Spectro-photometric close pairs in GOODS-S: major and minor companions of intermediate-mass galaxies

¹ Instituto de Astrofísica de Canarias, Calle vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
² Departamento de Astrofísica, Universidad de La Laguna, 38200 La Laguna, Tenerife, Spain
³ Departamento de Astrofísica y Ciencias de la Atmósfera, Facultad de C.C. Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
⁴ Laboratoire d'Astrophysique de Marseille, Pôle de l'Étoile Site de Château-Gombert, 38 rue Frédéric Joliot-Curie, 13388 Marseille, France e-mail: carlos.lopez@oamp.fr
⁵ Isaac Newton Group of Telescopes, Aptdo. Correos 321, 38700 Santa Cruz de La Palma, Tenerife, Spain
⁶ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA

Received: 10 February 2010
Accepted: 6 April 2010

Abstract

Aims. Recent work has shown that major mergers of disc galaxies can only account for ~20% of the growth of the galaxy red sequence between z = 1 and z = 0. Our goal here is to provide merger frequencies that encompass both major and minor mergers, derived from close pair statistics. We aim to show that reliable close pair statistics can be derived from galaxy catalogues with mixed spectroscopic and photometric redshifts.

Methods. We use B-band luminosity- and mass-limited samples from a Spitzer/IRAC-selected catalogue of GOODS-S. We present a new methodology for computing the number of close companions, , when spectroscopic redshift information is partial. The methodology extends the one used in spectroscopic surveys to make use of photometric redshift information. We select as close companions those galaxies separated by 6  h^-1 kpc < r_p < 21 h^-1 kpc in the sky plane and with a difference Δ v ≤ 500 km s^-1 in redshift space.

Results. We provide for four different B-band-selected samples. It increases with luminosity, in good agreement with previous estimations from spectroscopic surveys. The evolution of with redshift is faster in more luminous samples. We provide of ≥ 10¹⁰  galaxies, finding that the number including minor companions (, mass ratio μ ≥ 1/10) is roughly two times the number of major companions alone (, mass ratio μ ≥ 1/3) in the range 0.2 ≤ z < 1.1. We compare the major merger rate derived by close pairs with the one computed by morphological criteria, finding that both approaches provide similar merger rates for field galaxies when the progenitor bias is taken into account. Finally, we estimate that the total (major+minor) merger rate is ~1.7 times the major merger rate.

Conclusions. Only 30% to 50% of the ≥ 10¹⁰  early-type (E/S0/Sa) galaxies that appear between z = 1 and z = 0 may have undergone a major or a minor merger. Half of the red sequence growth since z = 1 is therefore unrelated to mergers.