The dominant role of mergers in the size evolution of massive early-type galaxies since z  ~  1^⋆

¹ Laboratoire d’Astrophysique de Marseille – LAM, Université d’Aix-Marseille & CNRS, UMR 7326, 38 rue F. Joliot-Curie, 13388 Marseille Cedex 13, France
² Centro de Estudios de Física del Cosmos de Aragón, Plaza San Juan 1, planta 2, 44001 Teruel, Spain
e-mail: clsj@cefca.es
³ California Institute of Technology, MC 105-24, 1200 East California Boulevard, Pasadena, CA 91125, USA
⁴ INAF Osservatorio Astronomico di Trieste, via Tiepolo, 11, 34143 Trieste, Italy
⁵ Institute of Astronomy, ETH Zurich, 8093 Zurich, Switzerland
⁶ INAF Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁷ Infrared Processing and Analysis Center, California Institute of Technology 100-22, Pasadena, CA 91125, USA
⁸ Institut de Recherche en Astrophysique et Planétologie (IRAP), CNRS, 14 avenue Édouard Belin, 31400 Toulouse, France
⁹ IRAP, Université de Toulouse, UPS-OMP, Toulouse, France
¹⁰ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
¹¹ Dipartimento di Astronomia, Universitá di Padova, vicolo Osservatorio 3, 35122 Padova, Italy
¹² Institute for Astronomy, 2680 Woodlawn Drive, University of Hawaii, Honolulu, HI 96822, USA
¹³ INAF – IASF, via Bassini 15, 20133 Milano, Italy
¹⁴ Research Center for Space and Cosmic Evolution, Ehime University, Bunkyo-cho 2-5, 790-8577 Matsuyama, Japan
¹⁵ CNRS, AIM-Unite Mixte de Recherche CEA-CNRS-Université Paris VII-UMR 7158, 91191 Gif-sur-Yvette, France
¹⁶ Max-Planck-Institut für Extraterrestrische Physik, 84571 Garching b. Muenchen, Germany
¹⁷ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
¹⁸ SUPA, Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
¹⁹ INAF Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
²⁰ MPA – Max Planck Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany
²¹ University of Vienna, Department of Astronomy, Tuerkenschanzstrasse 17, 1180 Vienna, Austria
²² Institut d’Astrophysique de Paris, UMR 7095 CNRS, Université Pierre et Marie Curie, 98bis boulevard Arago, 75014 Paris, France
²³ Instituto de Astrofísica de Andalucía, CSIC, Apdo. 3004, 18080 Granada, Spain
²⁴ Universitá degli Studi dellInsubria, via Valleggio 11, 22100 Como, Italy
²⁵ Instituto de Astrofísica de Canarias, vía Lactea s/n, 38200 La Laguna, Tenerife, Spain
²⁶ Departamento de Astrofísica, Universidad de La Laguna, 38205 Tenerife, Spain
²⁷ IPMU, Institute for the Physics and Mathematics of the Universe, 5-1-5 Kashiwanoha, 277-8583 Kashiwa, Japan
²⁸ INAF – IASF Bologna, via P. Gobetti 101, 40129 Bologna, Italy
²⁹ Dipartimento di Astronomia, Universitá di Bologna, via Ranzani 1, 40127 Bologna, Italy
³⁰ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
³¹ Astrophysical Observatory, City University of New York, College of Staten Island, 2800 Victory Blvd, Staten Island, NY 10314, USA
³² UCO/Lick Observatory, Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA
³³ Department of Physics, Yale University, New Haven, CT 06511, USA
³⁴ Yale Center for Astronomy and Astrophysics, Yale University, PO Box 208121, New Haven, CT 06520, USA
³⁵ Institut d’Astrophysique Spatiale, Bâtiment 121, CNRS & Université Paris Sud XI, 91405 Orsay Cedex, France

Received: 21 February 2012
Accepted: 26 August 2012

Abstract

Aims. The role of galaxy mergers in massive galaxy evolution, and in particular to mass assembly and size growth, remains an open question. In this paper we measure the merger fraction and rate, both minor and major, of massive early-type galaxies (M _⋆  ≥ 10¹¹  M_⊙) in the COSMOS field, and study their role in mass and size evolution.

Methods. We used the 30-band photometric catalogue in COSMOS, complemented with the spectroscopy of the zCOSMOS survey, to define close pairs with a separation on the sky plane 10   h^-1 kpc  ≤ r_p ≤ 30   h^-1 kpc and a relative velocity Δv ≤ 500 km s^-1 in redshift space. We measured both major (stellar mass ratio μ ≡ M _⋆ ,2/M _⋆ ,1 ≥ 1/4) and minor (1/10 ≤ μ < 1/4) merger fractions of massive galaxies, and studied their dependence on redshift and on morphology (early types vs. late types).

Results. The merger fraction and rate of massive galaxies evolves as a power-law (1 + z)ⁿ, with major mergers increasing with redshift, n_MM = 1.4, and minor mergers showing little evolution, n_mm ~ 0. When split by their morphology, the minor merger fraction for early-type galaxies (ETGs) is higher by a factor of three than that for late-type galaxies (LTGs), and both are nearly constant with redshift. The fraction of major mergers for massive LTGs evolves faster (n_MM^LT ~ 4 ) than for ETGs (n_MM^ET= 1.8).

Conclusions. Our results show that massive ETGs have undergone 0.89 mergers (0.43 major and 0.46 minor) since z ~ 1, leading to a mass growth of  ~30%. We find that μ ≥ 1/10 mergers can explain  ~55% of the observed size evolution of these galaxies since z ~ 1. Another  ~20% is due to the progenitor bias (younger galaxies are more extended) and we estimate that very minor mergers (μ < 1/10) could contribute with an extra  ~20%. The remaining  ~5% should come from other processes (e.g., adiabatic expansion or observational effects). This picture also reproduces the mass growth and the velocity dispersion evolution of these galaxies. We conclude from these results, and after exploring all the possible uncertainties in our picture, that merging is the main contributor to the size evolution of massive ETGs at z ≲ 1, accounting for  ~50−75% of that evolution in the last 8 Gyr. Nearly half of the evolution due to mergers is related to minor (μ < 1/4) events.