Evidence for major mergers of galaxies at 2 ≲ z < 4 in the VVDS and VUDS surveys^⋆

¹ Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
e-mail: lidia.tasca@oamp.fr
² Centro de Estudios de Física del Cosmos de Aragón, Plaza San Juan 1, planta 2, 44001 Teruel, Spain
³ INAF – IASF, via Bassini 15, 20133 Milano, Italy
⁴ INAF – Osservatorio Astronomico di Bologna, via Ranzani, 1, 40127 Bologna, Italy
⁵ Institut de Recherche en Astrophysique et Planétologie – IRAP, CNRS, Université de Toulouse, UPS-OMP, 14 avenue E. Belin, 31400 Toulouse, France
⁶ Institut d’Astrophysique de Paris, UMR 7095 CNRS, Université Pierre et Marie Curie, 98bis boulevard Arago, 75014 Paris, France
⁷ INAF – Osservatorio Astronomico di Roma, via di Frascati 33, 00040, Monte Porzio Catone, Italy
⁸ Astronomy Department, University of Massachusetts, Amherst, MA 01003, USA
⁹ LASTRO, École polytechnique fédérale de Lausanne, Suisse
¹⁰ Max-Planck-Institut fur extraterrestrische Physik, Giessenbachstrasse, 85748 Garching bei Munchen, Germany
¹¹ Research Center for Space and Cosmic Evolution, Ehime University, Bunkyo–cho, 790–8577 Matsuyama, Japan
¹² INAF–IASF Bologna, via P. Gobetti 101, 40129 Bologna, Italy

Received: 19 March 2013
Accepted: 25 February 2014

Abstract

Context. The mass assembly of galaxies can proceed through different physical processes. Here we report on the spectroscopic identification of close physical pairs of galaxies at redshifts 2 ≲ z< 4 and discuss the impact of major mergers in building galaxies at these early cosmological times.

Aims. We aim to identify and characterize close physical pairs of galaxies destined to merge and use their properties to infer the contribution of merging processes to the early mass assembly of galaxies.

Methods. We searched for galaxy pairs with a transverse separation r_p ≤ 25h^-1 kpc and a velocity difference Δv ≤ 500 km s^-1 using early data from the VIMOS Ultra Deep Survey (VUDS) that comprise a sample of 1111 galaxies with spectroscopic redshifts measurements at redshifts 1.8 ≤ z ≤ 4 in the COSMOS, ECDFS, and VVDS–02h fields, combined with VVDS data. We analysed their spectra and associated visible and near-infrared photometry to assess the main properties of merging galaxies that have an average stellar mass M_⋆ = 2.3 × 10¹⁰ M_⊙ at these redshifts.

Results. Using the 12 physical pairs found in our sample we obtain a first robust measurement of the major merger fraction at these redshifts, f_MM = 19.4_-6⁺⁹%. These pairs are expected to merge within 1 Gyr on average each producing a more massive galaxy by the time the cosmic star formation peaks at z ~ 1 − 2. Using the pairs’ merging time scales, we derive a merging rate of R_MM = 0.17_-0.05^+0.08 Gyr⁻¹. From the average mass ratio between galaxies in the pairs, the stellar mass of the resulting galaxy after merging will be ~60% higher than the most massive galaxy in the pair before merging. We conclude that major merging of galaxy pairs is on-going at 2 ≲ z< 4 and is significantly contributing to the major mass assembly phase of galaxies at this early epoch.