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 Coma cluster in magnitude range $-21 < M_I <-15$.
Taking advantage of high resolution DEIMOS spectrograph on Keck II for
measuring the internal velocity dispersion of galaxies and high resolution
imaging of HST/ACS, which allows an accurate surface brightness modeling, we
extend the fundamental plane (FP) of galaxies to $\sim$1 magnitude fainter
luminosities than all the previous studies of the FP in Coma cluster. We find
that, the scatter about the FP depends on the faint-end luminosity cutoff, 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 FP.
  We find $M/L \propto M^{-0.15\pm0.22}$ in 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.
