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

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 λλ4434–4684)/F(Hβ) > 1]. Our K-band spectrum clearly shows emission lines of Hβ and [O III] λλ4959, 5007, as well as optical Fe II emission. Our computed value of F(Fe II λλ4434–4684)/F(Hβ) ≃ 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 () of the high-z quasars observed to date are still classified as strong optical Fe II emitters, their Fe II/Hβ ratios, for the most part, follow the same trend as do those of low-z quasars, i.e., an anticorrelation in EW(Fe II)/EW(Hβ) versus EW([O III])/EW(Hβ), with radio-loud quasars having a mean value of EW(Fe II)/EW(Hβ) approximately half that of radio-quiet quasars at comparable values of EW([O III])/EW(Hβ).