Dwarf galaxies in the Coma cluster - II. Spectroscopic and photometric fundamental planes

We present a study of the Fundamental Plane (FP) for a sample of 71 dwarf galaxies in the core of the Coma cluster in the magnitude range -21 < MI < -15. Taking advantage of the high-resolution DEIMOS spectrograph on Keck II for measuring the internal velocity dispersion of galaxies and high-resolution imaging of the Hubble Space Telescope (HST)/ACS, which allows an accurate surface brightness modelling, we extend the FP of galaxies to luminosities of similar to 1 mag fainter than all the previous studies of the FP in the Coma cluster. We find that the scatter about the FP depends on the faint-end luminosity cut-off, such that the scatter increases for fainter galaxies. The residual from the FP correlates with the galaxy colour, with bluer galaxies showing larger residuals from the FP. We find M/L proportional to M-0.15 +/- 0.22 in the F814W band, indicating that in faint dwarf ellipticals, the M/L ratio is insensitive to the mass. We find that less massive dwarf ellipticals are bluer than their brighter counterparts, possibly indicating ongoing star formation activity. Although tidal encounters and harassment can play a part in removing stars and dark matter from the galaxy, we believe that the dominant effect will be the stellar wind associated with the star formation, which will remove material from the galaxy, resulting in larger M/L ratios. We attribute the deviation of a number of faint blue dwarfs from the FP of brighter ellipticals to this effect. We also study other scaling relations involving galaxy photometric properties including the Photometric Plane. We show that compared to the FP, the scatter about the Photometric Plane is smaller at the faint end.