Very deep spectroscopy of the Coma cluster line of sight : exploring new territories ⋆

Context. Environmental effects are known to have an important influence on cluster galaxies, but studies at very faint magnitudes (R> 21) are almost exclusively based on imaging. We present here a very deep spectroscopic survey of galaxies on the line of sight to the Coma cluster. Aims. After a series of papers based on deep multi–band imaging of the Coma cluster, we explore spectroscopically part of the central regions of Coma, in order to confirm and generalize previous results, concerning in particular the galaxy luminosity function, red sequence, stellar populations and the most likely formation scenario for the Coma cluster. Methods. We have obtained reliable VIMOS redshifts for 715 galaxies in the direction of the Coma cluster centre in the unprecedented magnitude range 21 ≤R≤ 23, corresponding to the absolute magnitude range −14 ≤ MR ≤ −12. Results. We confirm the substructures previously identified in Coma by Adami et al. (2005a), and identify three new substruc-tures. We detect a large number of groups behind Coma, in particular a large structure at z∼0.5, the SDSS Great Wall, and a large and very young previously unknown structure at z∼0.054, which we named the background massive group (BMG). These structures account for the mass maps derived from a recent weak lensing analysis by Gavazzi et al. (2009). The orbits of dwarf galaxies are probably anisotropic and radial, and could originate from field galaxies radially falling into the cluster along the numerous cosmological filaments surrounding Coma. Spectral characteristics of Coma dwarf galaxies show that red or absorption line galaxies have larger stellar masses and are older than blue or emission line galaxies. R≤22 galaxies show less prominent absorption lines than R≥22 galaxies. This trend is less clear for field galaxies which are similar to R≥22 Coma galaxies. This suggests that part of the faint Coma galaxies could have been recently injected from the field following the NGC 4911 group infall. We present a list of five Ultra Compact Dwarf galaxy candidates which need to be confirmed with high spatial resolution imaging with the HST. We also globally confirm spectroscopically our previous results concerning the galaxy luminosity functions based on imaging down to R=23 (MR = −12) and find that dwarf galaxies follow a red sequence similar to that drawn by bright Coma galaxies. Conclusions. Spectroscopy of faint galaxies in Coma confirms that dwarf galaxies are very abundant in this cluster, and that they are …