The VIMOS Ultra-Deep Survey: A major merger origin for the high fraction of galaxies at 2 < z < 6 with two bright clumps^⋆

¹ Aix-Marseille Univ., CNRS, LAM, Laboratoire d’Astrophysique de Marseille, 13284 Marseille, France
e-mail: bruno.ribeiro@lam.fr
² Instituto de Fisica y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Gran Bretaña 1111, Playa Ancha, Valparaíso, Chile
³ INAF–IASF Milano, via Bassini 15, 20133 Milano, Italy
⁴ Department of Physics, University of California, Davis, One Shields Ave., Davis, CA 95616, USA
⁵ Geneva Observatory, University of Geneva, Ch. des Maillettes 51, 1290 Versoix, Switzerland
⁶ Institut de Recherche en Astrophysique et Planétologie, IRAP, CNRS, Université de Toulouse, UPS-OMP, 14 avenue E. Belin, 31400 Toulouse, France
⁷ INAF–Osservatorio Astronomico di Bologna, via Ranzani, 40127 Bologna, Italy
⁸ Cavendish Laboratory, University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 0HE, UK
⁹ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁰ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹¹ Scientific Support Office, Directorate of Science and Robotic Exploration, ESA-ESTEC, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands

Received: 14 November 2016
Accepted: 21 August 2017

Abstract

The properties of stellar clumps in star-forming galaxies and their evolution over the redshift range 2 ≲ z ≲ 6 are presented and discussed in the context of the build-up of massive galaxies at early cosmic times. We focused on galaxies with spectroscopic redshifts from the VIMOS Ultra Deep Survey (VUDS) and stellar masses log ₁₀(M_⋆/M_⊙) > −0.204 × (z−4.5) + 9.35. We analyzed HST-ACS images to identify clumps within a 20 kpc radius using a method taking into account differential surface brightness dimming and luminosity evolution with redshift. We find that the population of galaxies with more than one clump is dominated by galaxies with two clumps, representing ~21–25% of the population, while the fraction of galaxies with three, or four and more, clumps is 8–11% and 7–9%, respectively. The fraction of clumpy galaxies is in the range ~35–55% over 2 < z < 6, increasing at higher redshifts, indicating that the fraction of irregular galaxies remains high up to the highest redshifts. The large and bright clumps (M_⋆ ~ 10⁹ up to ~10¹⁰ M_⊙) are found to reside predominantly in galaxies with two clumps. Smaller and lower luminosity clumps (M_⋆ < 10⁹ M_⊙) are found in galaxies with three clumps or more. We interpret these results as evidence for two different modes of clump formation working in parallel. The small low luminosity clumps are likely the result of disk fragmentation, with violent disk instabilities (VDI) forming several long-lived clumps in-situ as suggested from simulations. A fraction of these clumps is also likely coming from minor mergers as confirmed from spectroscopy in several cases. The clumps in the dominating population of galaxies with two clumps are significantly more massive and have properties akin to those in galaxy pairs undergoing massive merging observed at similar redshifts; they appear as more massive than the most massive clumps observed in numerical simulations of disks with VDI. We infer from these properties that the bright and large clumps are most likely the result of major mergers bringing-in ex situ matter onto a galaxy, and we derive a high major merger fraction of ~20%. The diversity of clump properties therefore suggests that the assembly of star-forming galaxies at z ~ 2–6 proceeds from several different dissipative processes including an important contribution from major and minor mergers.