The dominant role of mergers in the size evolution of massive early-type galaxies sincez ~  1

Abstract

Aims. The role of galaxy mergers in massive galaxy evolution, and in particular to mass assembly and size growth, remains an open question. In this paper we measure the merger fraction and rate, both minor and major, of massive early-type galaxies (M ≥ 10 11 M ) in the COSMOS field, and study their role in mass and size evolution. Methods. We used the 30-band photometric catalogue in COSMOS, complemented with the spectroscopy of the zCOSMOS survey, to define close pairs with a separation on the sky plane 10 h −1 kpc ≤ r p ≤ 30 h −1 kpc and a relative velocity Δv ≤ 500 km s −1 in redshift space. We measured both major (stellar mass ratio μ ≡ M ,2 /M ,1 ≥ 1/4) and minor (1/10 ≤ μ < 1/4) merger fractions of massive galaxies, and studied their dependence on redshift and on morphology (early types vs. late types). Results. The merger fraction and rate of massive galaxies evolves as a power-law (1 + z) n , with major mergers increasing with redshift, n MM = 1.4, and minor mergers showing little evolution, n mm ∼ 0. When split by their morphology, the minor merger fraction for early-type galaxies (ETGs) is higher by a factor of three than that for late-type galaxies (LTGs), and both are nearly constant with redshift. The fraction of major mergers for massive LTGs evolves faster (n LT MM ∼ 4) than for ETGs (n ET MM = 1.8). Conclusions. Our results show that massive ETGs have undergone 0.89 mergers (0.43 major and 0.46 minor) since z ∼ 1, leading to a mass growth of ∼30%. We find that μ ≥ 1/10 mergers can explain ∼55% of the observed size evolution of these galaxies since z ∼ 1. Another ∼20% is due to the progenitor bias (younger galaxies are more extended) and we estimate that very minor mergers (μ < 1/10) could contribute with an extra ∼20%. The remaining ∼5% should come from other processes (e.g., adiabatic expansion or observational effects). This picture also reproduces the mass growth and the velocity dispersion evolution of these galaxies. We conclude from these results, and after exploring all the possible uncertainties in our picture, that merging is the main contributor to the size evolution of massive ETGs at z 1, accounting for ∼50−75% of that evolution in the last 8 Gyr. Nearly half of the evolution due to mergers is related to minor (μ < 1/4) events.

The many flavours of photometric redshifts

“…Some of the recent applications heavily relying on accurate photoz are: the cosmic time evolution of galaxy properties [e.g., 6] or the search of primordial galaxies [e.g., 7]; the study of the relation between galaxy properties and their dark matter halos [e.g., 8] or cluster identifications [e.g., 9]. Galaxy environment or galaxy pairs evolution can also be investigated with photo-z [e.g., [10][11][12], although with a limited precision [e.g., 13,14]. Similarly, photo-z are used for studying the evolution of galaxies hosting an Active Galactic Nucleus (AGN) [e.g., 15] or more peculiar objects [e.g., Blazars: 16].…”

An Evolving and Mass-dependent σsSFR–M⋆ Relation for Galaxies

“…Qu et al (2017) and Lagos et al (2018b) studied the impact of mergers on mass assembly and angular momentum on the EAGLE galaxies. The authors found that the reference model is able to reproduce the observed merger rates and merger fractions of galaxies at various redshifts (Conselice et al 2003;Kartaltepe et al 2007;Ryan et al 2008;Lotz et al 2008;Conselice et al 2009;de Ravel et al 2009;Williams et al 2011;López-Sanjuan et al 2012;Bluck et al 2012;Stott et al 2013;Robotham et al 2014;Man et al 2016). Thus EAGLE can be used to study the effect of mergers on the σ sSFR -M ⋆ relation.…”

The Coevolution of Massive Quiescent Galaxies and Their Dark Matter Halos over the Last 6 Billion Years

“…For the redshift range probed by SHELS, this rate translates to ∼ 0.05 dex increase in stellar mass. Newman et al (2012b) estimate the growth rate for less massive galaxies (M * > 10 10.7 M ⊙ ) at z < 1 by identifying companions of massive quiescent host galaxies (see also López-Sanjuan et al 2012). Adopting a merger timescale of 1 Gyr and a ∆t = 4 Gyr, Newman et al (2012b) estimate a growth of ∼ 0.1 dex for galaxies with M * ∼ 10 10.7 M ⊙ .…”