Evidence for feedback in action from the molecular gas content in the z ~ 1.6 outflowing QSO XID2028^⋆,⋆⋆

¹ Dipartimento di Fisica e AstronomiaUniversità di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy
e-mail: marcella.brusa3@unibo.it
² Max-Planck Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching bei München, Germany
³ INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁴ IRAM–Institut de RadioAstronomie Millimétrique, 300 rue de la Piscine, 38406 Saint-Martin d’ Hères, France
⁵ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
⁶ INAF–Osservatorio Astronomico di Roma, via Frascati 33, 00044 Monte Porzio Catone (RM), Italy
⁷ INAF–Osservatorio Astronomico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
⁸ European Southern Observatory, Karl-Schwarzschild-str. 2, 85748 Garching bei München, Germany
⁹ Astronomy Centre, Department of Physics and Astronomy, University of Sussex, Brighton, BN1 9QH, UK
¹⁰ Dipartimento di Astronomia e Scienza dello Spazio, Università degli Studi di Firenze, Largo E. Fermi 2, 50125 Firenze, Italy
¹¹ Cavendish Laboratory, University of Cambridge, 19 J. J. Thomson Ave., Cambridge CB3 0HE, UK
¹² Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot, IRFU/Service d’Astrophysique, Bât. 709, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France

Received: 9 December 2014
Accepted: 3 March 2015

Abstract

Aims. Gas outflows are believed to play a pivotal role in shaping galaxies, as they regulate both star formation and black hole growth. Despite their ubiquitous presence, the origin and the acceleration mechanism of these powerful and extended winds is not yet understood. Direct observations of the cold gas component in objects with detected outflows at other wavelengths are needed to assess the impact of the outflow on the host galaxy interstellar medium (ISM).

Methods. We observed with the Plateau de Bure Interferometer an obscured quasar at z ~ 1.5, XID2028, for which the presence of an ionized outflow has been unambiguously signalled by NIR spectroscopy. The detection of ¹²CO(3−2) emission in this source allows us to infer the molecular gas content and compare it to the ISM mass derived from the dust emission. We then analyzed the results in the context of recent insights on scaling relations, which describe the gas content of the overall population of star-forming galaxies at a similar redshifts.

Results. The star formation efficiency (~100) and gas mass (M_gas = 2.1−9.5 × 10¹⁰ M_⊙) inferred from the CO(3−2) line depend on the underlying assumptions on the excitation of the transition and the CO-to-H2 conversion factor. However, the combination of this information and the ISM mass estimated from the dust mass suggests that the ISM/gas content of XID2028 is significantly lower than expected for its observed M_⋆, sSFR and redshift, based on the most up-to-date calibrations (with gas fraction <20% and depletion timescale <340 Myr).

Conclusions. Overall, the constraints we obtain from the far infrared and millimeter data suggest that we are observing QSO feedback able to remove the gas from the host.