Feii/MgiiEmission‐Line Ratio in High‐Redshift Quasars

We present results of the analysis of near infrared spectroscopicobservations of 6 high-redshift quasars (z > 4), emphasizing the measurement ofthe ultraviolet FeII/MgII emission line strength in order to estimate thebeginning of intense star formation in the early universe. To investigate theevolution of the FeII/MgII ratio over a wider range in cosmic time, we measuredthis ratio for composite quasar spectra which cover a redshift range of 0 < z <5 with nearly constant luminosity, as well as for those which span ~6 orders ofmagnitude in luminosity. A detailed comparison of the high-redshift quasarspectra with those of low-redshift quasars with comparable luminosity showsessentially the same FeII/MgII emission ratios and very similar continuum andline spectral properties, i.e. a lack of evolution of the relative iron tomagnesium abundance of the gas in bright quasars since z=5. Currentnucleosynthesis and stellar evolution models predict that alpha-elements likemagnesium are produced in massive stars ending in type II SNe, while iron isformed predominantly in SNe of type Ia with intermediate mass progenitors. Thisresults in an iron enrichment delay of 0.2 to 0.6 Gyr. We conclude that intensestar formation activity in the host galaxies of z>4 quasars must have startedalready at an epoch corresponding to z_f = 6 to 9, when the age of the universewas ~0.5 Gyr (H_o = 72 km/s/Mpc, Omega_M = 0.3, Omega_Lambda = 0.7). This epochcorresponds well to the re-ionization era of the universe.Comment: 22 pages, 5 figures, accepted for publication in ApJ (vol.596, Oct03