Black holes in pseudobulges: demography and models

¹ GEPI, Observatoire de Paris, CNRS, Univ. Paris Diderot, 5 Place Jules Janssen, 92195 Meudon, France
e-mail: francesco.shankar@obspm.fr
² Dipartimento di Astronomia, Universitá degli Studi di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁴ INFN/National Institute for Nuclear Physics, Sezione di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy
⁵ Department of Astronomy, Ohio State University, McPherson Laboratory, 140 W. 18th Ave., Columbus, OH 43210-1173, USA
⁶ Center for Cosmology and Astropharticle Physics, Ohio State University, Columbus, OH 43210, USA
⁷ Department of Physics and Astronomy, University of Pennsylvania, 209 South 33rd St, Philadelphia, PA 19104, USA
⁸ CEA, IRFU, SAp, 91191 Gif-sur-Yvette, France

Received: 3 November 2011
Accepted: 30 January 2012

Abstract

Context. There is mounting evidence that a significant fraction of black holes (BHs) today live in late-type galaxies, including bulge-less galaxies and those hosting pseudobulges, and are significantly undermassive with respect to the scaling relations followed by their counterpart BHs in classical bulges of similar stellar (or even bulge) mass.

Aims. Here we discuss the predictions of two state-of-the-art hierarchical galaxy formation models in which BHs grow via mergers and, in one, also via disc instability. Our aim is to understand if the wealth of new data on local BH demography is consistent with standard models.

Methods. We follow the merger trees of representative subsamples of BHs and compute the fractional contributions of different processes to the final BH mass.

Results. We show that the model in which BHs always closely follow the growth of their host bulges, also during late disk instabilities (i.e., bars), produces too narrow a distribution of BHs at fixed stellar mass to account for the numerous low-mass BHs now detected in later-type galaxies. Models with a looser connection between BH growth and bar instability instead predict the existence of a larger number of undermassive BHs, in better agreement with the observations.

Conclusions. The scatter in the updated local BH-bulge mass relation (with no restriction on galaxy type) appears to be quite large when including later-type systems, but it can still be managed to be reproduced within current hierarchical models. However, the fuelling of BHs during the late bar-instability mode needs to be better quantified/improved to properly fit the data. We conclude discussing how the possibly large number of BHs in later type galaxies demands for an in-depth revision of the local BH mass function and its modelling.