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

Abstract

We determine the first fossil group luminosity function based on spectroscopy of the member galaxies. The fossil group RX J1552.2+2013 has 36 confirmed members, it is at a mean redshift of 0.136 and has a velocity dispersion of 623 km/s (or 797 km/s if four emission lines galaxies in the outskirts of the velocity distribution are included). The luminosity function of RX J1552.2+2013, measured within the inner region of the system ~1/3 R_vir), in the range -23.5< M_i'<-17.5, is well fitted by a Schechter function with M*i'=-21.3 +/- 0.4 and alpha = -0.6 +/- 0.3 or a Gaussian function centered on M_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 a photometric survey in g', r', i'-bands (and statistical background correction) confirms the spectroscopically determined results. There is a significant dip in the luminosity function at M_r'=-18 mag, as also observed for other clusters. RX~J1552.2+2013 is a rich, strongly red-galaxy dominated system, with at least 19 galaxies with magnitudes between m_3 and m_3 + 2, within a surveyed circular area of radius 625 kpc centered on the peak of the x-ray emission. Its mass, ~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 fossil group. The central object of RX J1552.2+2013 is a cD galaxy which may have accreted the more luminous ~L* former members of the group. Although dynamical friction and subsequent merging are probably the processes responsible for the lack of bright galaxies in the system, for the fainter members, there must be another mechanism in action (perhaps tidal disruption) to deplete the fossil group from intermediate-luminosity galaxies M_r' ~ -18.