The early-type dwarf galaxy population of the Centaurus cluster
  We present a photometric study of the early-type dwarf galaxy population of
the Centaurus cluster, aiming at investigating the galaxy luminosity function
(LF) and galaxy scaling relations down to the regime of galaxies with M_V~-10
mag. On deep VLT/FORS1 V- and I-band images of the central part of the cluster,
we identify cluster dwarf-galaxy candidates using both morphological and
surface brightness selection criteria. Photometric and structural parameters of
the candidates are derived from analysis of their surface brightness profiles.
Fundamental scaling relations, such as the colour-magnitude and the
magnitude-surface brightness relation, are used to distinguish the cluster from
the background. We find a flat LF with a slope of \alpha = -1.14 \pm 0.12 for
M_V>-14 mag, when fitting a power law to the completeness-corrected galaxy
number counts. When plotting the central surface brightness of a Sersic model
vs. the galaxy magnitude, we find a continuous relation for magnitudes
-20<M_V<-10 mag, with only the brightest core galaxies deviating from this
relation, in agreement with previous studies of other clusters. In a
size-luminosity diagram of early-type galaxies from a range of environments, we
observe that R_eff slowly decreases with decreasing luminosity for -21<M_V<-13
mag and decreases more rapidly at fainter magnitudes. This trend continues to
the ultra-faint Local Group dwarf galaxies (M_V~-4 mag). The continuous central
surface brightness vs. absolute magnitude relation and the smooth relation in
the size-luminosity diagram over a wide range of magnitudes are consistent with
the interpretation of dwarf galaxies and more massive elliptical galaxies being
one family of objects with gradually changing structural properties. The most
massive core galaxies and the rare cE galaxies are the only exceptions.
