Letter to the Editor

Molecular gas in QSO host galaxies at z > 5

¹ INAF – Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy e-mail: maiolino@oa-roma.inaf.it
² IRAM, 300 rue de la Piscine, 38406 St.-Martin-d'Hères, France
³ Institut d'Astrophysique Spatiale, Université Paris-Sud, 91405 Orsay, France
⁴ Argelander-Institut für Astronomie, University of Bonn, Auf dem Hugel 71, 53121 Bonn, Germany
⁵ National Radio Astronomy Observatory, PO Box O, Socorro, NM 87801, USA
⁶ INAF – Osservatorio Astrofisico di Arcetri, L.go E. Fermi 5, 50125 Firenze, Italy
⁷ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 42, Cambridge, MA 02138, USA
⁸ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁹ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁰ Institut d'Astrophysique de Paris, Université Pierre & Marie Curie, 98bis boulevard Arago, 75014 Paris, France
¹¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

Received: 22 June 2007
Accepted: 17 July 2007

Abstract

We present observations with the IRAM Plateau de Bure Interferometer of three QSOs at aimed at detecting molecular gas in their host galaxies as traced by CO transitions. CO (5–4) is detected in SDSS J033829.31+002156.3 at .0267, placing it amongst the most distant sources detected in CO. The CO emission is unresolved with a beam size of ~, implying that the molecular gas is contained within a compact region, less than ~3 kpc in radius. We infer an upper limit on the dynamical mass of the CO emitting region of ~. The comparison with the Black Hole mass inferred from near-IR data suggests that the BH-to-bulge mass ratio in this galaxy is significantly higher than in local galaxies. From the CO luminosity we infer a mass reservoir of molecular gas as high as , implying that the molecular gas accounts for a significant fraction of the dynamical mass. When compared to the star formation rate derived from the far-IR luminosity, we infer a very short gas exhaustion timescale (~10⁷ years), comparable to the dynamical timescale. CO is not detected in the other two QSOs (SDSS J083643.85+005453.3 and SDSS J163033.90+401209.6) and upper limits are given for their molecular gas content. When combined with CO observations of other type 1 AGNs, spanning a wide redshift range (), we find that the host galaxy CO luminosity (hence molecular gas content) and the AGN optical luminosity (hence BH accretion rate) are correlated, but the relation is not linear: . Moreover, at high redshifts (and especially at ) the CO luminosity appears to saturate. We discuss the implications of these findings in terms of black hole-galaxy co-evolution.