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Cibinel, A.; Daddi, E.; Sargent, M. T.; Le Floc'h, E.; Liu, D.; Bournaud, F.; Oesch, P. A.; Amram, P.; Calabro, A.; Duc, P. -A.; Pannella, M.; Puglisi, A.; Perret, V.; Elbaz, D.; Kokorev, V. (2019): Early- and late-stage mergers among main sequence and starburst galaxies at 0.2 <= z <= 2. In: Monthly Notices of the Royal Astronomical Society, Vol. 485, No. 4: pp. 5631-5651
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Abstract

We investigate the fraction of close pairs and morphologically identified mergers on and above the star-forming main sequence (MS) at 0.2 <= z <= 2.0. The novelty of our work lies in the use of a non-parametric morphological classification performed on resolved stellar mass maps, reducing the contamination by non-interacting, high-redshift clumpy galaxies. We find that the merger fraction rapidly rises to >= 70 per cent above the MS, implying that - already at z greater than or similar to 1 -starburst (SB) events (Delta(MS) >= 0.6) are almost always associated with a major merger (1: 1 to 1: 6 mass ratio). The majority of interacting galaxies in the SB region are morphologically disturbed, late-stage mergers. Pair fractions show little dependence on MS offset and pairs are more prevalent than late-stage mergers only in the lower half of the MS. In our sample, major mergers on the MS occur with a roughly equal frequency of similar to 5-10 per cent at all masses greater than or similar to 10(10) M-circle dot. The MS major merger fraction roughly doubles between z = 0.2 and 2, with morphological mergers driving the overall increase at z greater than or similar to 1. The differential redshift evolution of interacting pairs and morphologically classified mergers on the MS can be reconciled by evolving observability time-scales for both pairs and morphological disturbances. The observed variation of the late-stage merger fraction with Delta(MS) follows the perturbative 2-Star Formation Mode model, where any MS galaxy can experience a continuum of different star formation rate enhancements. This points to an SB-merger connection not only for extreme events, but also more moderate bursts which merely scatter galaxies upward within the MS, rather than fully elevating them above it.