X-ray spectral evolution of high redshift quasars

Jill Bechtold,Olga Kuhn,Roc M Cutri,Jonathan C Mcdowell,Aneta Siemiginowska,Marcia Rieke,Martin Elvis,Belinda J Wilkes,Fabrizio Fiore

doi:10.1086/117111

Abstract

At z approx. equals 3, the x-ray spectra of radio-loud and radio-quiet quasars are different. High-redshift radio-quiet quasars either have large absorbing columns, N(sub H), and steeper power law spectral indices, alpha(sub epsilon), than low redshift quasars, or no absorption and similar alpha(sub epsilon)'s. In contrast, the radio-loud quasars at high redshift have substantial absorption and similar alpha(sub epsilon)'s to low redshift quasars. Implications for the interpretation of the evolution of the luminosity function of quasars are discussed. If the absorption arises outside the central engine for both radio-loud and radio-quiet quasars, then radio-quiet quasars differ from the radio-loud quasars in that their emitted power law spectrum has evolved with redshift. We argue that this favors models where quasars are numerous and short-lived, rather than rare and long-lived.

Highlights

A basic characteristic of quasars is the scale-free nature of their emission
Since there is no evidence for very large black hole masses in nearby Seyferts or ellipticals and since there are indications that the ratio of the luminosity to its Eddington value in nearby Seyferts and intermediate redshift quasars is between 0.01 and 0.1 (Wandel & Mushotzky 1986; Padovani & Rafanelli 1988; Chokshi & Turner 1992; Small & Blandford 1992), it seems unlikely that the evolution of the luminosity function could be explained by the gradual decline in luminosity of individual, rare objects
A further test for the long-lived option is the evolution of the quasar continuum, since as the quasar black holes grow larger and become more starved of accreting matter, the continuum they produce is likely to change in form

Summary

INTRODUCTION

From the lowest luminosity AGN (e.g., M81) to the highest redshift, most luminous quasars, the quasar continuum and emission lines scale almost linearly with luminosity, regardless of redshift (e.g., Netzer 1990; Blandford 1990, and references therein) This is true within both the radio-loud and the radio-quiet classes of quasar. Radio-quiet quasars instead continue to follow the correlation of aox increasing with increasing Lopt seen at lower redshift. These observations suggest that breaks in the scaling laws for quasars are beginning to appear which may be related to the evolution of the quasar population as a whole. In this paper we review the data supporting these claims, and discuss their potential value for understanding quasar emission mechanisms

X-RAY SPECTRA AND HARDNESS RATIOS

X-RAY SPECTRAL EVOLUTION

DISCUSSION

Findings

X-ray Spectral Evolution

CONCLUSIONS