Probabilistic model for dynamic galaxy decomposition

Abstract

In the era of precision cosmology and ever-improving cosmological simulations, a better understanding of different galaxy components such as bulges and discs will give us new insight into galactic formation and evolution. Based on the fact that the stellar populations of the constituent components of galaxies differ by their dynamical properties, we develop two simple models for galaxy decomposition using the IllustrisTNG cosmological hydrodynamical simulation. The first model uses a single dynamical parameter and can distinguish 4 components: thin disc, thick disc, counter-rotating disc and bulge. The second model uses one more dynamical parameter, was defined in a probabilistic manner, and distinguishes two components: bulge and disc. The number fraction of disc-dominated galaxies at a given stellar mass obtained by our models agrees well with observations for masses exceeding log 10 (𝑀 * /𝑀 ) = 10. Sérsic indices and half-mass radii for the bulge components agree well with those for real galaxies. The mode of the distribution of Sérsic indices for the disc components is at the expected value of 𝑛 = 1. However, disc half-mass radii are smaller than those for real galaxies, in accordance with previous findings that the IllustrisTNG simulation produces undersized discs.

Intrinsic alignments of bulges and discs

“…We employ a minimum stellar mass threshold of log 10 (𝑀 * /𝑀 ) = 10 for all galaxies, using their stellar mass from the SUBFIND catalog. A previous study (Jagvaral et al 2021) motivates the mass cut on the shape sample, where we concluded that the disc fractions from TNG100-1 agree well with observational values down to log 10 (𝑀 * /𝑀 ) = 10, below which we believe the resolution of the simulation may be affecting the results. Also, we have checked that using a lower mass on the density tracers, to include more galaxies, did not change our 𝐴 𝐼 amplitudes.…”

“…For the Red sample we present both the mass-controlled and the full sample; however the Blue sample was not mass controlled because of its very narrow mass range. Here, we should note that the simulation produces a slightly higher number of red disc galaxies compared to observations, which complicates the interpretation of these results (Nelson et al 2018;Jagvaral et al 2021). The Blue sample has an IA signal that is consistent with zero, similar to previous studies in real data such as Johnston et al (2019); Mandelbaum et al (2011).…”

“…In this subsection, we describe the dynamical model that probabilistically assigns each particle to either the bulge or the disc component of the galaxy (for more details, see Jagvaral et al 2021). The model identifies the two galaxy components through two physicallymotivated assumptions:…”

“…The 2D model is built deterministically, after which we can generate Monte Carlo realizations of the model, assigning star particles to the bulge or to the disc. In this work, we will focus on a single realization of the Monte Carlo simulation; the probabilistic nature of the Monte Carlo method was explored in Jagvaral et al (2021).…”

Bayesian fitting of multi-Gaussian expansion models to galaxy images

“…First off it is simpler and more direct to compare observation with simulations or semianalytic models. Similar to observations it is non-trivial to separate galaxies into different components in simulations (Obreja et al 2018;Jagvaral et al 2021) and the methods used are different then in observations. In the MGE framework it is simple to compare mass or light profiles or focus on specific parts of the galaxy that are of interest like the central regions or outskirts.…”