Julien Devriendt

Abstract:

Using the Horizon-AGN simulation we find a mass dependent spin orientation trend for galaxies: the spin of low-mass, rotation-dominated, blue, star-forming galaxies are preferentially aligned with their closest filament, whereas high-mass, velocity dispersion- supported, red quiescent galaxies tend to possess a spin perpendicular to these filaments. We explore the physical mechanisms driving galactic spin swings and quantify how much mergers and smooth accretion re-orient them relative to their host filaments.

Abstract:

Intrinsic alignments are believed to be a major source of systematics for future generation of weak gravitational lensing surveys like Euclid or LSST. Direct measurements of the alignment of the projected light distribution of galaxies in wide field imaging data seem to agree on a contamination at a level of a few per cent of the shear correlation functions, although the amplitude of the effect depends on the population of galaxies considered. Given this dependency, it is difficult to use dark matter-only simulations as the sole resource to predict and control intrinsic alignments. We report here estimates on the level of intrinsic alignment in the cosmological hydrodynamical simulation Horizon-AGN that could be a major source of systematic errors in weak gravitational lensing measurements. In particular, assuming that the spin of galaxies is a good proxy for their ellipticity, we show how those spins are spatially correlated and how they couple to the tidal field in which they are embedded. We also present theoretical calculations that illustrate and qualitatively explain the observed signals.

Abstract:

© 2016 The Authors. We use the Lya Mass Association Scheme (LyMAS) to predict cross-correlations at z = 2.5 between dark matter haloes and transmitted flux in the Lya forest, and compare to crosscorrelations measured for quasars and damped Lya systems (DLAs) from the Baryon Oscillation Spectroscopic Survey (BOSS) by Font-Ribera et al. We calibrate LyMAS using Horizon-AGN hydrodynamical cosmological simulations of a (100 h -1 Mpc) 3 comoving volume. We apply this calibration to a (1 h -1 Gpc) 3 simulation realized with 2048 3 dark matter particles. In the 100 h -1 Mpc box, LyMAS reproduces the halo-flux correlations computed from the full hydrodynamic gas distribution very well. In the 1 h -1 Gpc box, the amplitude of the large-scale cross-correlation tracks the halo bias b h as expected. We provide empirical fitting functions that describe our numerical results. In the transverse separation bins used for the BOSS analyses, LyMAS cross-correlation predictions follow linear theory accurately down to small scales. Fitting the BOSS measurements requires inclusion of random velocity errors; we find best-fitting rms velocity errors of 399 and 252 km s-1 for quasars and DLAs, respectively. We infer bias-weighted mean halo masses of M h /10 12 h -1 M⊙ = 2.19 +0.16 -0.15 and 0.69 +0.16 -0.14 for the host haloes of quasars and DLAs, with ~0.2 dex systematic uncertainty associated with redshift evolution, intergalactic medium parameters, and selection of data fitting range.