Letter to the Editor

The M_BH − M_star relation of obscured AGNs at high redshift^⋆

¹ Dipartimento di FisicaUniversità Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
² INAF - Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
e-mail: roberto.maiolino@oa-roma.inaf.it
³ Dipartimento di Astronomia, Università degli Studi di Bologna, via Ranzani1, 40127 Bologna, Italy
⁴ INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁵ Dipartimento Fisica e Astronomia, Università degli Studi di Firenze, Largo E. Fermi 2, 50125 Firenze, Italy

Received: 6 September 2010
Accepted: 4 October 2010

Abstract

We report the detection of broad Hα emission in three X-ray selected obscured AGNs at z ~ 1–2. By exploiting the Hα width and the intrinsic X-ray luminosity, we estimate their black hole masses, which are in the range 0.1–3 × 10⁹   M_⊙. By means of multi-band photometric data, we measure the stellar mass of their host galaxy and, therefore, infer their M_BH/M_star ratio. These are the first obscured AGNs at high-z, selected based on their black hole accretion (i.e. on the basis of their X-ray luminosity), that can be located on the M_BH − M_star relation at high-z. All of these obscured high-z AGNs are fully consistent with the local M_BH − M_star relation. This result conflicts with those for other samples of AGNs in the same redshift range, whose M_BH/M_star ratio departs significantly from the value observed in local galaxies. We suggest that the obscured AGNs in our sample are in an advanced evolutionary stage, have already settled onto the local M_BH-M_star relation, and whose nuclear activity has been temporarily revived by recent galaxy interactions.