Near-Infrared Spectroscopy of the High Redshift Quasar S4 0636+68 at z=3.2

Abstract

We present near-infrared (observed frame) spectra of the high-redshift quasar S4 0636+68 at z=3.2 which was previously thought to be one of a group of ``strong'' Fe II emitters [i.e., F(Fe II 4434-4684)/F(Hbeta) > 1]. Our K-band spectrum clearly shows emission lines of Hbeta and [O III]4959, 5007 as well as optical Fe II emission. Our computed value of F(Fe II 4434-4684)/F(Hbeta) ~ 0.8 for S4 0636+68 is less than previously thought, and in fact is comparable to values found for radio-loud, flat-spectrum, low-z quasars. Therefore S4 0636+68 appears not to be a strong optical Fe II emitter. Although more than half (5/8) of the high-z quasars observed to date are still classified as strong optical Fe II emitters, their Fe II/Hbeta ratios, for the most part, follow the same trend as that of low-$z$ quasars, i.e., an anticorrelation in EW(Fe II)/EW(Hbeta) versus EW([O III])/EW(Hbeta), with radio-loud quasars having a mean value of EW(Fe II)/EW(Hbeta) approximately half that of radio-quiet quasars at comparable values of EW([O III])/EW(Hbeta).