Highly accreting quasars at low and high z.

Abstract

Abstract This paper aims at reviewing and discussing the multifrequency properties of quasars accreting at very high rate in the context of the 4DE1 conceptual framework. The 4DE1 approach allows us to define luminosity-independent selection criteria that go beyond the restriction to Narrow Line Seyfert tis identified at low redshift. Highly accreting quasars are characterised by observational properties (steep soft-and hard X continuum, strong Fell emission, and extreme high ionization line profile blueshifts) that make them easily recognisable, and indicate extreme physical properties with respect to the general quasar population as well: an optically thick ADAF, very high metallicity, and a strong high ionization wind. We discuss some puzzling aspects related to their formation and evolution. We stress that the importance of the sources goes beyond the understanding of quasar physics: their Eddington ratio is expected to saturate toward values of order unity, making them possible cosmological probes.