The Luminosity Function of the Fossil Group RX J1552.2+2013

We determine the first fossil group luminosity function based on spectroscopyof the member galaxies. The fossil group RX J1552.2+2013 has 36 confirmedmembers, it is at a mean redshift of 0.136 and has a velocity dispersion of 623km/s (or 797 km/s if four emission lines galaxies in the outskirts of thevelocity distribution are included). The luminosity function of RXJ1552.2+2013, measured within the inner region of the system ~1/3 R_vir), inthe range -23.5< M_i'<-17.5, is well fitted by a Schechter function withM*i'=-21.3 +/- 0.4 and alpha = -0.6 +/- 0.3 or a Gaussian function centered onM_i'= -20.0 +/- 0.4 and with sigma=1.29 +/- 0.24 i' mag. (H_0 = 70 km/s Mpc,Omega_M=0.3, Omega_Lambda=0.7. The luminosity function obtained from aphotometric survey in g', r', i'-bands (and statistical background correction)confirms the spectroscopically determined results. There is a significant dipin the luminosity function at M_r'=-18 mag, as also observed for otherclusters. RX~J1552.2+2013 is a rich, strongly red-galaxy dominated system, withat least 19 galaxies with magnitudes between m_3 and m_3 + 2, within a surveyedcircular area of radius 625 kpc centered on the peak of the x-ray emission. Itsmass, ~3.0 10^14 M_0, M/L of 507 M_sol/L_B_sol and L_X of 6.3 10^43 ergs/s(bolometric) are more representative of a fossil cluster than of a fossilgroup. The central object of RX J1552.2+2013 is a cD galaxy which may haveaccreted the more luminous ~L* former members of the group. Although dynamicalfriction and subsequent merging are probably the processes responsible for thelack of bright galaxies in the system, for the fainter members, there must beanother mechanism in action (perhaps tidal disruption) to deplete the fossilgroup from intermediate-luminosity galaxies M_r' ~ -18.Comment: 14 pages, 7 Figures. accepted by A