Why do extremely massive disc galaxies exist today?

Abstract

Galaxy merger histories correlate strongly with stellar mass, largely regardless of morphology. Thus, at fixed stellar mass, spheroids and discs share similar assembly histories, both in terms of the frequency of mergers and the distribution of their mass ratios. Since mergers are the principal drivers of disc-to-spheroid morphological transformation, and the most massive galaxies typically have the richest merger histories, it is surprising that discs exist at all at the highest stellar masses (e.g. beyond the knee of the mass function). Using Horizon-AGN, a cosmological hydro-dynamical simulation, we show that extremely massive (M * > 10 11.4 M ) discs are created via two channels. In the primary channel (accounting for 70% of these systems and 8% of massive galaxies) the most recent, significant merger (stellar mass ratio > 1:10) between a massive spheroid and a gas-rich satellite 'spins up' the spheroid by creating a new rotational stellar component, leaving a massive disc as the remnant. In the secondary channel (accounting for 30% of these systems and 3% of massive galaxies), a system maintains a disc throughout its lifetime, due to an anomalously quiet merger history. Not unexpectedly, the fraction of massive discs is larger at higher redshift, due to the Universe being more gas-rich. The morphological mix of galaxies at the highest stellar masses is, therefore, a strong function of the gas fraction of the Universe. Finally, these massive discs have similar black-hole masses and accretion rates to massive spheroids, providing a natural explanation for why a minority of powerful AGN are surprisingly found in disc galaxies.

Massive disc galaxies too dominated by dark matter in cosmological hydrodynamical simulations

“…The results of PFM19 suggest the existence of multiple pathways for the creation of massive late-type and early-type systems; the former result from more gentle merging histories, which would lead to lower dark matter content and higher star formation efficiencies; although rejuvenation of ancient spheroids via gas-rich mergers is also a possibility (see Jackson et al 2020).…”

Using angular momentum maps to detect kinematically distinct galactic components

“…Dense environments are prone to frequent mergers (Naab et al 2014;Tacchella et al 2019) which are the natural culprits for converting ordered motion to random orbits and lowering the angular momentum of the remnant (Hopkins et al 2010;Lagos et al 2018a;Jackson et al 2020); this will also affect the D/T θ<30 • ratios.…”

Introducing the NEWHORIZON simulation: Galaxy properties with resolved internal dynamics across cosmic time

“…Low-mass and low surface-brightness regimes are becoming important frontiers for the study of galaxy evolution (e.g., Martin et al 2019) as surveys such as the LSST will allow us to observe very faint structures such as tidal streams and, for the first time, thousands of dwarfs at cosmological distances (mostly at z < 0.5). Complementary high-resolution cosmological simulations and deep observational datasets will enable us to start addressing the considerable tension between theory and observations in the dwarf regime (e.g., Boylan-Kolchin et al 2011;Pontzen & Governato 2012;Naab & Ostriker 2017;Silk 2017;Kaviraj et al 2019;Jackson et al 2021a) as well as in the highmass regime, where faint tidal features encode information that can aid in understanding the role of galaxy mergers and interactions in the formation, evolution, and survival of discs (Jackson et al 2020;Park et al 2019) and spheroids (Toomre & Toomre 1972;Bournaud et al 2007;Naab et al 2009;Kaviraj 2014;Dubois et al 2016;Martin et al 2018a).…”