The VIMOS VLT Deep Survey^⋆

The different assembly history of passive and star-forming L_B ≳ L_B^B galaxies in the group environment at z < 1

¹ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
e-mail: clsj@cefca.es
² Centro de Estudios de Física del Cosmos de Aragón, Plaza San Juan 1, planta 2, 44001 Teruel, Spain
³ INAF-Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁴ Institut de Recherche en Astrophysique et Planétologie (IRAP), CNRS, 14 avenue Édouard Belin, 31400 Toulouse, France
⁵ IRAP, Université de Toulouse, UPS-OMP, 31400 Toulouse, France

Received: 5 March 2013
Accepted: 15 August 2013

Abstract

Aims. Understanding the role of environment in galaxy evolution is an important but still open issue. In the present work we study the close environment of red and blue L_B ≳ L_B^* galaxies hosted by VVDS-Deep groups.

Methods. We use the VIMOS VLT Deep Survey to study the close environment of galaxies in groups at 0.2 ≤ z < 0.95. Close neighbours of L_B ≳ L_B^* galaxies (M_B^e = M_B + 1.1z ≤ -20) are identified with M_B^e ≤ −18.25 and within a relative distance 5  h^-1 kpc ≤ r_p ≤ 100  h^-1 kpc and relative velocity Δv ≤ 500 km s^-1. The richness 𝒩 of a group is defined as the number of M_B^e ≤ −18.25 galaxies belonging to that group. We split our principal sample into red, passive galaxies with NUV − r ≥ 4.25 and blue, star-forming galaxies with NUV − r < 4.25. We study how the number of close neighbours per L_B ≳ L_B^* galaxy depends on 𝒩, colour, and redshift.

Results. Blue galaxies with a close neighbour are primarily located in poor groups, while the red ones are in rich groups. The number of close neighbours per red galaxy increases with 𝒩, ‾n_red ∝ 0.11𝒩, while that of blue galaxies does not depend on 𝒩 and is roughly constant. In addition, these trends are found to be independent of redshift, and only the average ‾n_blue evolves, decreasing with cosmic time.

Conclusions. Our results support the following assembly history of L_B ≳ L_B^* galaxies in the group environment: red, massive (M_⋆ ~ 10^10.8  M_⊙) galaxies were formed in/accreted by the dark matter halo of the group at early times (z ≳ 1), therefore their number of neighbours provides a fossil record of the stellar mass assembly of groups, traced by their richness 𝒩. On the other hand, blue, less massive (M_⋆ ~ 10^10.2  M_⊙) galaxies have recently been accreted by the group potential and are still in their parent dark matter halo, having the same number of neighbours irrespective of 𝒩. As time goes by, these blue galaxies settle in the group potential and turn red and/or fainter, thus becoming satellite galaxies in the group. With a toy quenching model, we estimate an infall rate of field galaxies into the group environment of ℜ_infall = 0.9−1.5 × 10^-4 Mpc^-3 Gyr^-1 at z ~ 0.7.