The VIMOS VLT Deep Survey^*

Evolution of the major merger rate since z ~ 1 from spectroscopically confirmed galaxy pairs

¹ Laboratoire d'Astrophysique de Marseille, UMR 6110 CNRS-Université de Provence, BP8, 13376 Marseille Cedex 12, France e-mail: loic.deravel@oamp.fr
² IASF - INAF, via Bassini 15, 20133, Milano, Italy
³ IRA - INAF, via Gobetti 101, 40129, Bologna, Italy
⁴ INAF - Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
⁵ Canada France Hawaii Telescope corporation, Mamalahoa Hwy, Kamuela, HI-96743, USA
⁶ INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, 40127, Bologna, Italy
⁷ Max Planck Institut für Astrophysik, 85741 Garching, Germany
⁸ Institut d'Astrophysique de Paris, UMR 7095, 98 bis Bvd Arago, 75014 Paris, France
⁹ Laboratoire d'Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 14 avenue E. Belin, 31400 Toulouse, France
¹⁰ School of Physics & Astronomy, University of Nottingham, University Park, Nottingham NG72RD, UK
¹¹ Astrophysical Institute Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
¹² INAF - Osservatorio Astronomico di Brera, via Brera 28, 20021 Milan, Italy
¹³ Institute for Astronomy, 2680 Woodlawn Dr., University of Hawaii, Honolulu, Hawaii, 96822, USA
¹⁴ Observatoire de Paris, LERMA, 61 avenue de l'Observatoire, 75014 Paris, France
¹⁵ Università di Bologna, Dipartimento di Astronomia, via Ranzani 1, 40127, Bologna, Italy
¹⁶ Centre de Physique Théorique, UMR 6207 CNRS-Université de Provence, 13288 Marseille, France
¹⁷ Max Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching bei München, Germany
¹⁸ Universitätssternwarte München, Scheinerstrasse 1, 81679 München, Germany
¹⁹ Integral Science Data Centre, Ch. d'Écogia 16, 1290 Versoix, Switzerland
²⁰ Geneva Observatory, Ch. des Maillettes 51, 1290 Sauverny, Switzerland
²¹ The Andrzej Soltan Institute for Nuclear Studies, ul. Hoza 69, 00-681 Warsaw, Poland
²² INAF - Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131, Napoli, Italy
²³ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
²⁴ Universitá di Milano-Bicocca, Dipartimento di Fisica, Piazza delle Scienze 3, 20126 Milano, Italy

Received: 11 July 2008
Accepted: 31 January 2009

Abstract

Context. The rate at which galaxies grow via successive mergers is a key element in understanding the main phases of galaxy evolution.

Aims. We measure the evolution of the fraction of galaxies in pairs and the merging rate since redshift z ~ 1 assuming a (H₀ = 70 km s^-1 Mpc^-1, = 0.3 and = 0.7) cosmology.

Methods. From the VIMOS VLT Deep Survey we use a sample of 6464 galaxies with I_AB≤ 24 to identify 314 pairs of galaxies, each member with a secure spectroscopic redshift, which are close in both projected separation and in velocity.

Results. We estimate that at z ~ 0.9, 10.9 ± 3.2% of galaxies with ≤ -18-Qz (Q = 1.11) are in pairs with separations ≤ 20 h^-1 kpc, ≤ 500 km s^-1, and with ≤ 1.5, significantly larger than 3.8 ± 1.7% at z ~ 0.5; thus, the pair fraction evolves as (1 + z)^m with m = 4.73 ± 2.01. For bright galaxies with ≤ -18.77, the pair fraction is higher and its evolution with redshift is flatter with m = 1.50 ± 0.76, a property also observed for galaxies with increasing stellar masses. Early-type pairs (dry mergers) increase their relative fraction from 3% at z ~ 0.9 to 12% at z ~ 0.5. The star formation rate traced by the rest-frame [OII] EW increases by 26 ± 4% for pairs with the smallest separation r_p≤ 20 h^-1 kpc. Following published prescriptions to derive merger timescales, we find that the merger rate of -18-Qz galaxies evolves as N_mg = (4.96 ± 2.07)10^-4 mergers Mpc^-3 Gyr^-1.

Conclusions. The merger rate of galaxies with ≤ -18-Qz has significantly evolved since z ~ 1 and is strongly dependent on the luminosity or stellar mass of galaxies. The major merger rate increases more rapidly with redshift for galaxies with fainter luminosities or stellar mass, while the evolution of the merger rate for bright or massive galaxies is slower, indicating that the slow evolution reported for the brightest galaxies is not universal. The merger rate is also strongly dependent on the spectral type of galaxies involved. Late-type mergers were more frequent in the past, while early-type mergers are more frequent today, contributing to the rise in the local density of early-type galaxies. About 20% of the stellar mass in present day galaxies with ≥ 9.5 has been accreted through major merging events since z = 1. This indicates that major mergers have contributed significantly to the growth in stellar mass density of bright galaxies over the last half of the life of the Universe.