Black hole growth by accretion

Astronomy Department, The Ohio State University, 140 West 18th Avenue, Columbus, OH 43210, USA e-mail: smita@astronomy.ohio-state.edu

Received: 22 July 2004
Accepted: 9 November 2004

Abstract

We show that black holes grow substantially by accretion at close to Eddington rates. Using a complete sample of soft X-ray selected AGNs, Grupe & Mathur (2004, ApJ, 606, L41) have shown that narrow line Seyfert 1 galaxies, as a class, lie below the M_BH–σ relation of normal galaxies. Some NLS1s, however, lie on or close to the M_BH–σ relation. Here we show that not all NLS1s accrete at close to Eddington rates: those with low are close to the M_BH–σ relation, and those with high are far from it. With various tests in this paper, we argue that black holes grow in mass substantially in their high-accretion phase and approach the M_BH–σ relation over time. The mass growth in a low accretion phase, as in BLS1s and also in some NLS1s, appears to be insignificant. Any theoretical model attempting to explain the M_BH–σ relation needs to explain the above observations.