What can we learn from quasar absorption spectra?

Abstract

We analyze optical-near infrared spectra of a large sample of quasars at high redshift with the aim of investigating both the cosmic reionization history at z ~ 6 and the properties of dust extinction at z > 4. In order to constrain cosmic reionization, we study the transmitted flux in the region blueward the Lya emission line in a sample of 17 quasars spectra at 5.7 ≤ zem ≤ 6.4. By comparing the properties of the observed spectra with the results of a semi-analytical model of the Lyα forest we find that actual data favor a model in which the Universe is highly ionized at z ∼ 6, thus being consistent with an epoch of reionization at higher redshifts. For what concerns the study of the high-z dust, we focus our attention on the region redward the Lya emission line of 33 quasars at 4 ≤ zem ≤ 6.4. We compute simulated dust-absorbed quasar spectra by taking into account a large grid of extinction curves. We find that the SMC extinction curve, which has been shown to reproduce the dust reddening of most quasars at z < 4, is not a good prescription for describing dust extinction also at higher redshifts.