The VIMOS VLT Deep Survey
Evolution of the major merger rate since z ∼ 1 from spectroscopically confirmed galaxy pairs

L. de Ravel, O. Le Fèvre, L. Tresse, D. Bottini, B. Garilli, V. Le Brun, D. Maccagni, R. Scaramella, M. Scodeggio, G. Vettolani, A. Zanichelli, C. Adami, S. Arnouts, S. Bardelli, M. Bolzonella, A. Cappi, S. Charlot, P. Ciliegi, T. Contini, S. Foucaud, P. Franzetti, I. Gavignaud, L. Guzzo, O. Ilbert, A. Iovino, F. Lamareille, H. J. McCracken, B. Marano, C. Marinoni, A. Mazure, B. Meneux, R. Merighi, S. Paltani, R. Pellò, A. Pollo, L. Pozzetti, M. Radovich, D. Vergani, G. Zamorani, E. Zucca, M. Bondi, A. Bongiorno, J. Brinchmann, O. Cucciati, S. de la Torre, L. Gregorini, P. Memeo, E. Pérez-Montero, Y. Mellier, P. Merluzzi, S. Temporin

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, Ω_M = 0.3 and Ω_Λ = 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 M_B(z) ≤ −18 − Q_z (Q = 1.11) are in pairs with separations Δr_p ≤ 20 h⁻¹ kpc, Δv ≤ 500 km s⁻¹, and with ΔM_B ≤ 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 M_B(z = 0) ≤ −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% a 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 rp ≤ 20 h⁻¹ kpc. Following published prescriptions to derive merger timescales, we find that the merger rate of M_B(z) ≤ −18− Qz galaxies evolves as N_mg = (4.96 ± 2.07) × 10⁻⁴ × (1 + z)^2.20±0.77 mergers Mpc⁻³ Gyr⁻¹.
Conclusions. The merger rate of galaxies with M_B(z) ≤ −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 log(M/M_☉) ≥ 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.

Keywords
galaxies: evolution – galaxies: interactions – galaxies: formation

Notes
based on observations obtained with the European Southern Observatory Telescopes at the Paranal Observatory, under programs 072.A-0586 and 073.A-0647.

Astronomy and Astrophysics
Volume 498, Page 379
May 2009

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