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Register a new userGroup-scale intrinsic galaxy alignments in the Illustris-TNG and MassiveBlack-II simulations
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We study the alignments of satellite galaxies, and their anisotropic distribution, with respect to location and orientation of their host central galaxy in MassiveBlack-II and IllustrisTNG simulations. We find that: the shape of the satellite system in halos of mass ($> 10^{13}h^{-1}M_{odot}$) is well aligned with the shape of the central galaxy at $z=0.06$ with the mean alignment between the major axes being $sim Delta theta = 12^{circ}$ when compared to a uniform random distribution; that satellite galaxies tend to be anisotropically distributed along the major axis of the central galaxy with a stronger alignment in halos of higher mass or luminosity; and that the satellite distribution is more anisotropic for central galaxies with lower star formation rate, which are spheroidal, and for red central galaxies.Radially we find that satellites tend to be distributed along the major axis of the shape of the stellar component of central galaxies at smaller scales and the dark matter component on larger scales. We find that the dependence of satellite anisotropy on central galaxy properties and the radial distance is similar in both the simulations with a larger amplitude in MassiveBlack-II. The orientation of satellite galaxies tends to point toward the location of the central galaxy at small scales and this correlation decreases with increasing distance, and the amplitude of satellite alignment is higher in high mass halos. However, the projected ellipticities do not exhibit a scale-dependent radial alignment, as has been seen in some observational measurements.
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We compare the shapes and intrinsic alignments of galaxies in the MassiveBlack-II cosmological hydrodynamic simulation (MBII) to those in a dark matter-only (DMO) simulation performed with the same volume (100$h^{-1}$Mpc)$^{3}$, cosmological parameters, and initial conditions. Understanding the impact of baryonic physics on galaxy shapes and alignments and their relation to the dark matter distribution should prove useful to map the intrinsic alignments of galaxies from hydrodynamic to dark matter-only simulations. We find that dark matter subhalos are typically rounder in MBII, and the shapes of stellar matter in low mass galaxies are more misaligned with the shapes of the dark matter of the corresponding subhalos in the DMO simulation. At $z=0.06$, the fractional difference in the mean misalignment angle between MBII and DMO simulations varies from $sim 28 % - 12 %$ in the mass range $10^{10.8} - 6.0 times 10^{14} h^{-1}M_{odot}$. We study the dark matter halo shapes and alignments as a function of radius, and find that while galaxies in MBII are more aligned with the inner parts of their dark matter subhalos, there is no radial trend in their alignments with the corresponding subhalo in the DMO simulation. This result highlights the importance of baryonic physics in determining the alignment of the galaxy with respect to the inner parts of the halo. Finally, we compare the ellipticity-direction (ED) correlation for galaxies to that for dark matter halos, finding that it is suppressed on all scales by stellar-dark matter misalignment. In the projected shape-density correlation ($w_{delta+}$), which includes ellipticity weighting, this effect is partially canceled by the higher mean ellipticities of the stellar component, but differences of order $30-40%$ remain on scales $> 1$ Mpc over a range of subhalo masses, with scale-dependent effects below $1$ Mpc.
We investigate the redshift evolution of the intrinsic alignments (IA) of galaxies in the texttt{MassiveBlackII} (MBII) simulation. We select galaxy samples above fixed subhalo mass cuts ($M_h>10^{11,12,13}~M_{odot}/h$) at $z=0.6$ and trace their progenitors to $z=3$ along their merger trees. Dark matter components of $z=0.6$ galaxies are more spherical than their progenitors while stellar matter components tend to be less spherical than their progenitors. The distribution of the galaxy-subhalo misalignment angle peaks at $sim10~mathrm{deg}$ with a mild increase with time. The evolution of the ellipticity-direction~(ED) correlation amplitude $omega(r)$ of galaxies (which quantifies the tendency of galaxies to preferentially point towards surrounding matter overdensities) is governed by the evolution in the alignment of underlying dark matter~(DM) subhaloes to the matter density of field, as well as the alignment between galaxies and their DM subhaloes. At scales $sim1~mathrm{cMpc}/h$, the alignment between DM subhaloes and matter overdensity gets suppressed with time, whereas the alignment between galaxies and DM subhaloes is enhanced. These competing tendencies lead to a complex redshift evolution of $omega(r)$ for galaxies at $sim1~mathrm{cMpc}/h$. At scales $>1~mathrm{cMpc}/h$, alignment between DM subhaloes and matter overdensity does not evolve significantly; the evolution of the galaxy-subhalo misalignment therefore leads to an increase in $omega(r)$ for galaxies by a factor of $sim4$ from $z=3$ to $0.6$ at scales $>1~mathrm{cMpc}/h$. The balance between competing physical effects is scale dependant, leading to different conclusions at much smaller scales($sim0.1~mathrm{Mpc}/h$).
The intrinsic correlations of galaxy shapes and orientations across the large-scale structure of the Universe are a known contaminant to weak gravitational lensing. They are known to be dependent on galaxy properties, such as their mass and morphologies. The complex interplay between alignments and the physical processes that drive galaxy evolution remains vastly unexplored. We assess the sensitivity of intrinsic alignments (shapes and angular momenta) to Active Galactic Nuclei -AGN- feedback by comparing galaxy alignment in twin runs of the cosmological hydrodynamical Horizon simulation, which do and do not include AGN feedback respectively. We measure intrinsic alignments in three dimensions and in projection at z=0 and z=1. We find that the projected alignment signal of all galaxies with resolved shapes with respect to the density field in the simulation is robust to AGN feedback, thus giving similar predictions for contamination to weak lensing. The relative alignment of galaxy shapes around galaxy positions is however significantly impacted, especially when considering high-mass ellipsoids. Using a sample of galaxy twins across simulations, we determine that AGN changes both the galaxy selection and their actual alignments. Finally, we measure the alignments of angular momenta of galaxies with their nearest filament. Overall, these are more significant in the presence of AGN as a result of the higher abundance of massive pressure-supported galaxies.
We report results for the alignments of galaxies in the EAGLE and cosmo-OWLS simulations as a function of galaxy separation and halo mass. The combination of these hydro-cosmological simulations enables us to span four orders of magnitude in halo mass ($10.7<log_{10}(M_{200}/[h^{-1}M_odot])<15$) and a large range of separations ($-1<log_{10}(r/[h^{-1}Mpc])< 2$). We focus on two classes of alignments: the orientations of galaxies with respect to either the directions to, or the orientations of, surrounding galaxies. We find that the strength of the alignment is a strongly decreasing function of the distance between galaxies. The orientation-direction alignment can remain significant up to ~100 Mpc, for galaxies hosted by the most massive haloes in our simulations. Galaxies hosted by more massive subhaloes show stronger alignment. At a fixed halo mass, more aspherical or prolate galaxies exhibit stronger alignments. The spatial distribution of satellites is anisotropic and significantly aligned with the major axis of the main host halo. The major axis of satellite galaxies, when all stars are considered, are preferentially aligned towards the centre of the main host halo. The predicted projected direction-orientation alignment, $epsilon_{g+}(r_{p})$, is in broad agreement with recent observations when only stars within the typical observable extent of a galaxy are used to define galaxy orientations. We find that the orientation-orientation alignment is weaker than the orientation-direction alignment on all scales. Overall, the strength of galaxy alignments depends strongly on the subset of stars that are used to measure the orientations of galaxies and it is always weaker than the alignment of the dark matter haloes. Thus, alignment models that use halo orientation as a direct proxy for galaxy orientation will overestimate the impact of intrinsic alignments on weak lensing analyses.
Based on galaxies from the Sloan Digital Sky Survey (SDSS) and subhalos in the corresponding reconstructed region from the constrained simulation of ELUCID, we study the alignment of central galaxies relative to their host groups in the group catalog, as well as the alignment relative to the corresponding subhalos in the ELUCID simulation. Galaxies in observation are matched to dark matter subhalos in the ELUCID simulation using a novel neighborhood abundance matching method. In observation, the major axes of galaxies are found to be preferentially aligned to the major axes of their host groups. There is a color dependence of galaxy-group alignment that red centrals have a stronger alignment along the major axes of their host groups than blue centrals. Combining galaxies in observation and subhalos in the ELUCID simulation, we also find that central galaxies have their major axes to be aligned to the major axes of their corresponding subhalos in the ELUCID simulation. We find that the galaxy-group and galaxy-subhalo alignment signals are stronger for galaxies in more massive halos. We find that the alignments between main subhalos and the SDSS matched subhalo systems in simulation are slightly stronger than the galaxy-group alignments in observation.
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