The cluster Abell 780: an optical view
  The Abell 780 cluster, better known as the Hydra A cluster, has been
thouroughly analyzed in X-rays. However, little is known on its optical
properties. We derive the galaxy luminosity function (GLF) in this apparently
relaxed cluster, and search for possible environmental effects by comparing the
GLFs in various regions, and by looking at the galaxy distribution at large
scale around Abell 780. Our study is based on optical images obtained with the
ESO 2.2m telescope and WFI camera in the B and R bands, covering a total region
of 67.22x32.94 arcmin^2, or 4.235x2.075 Mpc^2 for a cluster redshift of 0.0539.
In a region of 500 kpc radius around the cluster centre, the GLF in the R band
shows a double structure, with a broad and flat bright part and a flat faint
end that can be fit by a power law with an index alpha=-0.85+-0.12 in the
20.25<R<21.75 interval. If we divide this 500 kpc radius region in North+South
or East+West halves, we find no clear difference between the GLFs in these
smaller regions. No obvious large scale structure is apparent within 5 Mpc from
the cluster, based on galaxy redshifts and magnitudes collected from the NED
database in a much larger region than that covered by our data, suggesting that
there is no major infall of material in any preferential direction. However,
the Serna-Gerbal method reveals the presence of a gravitationally bound
structure of 27 galaxies, which includes the cD, and of a more strongly
gravitationally bound structure of 14 galaxies. These optical results agree
with the overall relaxed structure of Abell 780 previously derived from X-ray
analyses.
