The cluster Abell 780: An optical view

Abstract

Context. The Abell 780 cluster, better known as the Hydra A cluster, has been thouroughly analyzed in X-rays. However, little is known about its optical properties. Aims.We propose to derive the galaxy luminosity function (GLF) in this apparently relaxed cluster and to 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. Methods.Our study is based on optical images obtained with the ESO 2.2m telescope andWFI camera in the B and R bands, covering a total region of 67.22 × 32.94 arcmin2, or 4.235 × 2.075 Mpc2 for a cluster redshift of 0.0539. Results. In a region of 500 kpc radius around the cluster center, 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 a ∼ -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 a gravitationally bound structure of 27 galaxies, which includes the cD, and of a more strongly gravitationally bound structure of 14 galaxies. Conclusions. These optical results agree with the overall relaxed structure of Abell 780 previously derived from X-ray analyses. © 2009 ESO.