The connection between surface brightness and satellite systems for central galaxies through Illustris TNG

TL;DR

This work investigates how central galaxy surface brightness relates to satellite system properties in a $\Lambda$CDM cosmology by analyzing central galaxies and their satellites in IllustrisTNG-100, focusing on LSBG vs HSBG differences at fixed stellar mass. It defines LSBGs as $\mu^{cen}_r>23$ mag arcsec$^{-2}$ and compares them to HSBGs with $\mu^{cen}_r<21$ mag arcsec$^{-2}$ for centrals in the range $M^{cen}_{\star} \in [10^{10},10^{11}] M_\odot$, and examines satellite abundance, velocity dispersion, and angular-momentum alignment. The results show that LSBGs host more satellites within $R_{200}$ and exhibit higher $\sigma^{sat}$ at fixed central mass, with a satellite population biased toward counter-rotation relative to the central disc; continuous analyses reveal enhanced non-radial satellite motions and decreasing alignment with the disc as $\mu^{cen}_r$ increases. These findings imply distinct accretion histories and angular-momentum transfer for LSBGs, consistent with extended, rotation-supported discs formed through gentle, aligned accretion and fewer radial mergers, and provide concrete, testable predictions for upcoming surveys and gravity-model tests in galaxy formation areas.

Abstract

We analyse different properties of central low-surface-brightness galaxies (LSBGs) and their satellite systems using the simulation Illustris TNG-100, in order to deepen our understanding of the formation mechanism of LSBGs in a $Λ$CDM cosmology. We find differences in the spin and the concentrations of the LSBGs haloes and the host haloes of high-surface-brightness galaxies (HSBGs), consistent with previous studies. By analysing their spatial and kinematical distribution of satellites, we find that LSBGs tend to have a larger number of satellites than HSBGs and with a larger velocity dispersion. Moreover, we obtain a continuous relation between the number of satellites and surface brightness, particularly for massive central galaxies. We also find a relation between surface brightness and the relative tangential velocity of the satellites. For a given stellar mass, the existence of LSBGs is strongly correlated with their satellite system dominated by rotation. Furthermore, the satellite system is systematically in counter-rotation with respect to the primary disc in LSBGs. We propose that this fact reflects that these galaxies have not experienced a significantly high rate of mergers, which are more likely associated with radial orbits expected in systems of galaxies with a high surface brightness.

Figures (8)

• Figure 1: Mean surface brightness ($\mu^{cen}_r$) as a function of the stellar mass ($M^{cen}_{\star}$) for all central galaxies with $M^{cen}_{\star} = 10^{10} - 10^{11} ~ \hbox{M}_{\odot}$, coloured by $\kappa^{cen}_{rot}$. The horizontal dashed red lines indicate the threshold to select high and low-surface-brightness galaxies.
• Figure 2: Distributions of the mean surface brightness $\mu^{cen}_r$ (left panel), stellar mass $M^{cen}_{\star}$ (middle panel), and morphology parameter $\kappa^{cen}_{rot}$ (right panel), for LSBGs (solid blue lines) and comparison (dashed orange lines) samples. In each panel, vertical lines indicate the median of each distribution (in their respective colours).
• Figure 3: From left to right, distribution for the projected stellar half-mass radius ($r^{cen}_{50}$), halo mass ($M_{200}$), halo concentration ($R_{200}/R_s$), and the dimensionless spin parameter ($\lambda$). The colour code is the same as Fig. \ref{['fig:basic_dist']}. In each panel, vertical lines indicate the median of each distribution (in their respective colours), and the black arrow in the first panel indicates the half-light radius of the Milky Way.
• Figure 4: Cumulative number of satellites as a function of the distance to the central galaxy in units of the virial radius ($R_{200}$) of their host halo. The left panel corresponds to the LSBGs sample (blue shaded) the middle panel corresponds to the comparison sample (orange shaded). In both cases, the lines connecting dots correspond to each satellite system for a central galaxy. The colour intensity of each line indicates the mass of the host halo. The right panel shows the mean number of satellites of each sample (in blue, LSBGs and in orange, the comparison sample). The bootstrap errors of the median are indicated by the shadowed areas.
• Figure 5: Velocity dispersion ($\sigma^{sat}$) of the satellite system of each host halo as a function of the stellar mass of the central galaxy. The number of satellites is indicated by the shade of the colour for both samples, LSBGs (blue circles) and the comparison sample (orange circles). The solid lines correspond to a linear fit, and the shadowed region indicates the fit error.