Three-point intrinsic alignments of dark matter haloes in the IllustrisTNG simulation

We use the IllustrisTNG suite of cosmological simulations to measure intrinsic alignment (IA) bispectra of dark matter subhaloes between redshifts 0 and 1. We decompose the intrinsic shear field into E- and B- modes and find that the bispectra B_{δδE} and B_{δEE}, between the matter overdensity field, δ, and the E-mode field, are detected with high significance. We also model the IA bispectra analytically using a method consistent with the two-point non-linear alignment model. We use this model and the simulation measurements to infer the IA amplitude A_{IA} and find that values of A_{IA} obtained from IA power spectra and bispectra agree well at scales up to k_{max} = 2 hMpc^{−1}⁠. For example at z = 1, A_{IA} = 2.13 ± 0.02 from the cross power spectrum between the matter overdensity and E-mode fields and A_{IA} = 2.11 ± 0.03 from B_{δδE}. This demonstrates that a single physically motivated model can jointly model two-point and three-point statistics of IAs, thus enabling a cleaner separation between IAs and cosmological weak lensing signals

Shapes and Orientations of Galaxy Clusters

Galaxy clusters are large, gravitationally bound structures, primarily consisting of stars and dark matter. These two components form different shapes, which are oriented differently. Using data from IllustrisTNG, a publicly available set of hydrodynamical simulations, we calculate the 2D shapes from the inertia tensor of these clusters. Based on shapes measured from various methods, we find the misalignment between each shape. From these misalignments, we find that alignments between shapes measured based on individual particle positions tend to be much stronger than those measured with galaxy positions. We also find that shapes measured with these galaxy positions tend to be much more round as opposed to elliptical in shape

Galaxy Shapes and Intrinsic Alignments in The MassiveBlack-II Simulation

The intrinsic alignment of galaxy shapes with the large-scale density field is a contaminant to weak lensing measurements, as well as being an interesting signature of galaxy formation and evolution (albeit one that is difficult to predict theoretically). Here we investigate the shapes and relative orientations of the stars and dark matter of halos and subhalos (central and satellite) extracted from the MassiveBlack-II simulation, a state-of-the-art high resolution hydrodynamical cosmological simulation which includes stellar and AGN feedback in a volume of $(100{h^{-1}\mathrm{Mpc}})^3$. We consider redshift evolution from $z=1$ to $0.06$ and mass evolution within the range of subhalo masses, $10^{10} -6.0 \times 10^{14.0}{h^{-1}M_{\odot}}$. The shapes of the dark matter distributions are generally more round than the shapes defined by stellar matter. The projected root-mean-square (RMS) ellipticity per component for stellar matter is measured to be $e_\text{rms} = 0.28$ at $z=0.3$ for $M_{subhalo}> 10^{12.0}{h^{-1}M_{\odot}}$, which compares favourably with observational measurements. We find that the shapes of stellar and dark matter are more round for less massive subhalos and at lower redshifts. By directly measuring the relative orientation of the stellar matter and dark matter of subgroups, we find that, on average, the misalignment between the two components is larger for less massive subhalos. The mean misalignment angle varies from $\sim 30^{\circ}-10^{\circ}$ for $M \sim 10^{10} - 10^{14} {h^{-1}M_{\odot}}$ and shows a weak dependence on redshift. We also compare the misalignment angles in central and satellite subhalos at fixed subhalo mass, and find that centrals are more misaligned than satellites. We present fitting formulae for the shapes of dark and stellar matter in subhalos and also the probability distributions of misalignment angles.Comment: 18 pages, 18 figures, submitted to MNRA

Intrinsic alignments of galaxies in the MassiveBlack-II simulation: analysis of two-point statistics

The intrinsic alignment of galaxies with the large-scale density field is an important astrophysical contaminant in upcoming weak lensing surveys. We present detailed measurements of the galaxy intrinsic alignments and associated ellipticity-direction (ED) and projected shape ($w_{g+}$) correlation functions for galaxies in the cosmological hydrodynamic MassiveBlack-II (MB-II) simulation. We carefully assess the effects on galaxy shapes, misalignment of the stellar component with the dark matter shape and two-point statistics of iterative weighted (by mass and luminosity) definitions of the (reduced and unreduced) inertia tensor. We find that iterative procedures must be adopted for a reliable measurement of the reduced tensor but that luminosity versus mass weighting has only negligible effects. Both ED and $w_{g+}$ correlations increase in amplitude with subhalo mass (in the range of $10^{10} - 6.0\times 10^{14}h^{-1}M_{\odot}$), with a weak redshift dependence (from $z=1$ to $z=0.06$) at fixed mass. At $z \sim 0.3$, we predict a $w_{g+}$ that is in reasonable agreement with SDSS LRG measurements and that decreases in amplitude by a factor of $\sim 5$--18 for galaxies in the LSST survey. We also compared the intrinsic alignments of centrals and satellites, with clear detection of satellite radial alignments within their host halos. Finally, we show that $w_{g+}$ (using subhalos as tracers of density) and $w_{\delta+}$ (using dark matter density) predictions from the simulations agree with that of non-linear alignment models (NLA) at scales where the 2-halo term dominates in the correlations (and tabulate associated NLA fitting parameters). The 1-halo term induces a scale dependent bias at small scales which is not modeled in the NLA model.Comment: 25 pages, 27 figures, revised after referee comments, accepted for publication in MNRA