 1.5 where this line would be outside our wavelength range. Absorption features are seen in most ERG spectra.

We examine the morphologies of these ERGs on Hubble Space Telescope Advanced Camera for Surveys (HST-ACS) images and fit radii and Sersic indices. We find three broad classes: (i) spheroidals; (ii) mergers at a variety of stages (some are spheroidal-disc pairs, some have tidal tails), often with a high surface brightness and (iii) red spirals (which may have star forming regions in their outer discs).

We perform an age-dating analysis by fitting the spectra and nine-band photometry (BVIZJHK, plus 3.6/4.5-μm fluxes from Spitzer) of the ERGs with two-component models, consisting of passively evolving, old stellar populations combined with a younger, continuously star forming component, for which the age and dust extinction are allowed to vary from 10 to 800Myr and E(B - V) = 0.0 to 0.5mag. For only one ERG is the best-fitting model purely passive, for the others the best fit is obtained by including a star forming component, which typically forms a few (0.26-13.5) per cent of the stellar mass, and is subject to dust reddening averaging E(B - V) ~= 0.35. The ages of the star forming components tend to be youngest (10-40Myr) in the merging ERGs, and older (200-800Myr) in spiral ERGs, with mixed ages for the spheroidals.

The best-fitting mean ages for the old stellar populations range from 0.6 to 4.5Gyr, averaging 2.1Gyr, with masses from 3 × 10¹⁰ to 2 × 10¹¹M_solar. The mean stellar formation redshifts of ERGs are spread from ~0.5Gyr before the epoch of observation out to z ~ 5. We propose that most ERGs are galaxies, or mergers of galaxies, formed some Gyr earlier, in an early (z ~ 5) phase of massive galaxy formation, which since then have experienced a wide variety of merger and star formation (SF) histories, accounting for the wide range of observed stellar ages.

Finally, we examine the clustering of the ERGs on this field, as a function of the photometric redshifts estimated by Caputi et al. The comoving correlation radius is r₀ ~= 13h^-1Mpc for the full sample and, dividing by redshift, is constant or increasing with redshift, thus favouring comoving (ɛ = -1.2) evolution over a stable (ɛ = 0) clustering model.