Forming disc galaxies in major mergers – III. The effect of angular momentum on the radial density profiles of disc galaxies

Abstract

We study the effect of angular momentum on the surface density profiles of disc galaxies, using high resolution simulations of major mergers whose remnants have downbending radial density profiles (type II). As described in the previous papers of this series, in this scenario, most of the disc mass is acquired after the collision via accretion from a hot gaseous halo. We find that the inner and outer disc scalelengths, as well as the break radius, correlate with the total angular momentum of the initial merging system, and are larger for high angular momentum systems. We follow the angular momentum redistribution in our simulated galaxies, and find that, like the mass, the disc angular momentum is acquired via accretion, i.e. to the detriment of the gaseous halo. Furthermore, high angular momentum systems give more angular momentum to their discs, which affects directly their radial density profile. Adding simulations of isolated galaxies to our sample, we find that the correlations are valid also for disc galaxies evolved in isolation. We show that the outer part of the disc at the end of the simulation is populated mainly by inside-out stellar migration, and that in galaxies with higher angular momentum, stars travel radially further out. This, however, does not mean that outer disc stars (in type II discs) were mostly born in the inner disc. Indeed, generally the break radius increases over time, and not taking this into account leads to overestimating the number of stars born in the inner disc.

Disc galaxies formed from major mergers in Illustris

“…Using this sample we therefore get a general idea on how to form a disc from a major merger. The merger destroys the galaxies and forms a rather spheroidal component out of the old stars, but after the merger star formation creates a new young disc component, in good agreement with what was found in Peschken et al (2017) for an isolated pair of galaxies. This is only possible in a merging system with enough gas, i.e.…”

“…But where does this disc gas come from? Is this gas transformed into a disc component by the merger, or does it come from slow accretion from the outer parts into the disc over time, consistently with the results in Peschken et al (2017)? The amount of gas being constant in the disc over time suggests the latter explanation, and we will here verify this hypothesis by following the gas movements.…”

“…Springel & Hernquist 2005;Governato et al 2007;Hopkins et al 2009;Borlaff et al 2014) showed that the remnant of a major merger could have a small disc component, particularly if the two initial discs were on, or at least close to, the orbital plane. Athanassoula et al (2016) (see also Rodionov et al 2017;Peschken et al 2017;Athanassoula 2018) showed how to create a spiral galaxy from a major merger, using simula-Contact e-mail: npeschken@camk.edu.pl tions of an isolated pair of disc galaxies with a hot gaseous halo each, colliding at intermediate or high redshift. While the stars pile up in the center of the remnant to form a classical bulge, gas is accreted from the halo after the merger, and allows the formation of a dominating disc component.…”

On the environment of low surface brightness galaxies at different scales

“…The results presented in Figures 10 and 11 support the common knowledge (Dalcanton et al 1997;Boissier et al 2003;Kim & Lee 2013;Peschken et al 2017) that LSB galaxies are formed in fast-spinning dark matter haloes, and give constrains of the relative value of this parameter be-tween LSB and HSB galaxies, setting this parameter as the one responsible for establishing their low surface brightness nature.…”

Constraints on radial migration in spiral galaxies – II. Angular momentum distribution and preferential migration

“…For example, it is now recognized that disc stars can migrate radially, moving of order a disc scale-length over their lifetime, through successive resonant interactions with transient gravitational perturbations (for observational evidence, see e.g., Wielen et al (1996); Bovy et al (2012); Radburn-Smith et al (2012); Hayden et al (2015); Kordopatis et al (2015); Loebman et al (2016) and for theoretical investigations see e.g. Sellwood & Binney (2002); Roškar et al (2008); Schönrich & Binney (2009) ;Loebman et al (2011);Minchev et al (2012); Roškar et al (2013); Vera-Ciro et al (2016); Martinez-Medina et al (2017); Peschken et al (2017); Schönrich & McMillan (2017)). Lynden-Bell & Kalnajs (1972) established that stars on close to circular orbits can exchange orbital angular momentum with steady (or slowly growing) spiral waves at the major resonances, namely corotation and the inner/outer Lindblad resonances.…”