The very red halo of the cD galaxy in A 3284

Abstract

We present deep CCD surface photometry in the Gunn g, r, i system and spectroscopy of the cD galaxy at the centre of the cluster A 3284 at z=0.15. The brightness profile of the galaxy has been tracked up to 40 arcsec (142kpc) from the centre, and down to 26 mag/arcsec^+2^. The core and halo components in the galaxy have been singled out deriving geometrical parameters of the fitting isophotes as well as magnitudes and colours. The spectral properties of the galaxy core indicate a stellar population super metal-rich with [Fe/H]=+0.5. The cD halo is clearly dominant at 45kpc from the galactic centre and has exceedingly red colours (g-r=1.03, g-i=1.82), about 0.7mag redder than the core g-i. A match with the models for evolutionary population synthesis by Buzzoni (1989) shows that the halo is consistent with a population of unevolved M-dwarf stars lower than 0.7 M<SUB>sun</SUB>_. The M/L ratio in B for the halo is estimated to range between 50 and 200 implying a total mass for the cD galaxy of 1.6-3.1x10^13^M<SUB>sun</SUB>_ and a total B luminosity of 6.0x10^11^L<SUB>sun</SUB>_.