Velocity Dispersion Functions of Pressure-supported Galaxies in EAGLE Simulations with Varying AGN Feedback

TL;DR

This study probes how AGN feedback in cosmological simulations shapes the stellar velocity dispersion function (VDF) for pressure-dominated galaxies. By analyzing five EAGLE runs with varying AGN prescriptions and measuring $σ_{*}$ within a fixed 10 kpc aperture, the authors show that standard and enhanced feedback yield similar $M_{*}-σ_{*}$ relations and VDF shapes, while reduced or absent feedback produces an excess of high-$σ_{*}$ systems due to stronger central mass concentration. After aligning the simulated VDFs with the observed SDSS VDF by applying the mean $M_{*}-σ_{*}$ offset, the normal/enhanced AGN models reproduce the observed VDF shape, whereas NoAGN remains discrepant, highlighting the central role of AGN feedback in shaping galaxy kinematics. Overall, the VDF emerges as a sensitive testbed for AGN feedback strength and central mass assembly, providing theoretical benchmarks for forthcoming spectroscopic surveys.

Abstract

We investigate the stellar velocity dispersion functions (VDFs) of pressure-supported galaxies in the EAGLE cosmological simulations. The central stellar velocity dispersion is one of the fundamental dynamical tracers of the total mass of galaxy subhalos, alongside luminosity and stellar mass. Because it reflects the gravitational potential, the stellar velocity dispersion is expected to be relatively insensitive to feedback from Active Galactic Nuclei (AGN), a critical process that regulates the connection between other galaxy observables and subhalo mass. To examine the impact of AGN feedback, we analyze the VDFs from five EAGLE simulation runs, each adopting a different AGN feedback model: one "standard", two "enhanced", one "reduced", and one with no AGN feedback. We compute the stellar velocity dispersions of pressure-supported galaxies using member stellar particles, mimicking fiber spectroscopy. The VDFs from the standard and enhanced AGN feedback models show little difference. However, contrary to our initial expectation that the VDF shape would be largely insensitive to AGN feedback, the simulations with reduced and no AGN feedback show a significant excess of high velocity dispersion galaxies ($σ_{*}$ > 200 km s$^{-1}$) and a deficit of low dispersion galaxies (100 < $σ_{*}$ (km s$^{-1}$) < 200), compared to those with standard or enhanced AGN feedback. The presence of high velocity dispersion galaxies in the no-AGN model arises from enhanced central star formation, due to the absence of AGN-driven gas heating or expulsion. Our results demonstrate that the shape of the theoretical VDF is sensitive to the strength of AGN feedback. These predictions offer a theoretical benchmark for future observational studies of the galaxy velocity dispersion function using large-scale spectroscopic surveys.

Figures (13)

• Figure 1: Median black hole mass as a function of the stellar mass of subhalos in various EAGLE simulations at $z = 0$. Blue triangles, red diamonds, magenta squares, and orange hexagons show the relations from EAGLE-50, EAGLE-eAGN-dT, EAGLE-eAGN-Visc, and EAGLE-wAGN, respectively. At $11.6 < \log (M_{*}/ \rm {M}_{\odot}) < 11.8$, EAGLE-50, EAGLE-eAGN-dT, EAGLE-eAGN-Visc do not contain any subhalo in the mass bin; there is no subhalo with $\log (M_{*}/ \rm {M}_{\odot}) > 11.9$ in EAGLE-wAGN. Error bars show the 1$\sigma$ scatter in black hole mass within each stellar mass bin.
• Figure 2: The $v/\sigma$ ratio as a function of stellar mass for subhalos in (a) EAGLE-50, (b) EAGLE-eAGN-dT, (c) EAGLE-eAGN-Visc, (d) EAGLE-wAGN, and (e) EAGLE-NoAGN. Black circles display the median $v/\sigma$ as a function of stellar mass for subhalos in each simulation. Horizon dashed lines indicate $v/\sigma = 0.5$, where the rotation-dominated ($v/\sigma \geq 0.5$) and pressure-dominated ($v/\sigma < 0.5$) galaxies are separated.
• Figure 3: The median line-of-sight velocity dispersions (along with $z-$axis) of pressure-dominated galaxies in EAGLE-50 measured within a cylindrical volume with an aperture of 10 kpc (blue triangles). The error bar indicates the 1$\sigma$ standard deviation in each stellar mass bin. For comparison, the median velocity dispersions are measured within 3 kpc (the black solid line) and the half-mass radius defined by the stellar particles (the red dashed line).
• Figure 4: The stellar mass to 1D stellar velocity dispersion measured within a cylindrical volume with a 10 kpc aperture along with $z-$axis ($\sigma_{*, z, 10 \rm ~kpc}$) relations of galaxy subhalos in (a) EAGLE-50, (b) EAGLE-eAGN-dT, (c) EAGLE-eAGN-Visc, (d) EAGLE-wAGN, and (e) EAGLE-NoAGN. White circles show the median $\sigma_{*, z, 10\rm ~kpc}$ at each stellar mass bin. Black bold dashed lines show the central 95% of the $\sigma_{*, z, 10\rm ~kpc}$ distributions. Vertical dotted lines indicate the stellar mass limit we applied. Horizontal dotted lines mark the velocity dispersion limit where the stellar mass limit intersects with the 95% upper limit of the velocity dispersion distributions. Below this velocity dispersion limit, our EAGLE galaxy samples are incomplete in terms of velocity dispersion. The subhalos in the shaded region are excluded from the $\sigma_{*}$ complete sample. The blue dashed lines indicate the $M_{*} - \sigma_{*}$ relation for TNG100 Sohn2024a, and the gray solid lines show the observed $M_{*} - \sigma_{*}$ relation for SDSS quiescent subhalos in the local universe Zahid2016.
• Figure 5: (a) Stellar mass functions and (b) stellar velocity dispersion functions of pressure-dominated galaxies in five EAGLE simulations. Green crosses show the relations from EAGLE-NoAGN. The other symbols are the same in Figure \ref{['fig:bhmass']}. Error bars indicate Poisson errors. For bins with fewer than 10 galaxies, we display only the upper values, marked by arrows.