Ultraviolet Fe ii emission in fainter quasars: luminosity dependences, and the influence of environments

Abstract

We investigate the strength of ultraviolet Fe ii emission in fainter quasars compared with brighter quasars for 1.0 a parts per thousand currency sign z a parts per thousand currency sign 1.8, using the Sloan Digital Sky Survey (SDSS) DR7QSO catalogue and spectra of Schneider et al., and the SDSS Faint Quasar Survey (SFQS) catalogue and spectra of Jiang et al. We quantify the strength of the UV Fe ii emission using the W2400 equivalent width of Weymann et al., which is defined between two rest-frame continuum windows at 2240-2255 and 2665-2695 . The main results are the following. (1) We find that for W2400 a parts per thousand(3) 25 there is a universal (i.e. for quasars in general) strengthening of W2400 with decreasing intrinsic luminosity, L3000. (2) In conjunction with previous work by Clowes et al., we find that there is a further, differential, strengthening of W2400 with decreasing L3000 for those quasars that are members of Large Quasar Groups (LQGs). (3) We find that increasingly strong W2400 tends to be associated with decreasing full width at half maximum (FWHM) of the neighbouring Mg ii lambda 2798 broad emission line. (4) We suggest that the dependence of W2400 on L3000 arises from Ly alpha fluorescence. (5) We find that stronger W2400 tends to be associated with smaller virial estimates from Shen et al. of the mass of the central black hole, by a factor of similar to 2 between the ultrastrong emitters and the weak. Stronger W2400 emission would correspond to smaller black holes that are still growing. The differential effect for LQG members might then arise from preferentially younger quasars in the LQG environments.