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

TL;DR

This study quantifies both major and minor galaxy merger frequencies in GOODS-S using a new close pair methodology, revealing that minor mergers are roughly twice as common as major ones and contribute significantly to galaxy evolution.

Contribution

The paper introduces a novel method for estimating close companion numbers with partial spectroscopic data, incorporating minor mergers into merger rate calculations.

Findings

Minor mergers are about twice as frequent as major mergers for galaxies with M_star >= 10^10 M_Sun.

The total merger rate (major+minor) is approximately 1.7 times the major merger rate.

Only 30-50% of massive early-type galaxies have experienced mergers since z=1.

Abstract

(Abriged) Our goal here is to provide merger frequencies that encompass both major and minor mergers, derived from close pair statistics. We use B-band luminosity- and mass-limited samples from an Spitzer/IRAC-selected catalogue of GOODS-S. We present a new methodology for computing the number of close companions, Nc, when spectroscopic redshift information is partial. We select as close companions those galaxies separated by 6h^-1 kpc < rp < 21h^-1 kpc in the sky plane and with a difference Delta_v <= 500 km s^-1 in redshift space. We provide Nc for four different B-band-selected samples. Nc increases with luminosity, and its evolution with redshift is faster in more luminous samples. We provide Nc of M_star >= 10^10 M_Sun galaxies, finding that the number including minor companions (mass ratio >= 1/10) is roughly two times the number of major companions alone (mass ratio >= 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. Only 30% to 50% of the M_star >= 10^10 M_Sun early-type (E/S0/Sa) galaxies that appear 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.