Detailed characterization of Hβ emission line profile in low-<i>z</i>SDSS quasars

Abstract

We explore the properties of the H_beta emission line profile in a large,\nhomogeneous and bright sample of N~470 low redshift quasars extracted from\nSloan Digital Sky Survey (DR5). We approach the investigation from two\ncomplementary directions: composite/median spectra and a set of line diagnostic\nmeasures (asymmetry index, centroid shift and kurtosis) in individual quasars.\nThe project is developed and presented in the framework of the so-called 4D\nEigenvector 1 (4DE1) Parameter Space, with a focus on its optical dimensions,\nFWHM(H_beta) and the relative strength of optical FeII\n(R_FeII=W(FeII4434-4684)/W(H_beta)). We reenforce the conclusion that not all\nquasars are alike and spectroscopically they do not distribute randomly about\nan average typical optical spectrum. Our results give further support to the\nconcept of two populations A and B (narrower and broader than 4000 km/s\nFWHM(H_beta), respectively) that emerged in the context of 4DE1 space. The\nbroad H_beta profiles in composite spectra of Population A sources are best\ndescribed by a Lorentzian and in Population B by a double Gaussian model.\nMoreover, high and low accretion sources (an alternative view of the Population\nA/B concept) not only show significant differences in terms of Black Hole (BH)\nmass and Eddington ratio L_bol/L_Edd, but they also show distinct properties in\nterms of line asymmetry, shift and shapes. We finally suggest that a potential\nrefinement of the 4DE1 space can be provided by separating two populations of\nquasars at R_FeII~0.50 rather than at FWHM(H_beta)=4000 km/s. Concomitantly,\nthe asymmetry and centroid shift profile measures at 1/4 fractional intensity\ncan be reasonable surrogates for the FWHM(H_beta) dimension of the current\n4DE1.\n