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Early- and late-stage mergers among main sequence and starburst galaxies at 0.2 ≤ z ≤ 2

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 ≳ 1 - starburst (SB) events (∆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 ̃5-10 per cent at all masses ≳ 1010 M☉. The MS major merger fraction roughly doubles between z = 0.2 and 2, with morphological mergers driving the overall increase at z ≳ 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 ∆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.
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Submitted on : Friday, December 13, 2019 - 11:13:10 PM
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A. Cibinel, Emanuele Daddi, M. T. Sargent, E. Le Floc'H, D. Liu, et al.. Early- and late-stage mergers among main sequence and starburst galaxies at 0.2 ≤ z ≤ 2. Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP): Policy P - Oxford Open Option A, 2019, 485 (4), pp.5631-5651. ⟨10.1093/mnras/stz690⟩. ⟨hal-02390884⟩

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