Identifying the discs, bulges, and intra-halo light of simulated galaxies through structural decomposition

Abstract

We perform a structural decomposition of galaxies identified in three cosmological hydrodynamical simulations by applying Gaussian mixture models (GMMs) to the kinematics of their stellar particles. We study the resulting disc, bulge, and intra-halo light (IHL) components of galaxies whose host dark matter haloes have virial masses in the range M200 = 1011–$10^{15}\, {\rm M_\odot }$. Our decomposition technique isolates galactic discs whose mass fractions, fdisc, correlate strongly with common alternative morphology indicators; for example, fdisc is approximately equal to κco, the fraction of stellar kinetic energy in corotation. The primary aim of our study, however, is to characterize the IHL of galaxies in a consistent manner and over a broad mass range, and to analyse its properties from the scale of galactic stellar haloes up to the intra-cluster light. Our results imply that the IHL fraction, fIHL, has appreciable scatter and is strongly correlated with galaxy morphology: at fixed stellar mass, the IHL of disc galaxies is typically older and less massive than that of spheroids. Above $M_{200}\approx 10^{13}\, {\rm M_\odot }$, we find, on average, fIHL ≈ 0.37, albeit with considerable scatter. The transition radius beyond which the IHL dominates the stellar mass of a galaxy is roughly $30\, {\rm kpc}$ for disc galaxies, but depends strongly on halo mass for spheroids. However, we find that no alternative IHL definitions – whether based on the ex situ stellar mass, or the stellar mass outside a spherical aperture – reproduce our dynamically defined IHL masses.

The Connection between the Intracluster Light and its Host Halo: Formation Time and Contribution from Different Channels

“…Preprocessing appears to be more important than mergers, but much less than stellar stripping. This result is particularly in accordance with observational studies (e.g., Joo & Jee 2023;Proctor et al 2023) that give to preprocessing/accretion a relevant role in the ICL formation. However, as mentioned in this work and discussed in C23a, this channel is not a direct way to form ICL, because the stray stars coming from preprocessing have already formed elsewhere in the past, and they are just accreted during the hierarchical formation of a given halo.…”

Diffuse Light in Milky Way–like Haloes

“…Not only that, our model predicts fractions that are in good agreement with the observations by Ragusa et al (2023) and Kluge et al (2021) in that halo mass range. Overall, the model agrees very well with the predictions of the Eagle project (Proctor et al 2024) up to M log 12.1 halo ~, while those of TNG50 (Ahvazi et al 2023) are biased low with respect to ours, and predict DL fractions that decrease quite importantly toward low-mass haloes. The disagreement between Eagle and TNG50 simulations, in our opinion, has to be taken with caution.…”

“…This implies also a series of results that we are going to show through the analysis below. In order to achieve our goal, we first start by showing in Figure 1 the fraction of DL as a function of halo mass and compare the predictions of our model (black lines and circles with different colors) with the results of the Eagle project (Proctor et al 2024, purple line), those of TNG50 (Ahvazi et al 2023, blue crosses), and several observed fractions from different authors (Harmsen et al 2017;Deason et al 2019;Kluge et al 2021;Ragusa et al 2023, gray symbols). The plot shows that the predictions of our model are in better agreement with observations than the other two numerical predictions, and it does agree with the Eagle project on very small scales, up to M log 12.5 halo ~if we consider the scatter (but they start to diverge from M log 12.1 halo ~).…”

“…). However, in the past few years, there have been the first measurements of the DL in the Milky Way (Deason et al 2019) and M31 (Harmsen et al 2017), and, given the improvements in the spatial and mass resolutions of hydro-simulations, also the first theoretical predictions (e.g., Ahvazi et al 2023;Proctor et al 2024). Our model has never been tested on such small scales, and this Letter brings the first results on the formation of the DL in haloes below the typical halo masses usually probed.…”

“…While a substantial amount of work has been done in order to investigate the ICL and its properties in group/cluster size haloes, M 13 log 15 halo   (Burke et al 2015;Groenewald et al 2017;Iodice et al 2017;Morishita et al 2017;DeMaio et al 2018DeMaio et al , 2020Montes & Trujillo 2018;Zhang et al 2019;Spavone et al 2020;Ragusa et al 2022Ragusa et al , 2023Joo & Jee 2023, just to quote some of the most recent observational works), only very few authors (e.g., Purcell et al 2007, andvery recently Ahvazi et al 2023;Proctor et al 2024) have focused on much smaller scales, such as, e.g., Milky Way-like haloes (Harmsen et al 2017;Deason et al 2019). Understanding the properties of the DL in these haloes, as well as the mechanisms that add to its formation, is of fundamental importance in order to constrain the typical halo mass under which the formation of the DL becomes negligible, i.e., where either the efficiency of the DM potential well is no longer enough, or the haloes lack the primary ingredient, satellite galaxies orbiting around the central one.…”