AGN host galaxies at redshift z  ≈  0.7: peculiar or not?

¹ Institute for Astro- and Particle Physics, University of Innsbruck, Technikerstr. 25/8, 6020 Innsbruck, Austria
e-mail: Asmus.Boehm@uibk.ac.at
² Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
³ University of Michigan, Department of Astronomy, 830 Dennison Building, 500 Church Street, Ann Arbor, MI 48105, USA
⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁵ Department of Physics, Denys Wilkinson Building, University of Oxford, Keble Road, Oxford, OX1 3RH, UK
⁶ Institute of Cosmology and Gravitation, University of Portsmouth, Hampshire Terrace, Portsmouth, PO1 2EG, UK
⁷ School of Physics and Astronomy, The University of Nottingham, University Park, Nottingham NG7 2RD, UK
⁸ Department of Anthropology, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada
⁹ Department of Astronomy, University of Texas at Austin, 1 University Station, C1400 Austin, TX 78712-0259, USA
¹⁰ Department of Physics, University of Missouri-Kansas City, Kansas City, MO 64110, USA
¹¹ Giant Magellan Telescope Organization, 251 South Lake Avenue, Pasadena, CA 91101, USA
¹² Institut de Ciéncies del Cosmos, University of Barcelona, 08028 Barcelona, Spain
¹³ Department of Physics and Astronomy, University Of Waterloo, Waterloo, Ontario, N2L 3G1, Canada
¹⁴ Department of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland
¹⁵ University of Texas, McDonald Observatory, Fort Davis, TX 79734, USA
¹⁶ Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh, EH9 3HJ, UK
¹⁷ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
¹⁸ Centro Hispano Aleman de Calar Alto, C/Jesus Durban Remon 2-2, 04004 Almeria, Spain
¹⁹ Purple Mountain Observatory, National Astronomical Observatories, Chinese Academy of Sciences, 210008 Nanjing, PR China

Received: 21 July 2010
Accepted: 19 October 2012

Abstract

Aims. We perform a quantitative morphological comparison between the hosts of active galactic nuclei (AGN) and quiescent galaxies at intermediate redshifts (z ≈ 0.7). The imaging data are taken from the large HST/ACS mosaics of the GEMS and STAGES surveys. Our main aim is to test whether nuclear activity at this cosmic epoch is triggered by major mergers.

Methods. Using images of quiescent galaxies and stars, we created synthetic AGN images to investigate the impact of an optical nucleus on the morphological analysis of AGN hosts. Galaxy morphologies are parameterized using the asymmetry index A, the concentration index C, the Gini coefficient G, and the M₂₀ index. A sample of  ~200 synthetic AGN was matched to 21 real AGN in terms of redshift, host brightness, and host-to-nucleus ratio to ensure a reliable comparison between active and quiescent galaxies.

Results. The optical nuclei strongly affect the morphological parameters of the underlying host galaxy. Taking these effects into account, we find that the morphologies of the AGN hosts are clearly distinct from galaxies undergoing violent gravitational interactions. Indeed, the host galaxy distributions in morphological descriptor space are more similar to undisturbed galaxies than to major mergers.

Conclusions. Intermediate-luminosity (L_X ≲ 10⁴⁴   erg/s) AGN hosts at z ≈ 0.7 show morphologies similar to the general population of massive galaxies with significant bulges at the same redshifts. If major mergers are the driver of nuclear activity at this epoch, the signatures of gravitational interactions fade rapidly before the optical AGN phase starts, making them undetectable on single-orbit HST images, at least with usual morphological descriptors. This could be investigated in future synthetic observations created from numerical simulations of galaxy-galaxy interactions.