Black hole growth by accretion

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) have shown that narrow line Seyfert 1 galaxies, as a class, lie below the MBH–σ relation of normal galaxies. Some NLS1s, however, lie on or close to the MBH–σ relation. Here we show that not all NLS1s accrete at close to Eddington rates: those with low L/LEddington are close to the MBH–σ relation, and those with high L/LEddington are far. With various tests in this paper, we argue that black holes grow in mass substantially in their high-accretion phase and approach the MBH–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 MBH–σ relation will have to explain the above observations.