The extinction law at high redshift and its implications

We analyze the optical-near infrared spectra of 33 quasars with redshifts 3.9 ≤ z ≤ 6.4 to investigate the properties of dust extinction at these cosmic epochs. The SMC extinction curve has been shown to reproduce the dust reddening of most quasars at z 4, both by performing a simultaneous fit of all quasars and by averaging the extinction curves inferred for individual quasars. In the case of broad absorption line quasars (which are generally more absorbed by dust and possibly in a younger evolutionary stage), the mean extinction curve deviates from the SMC at a confidence level ≥95%. The difference between extinction curves in quasars at z > 4 and in quasars at lower redshift is indicative of either a different dust production mechanism at high redshift, or a different mechanism for processing dust into the ISM. We suggest that the same transitions may also apply to normal, star-forming galaxies at z > 4. In particular, the observed change in the average spectral slope of galaxies at z > 4 may be partially ascribed to a variation in the extinction curve, rather than a lower dust content at high redshift. In this scenario, the extinction curve inferred at z > 4 would imply a cosmic star-formation corrected for dust attenuation a factor of ~2 higher than estimated in the past.