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Intrinsic alignment of disks and ellipticals across hydrodynamical simulations

M. L. van Heukelum, N. E. Chisari

TL;DR

This study conducts a cross-simulation analysis of intrinsic alignments (IA) for disks and ellipticals using three major hydrodynamical suites—IllustrisTNG (TNG300), EAGLE, and Horizon-AGN—across $z=0$ and $z=1$. It defines galaxies with multiple morphology metrics ($\kappa_{\mathrm{rot}}$, $|v/\sigma|$, $r-i$, $\mathrm{BTR}$) and measures projected IA via the quadrupole $\tilde{\xi}_{g+,2}$ and, where relevant, $w_{g+}$, employing simple and reduced inertia tensors. The key results show ellipticals around galaxies consistently exhibit positive IA signals, while disks around galaxies are positive or consistent with null, with disks around ellipticals displaying more complex behavior, including a negative reduced-shape signal in Horizon-AGN at $z=1$, suggesting inner-region physics and sub-grid models matter on non-linear scales. The analyses reveal that IA amplitudes track underlying stellar mass distributions but are also shaped by sub-grid physics and measurement choices, underscoring the need for consistent, cross-simulation modeling when applying IA constraints to weak lensing analyses.

Abstract

The correlations between the positions and shapes of galaxies, i.e. intrinsic alignments, have been measured in many observational studies and hydrodynamical simulations. The alignments of disk galaxies in hydrodynamical simulations have been measured to be positive, null and negative with varying methodologies, samples and hydrodynamical simulations. This work compares the correlations of disks and ellipticals around all galaxies and disks around ellipticals at $z=0$ and $z=1$ for simple and reduced shapes in TNG300, Horizon-AGN and EAGLE for multiple morphological definitions in a consistent way. All types of signals are positive and robust in TNG300 and EAGLE and positive or null in Horizon-AGN, except for the disks around ellipticals correlation for reduced shapes at $z=1$ when defined by $|v/σ|$, which is negative. A re-weighting of the ellipticals around galaxies signals in TNG300, according to the underlying stellar mass distributions of the samples, highlights the importance of the influence of (sub-grid) physics at these non-linear scales.

Intrinsic alignment of disks and ellipticals across hydrodynamical simulations

TL;DR

This study conducts a cross-simulation analysis of intrinsic alignments (IA) for disks and ellipticals using three major hydrodynamical suites—IllustrisTNG (TNG300), EAGLE, and Horizon-AGN—across and . It defines galaxies with multiple morphology metrics (, , , ) and measures projected IA via the quadrupole and, where relevant, , employing simple and reduced inertia tensors. The key results show ellipticals around galaxies consistently exhibit positive IA signals, while disks around galaxies are positive or consistent with null, with disks around ellipticals displaying more complex behavior, including a negative reduced-shape signal in Horizon-AGN at , suggesting inner-region physics and sub-grid models matter on non-linear scales. The analyses reveal that IA amplitudes track underlying stellar mass distributions but are also shaped by sub-grid physics and measurement choices, underscoring the need for consistent, cross-simulation modeling when applying IA constraints to weak lensing analyses.

Abstract

The correlations between the positions and shapes of galaxies, i.e. intrinsic alignments, have been measured in many observational studies and hydrodynamical simulations. The alignments of disk galaxies in hydrodynamical simulations have been measured to be positive, null and negative with varying methodologies, samples and hydrodynamical simulations. This work compares the correlations of disks and ellipticals around all galaxies and disks around ellipticals at and for simple and reduced shapes in TNG300, Horizon-AGN and EAGLE for multiple morphological definitions in a consistent way. All types of signals are positive and robust in TNG300 and EAGLE and positive or null in Horizon-AGN, except for the disks around ellipticals correlation for reduced shapes at when defined by , which is negative. A re-weighting of the ellipticals around galaxies signals in TNG300, according to the underlying stellar mass distributions of the samples, highlights the importance of the influence of (sub-grid) physics at these non-linear scales.

Paper Structure

This paper contains 26 sections, 13 equations, 16 figures.

Table of Contents

  1. Introduction
  2. Measurements and Data
  3. Simulations
  4. IllustrisTNG
  5. EAGLE
  6. Horizon-AGN
  7. Shapes of galaxies
  8. Projected correlation functions
  9. Projected correlation function $w_{g+}$
  10. Multipoles
  11. Covariance
  12. Selections
  13. Results
  14. Distributions
  15. Redshift evolution
  16. ...and 11 more sections

Figures (16)

  • Figure 1: Normalised distributions of $\mathrm{BTR}$ (top left), $|v/\sigma|$ (top right), $\kappa_{\mathrm{rot}}$ (bottom left) and $r-i$ (bottom right), which are used to split galaxies according to morphology, in TNG100 (light blue), TNG300 (dark blue), EAGLE (orange) and Horizon-AGN (green) at $z\approx0$. The variables show a range of distributions with varying features across simulations.
  • Figure 2: Normalised distributions of $\mathrm{BTR}$ (top left), $|v/\sigma|$ (top right), $\kappa_{\mathrm{rot}}$ (bottom left) and $r-i$ (bottom right), which are used to split galaxies according to morphology, in TNG300 across a range of redshifts, $0\leq z\leq 1$. The $r-i$ distributions display a clear redshift evolution, whereas the other variables show only a mild evolution with redshift.
  • Figure 3: Normalised distributions of $|v/\sigma|$ (top), $\kappa_{\mathrm{rot}}$ (middle) and $r-i$ (bottom), which are used to split galaxies according to morphology, in EAGLE across a range of redshifts, $0\leq z\leq 1$. All variables show a redshift evolution in their distributions. The kinematic variables $|v/\sigma|$ and $\kappa_{\mathrm{rot}}$ evolve opposite to expectations.
  • Figure 4: Normalised distributions of $|v/\sigma|$ (top) and $r-i$ (bottom), which are used to split galaxies according to morphology, in Horizon-AGN across a range of redshifts, $0\leq z\leq 1$. Both variables show redshift evolutions in line with expectations.
  • Figure 5: Galaxy stellar mass distributions at $z=0$ (top) and $z=1$ (bottom) of the full galaxy sample (black) and elliptical (continuous line style) and disk (dashed) samples, defined based on a selection in $\mathrm{BTR}$ (medium blue), $|v/\sigma|$ (light blue), $\kappa_{\mathrm{rot}}$ (dark blue) or $r-i$ (orange), in TNG300 (left), Horizon-AGN (middle) and EAGLE (right). The choice of variable used in the selection of the elliptical sample has a large impact on the resulting galaxy stellar mass distribution of the sample. The stellar mass distributions of the selections evolve with redshift.
  • ...and 11 more figures