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

López-Sanjuan, C.; Balcells, M.; Pérez-González, P. G.; Barro, G.; Gallego, J.; Zamorano, J.
Bibliographical reference

Astronomy and Astrophysics, Volume 518, id.A20

Advertised on:
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, N_c, 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 < rp < 21 h-1 kpc in the sky plane and with a difference Δ v ≤ 500 km s-1 in redshift space. Results: We provide N_c for four different B-band-selected samples. It increases with luminosity, in good agreement with previous estimations from spectroscopic surveys. The evolution of N_c with redshift is faster in more luminous samples. We provide N_c of Mstar ≥ 1010 M&sun; galaxies, finding that the number including minor companions (N_c^m, mass ratio μ ≥ 1/10) is roughly two times the number of major companions alone (N_c^M, 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 Mstar ≥ 1010 M&sun; 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.
Related projects
Group members
Traces of Galaxy Formation: Stellar populations, Dynamics and Morphology

We are a large, diverse, and very active research group aiming to provide a comprehensive picture for the formation of galaxies in the Universe. Rooted in detailed stellar population analysis, we are constantly exploring and developing new tools and ideas to understand how galaxies came to be what we now observe.

Martín Navarro