Tidal alignment of galaxies

Abstract

Abstract.We develop an analytic model for galaxy intrinsic alignments (IA) based on the theory of tidal alignment. We calculate all relevant nonlinear corrections at one-loop order, including effects from nonlinear density evolution, galaxy biasing, and source density weighting. Contributions from density weighting are found to be particularly important and lead to bias dependence of the IA amplitude, even on large scales. This effect may be responsible for much of the luminosity dependence in IA observations. The increase in IA amplitude for more highly biased galaxies reflects their locations in regions with large tidal fields. We also consider the impact of smoothing the tidal field on halo scales. We compare the performance of this consistent nonlinear model in describing the observed alignment of luminous red galaxies with the linear model as well as the frequently used "nonlinear alignment model," finding a significant improvement on small and intermediate scales. We also show that the cross-correlation between density and IA (the "GI" term) can be effectively separated into source alignment and source clustering, and we accurately model the observed alignment down to the one-halo regime using the tidal field from the fully nonlinear halo-matter cross correlation. Inside the one-halo regime, the average alignment of galaxies with density tracers no longer follows the tidal alignment prediction, likely reflecting nonlinear processes that must be considered when modeling IA on these scales. Finally, we discuss tidal alignment in the context of cosmic shear measurements.

Cosmology from cosmic shear with Dark Energy Survey Science Verification data

“…We also consider a model called the complete tidal alignment (CTA) model [16]. This model includes all terms that contribute at next-to-leading order in the tidal alignment scenario, while also smoothing the tidal field.…”

“…We also consider a new model, described in Blazek et al [16], which includes all terms that contribute at next-to-leading order in the tidal alignment scenario, while simultaneously smoothing the tidal field (e.g. at the Lagrangian radius of the host halo).…”

“…As expected, imposing this lower bound significantly improves constraints when flexible redshift evolution of IA is allowed (see Table I and Figure 3). While allowing for mildly negative A within the tidal alignment paradigm may par-11 See Kirk et al [65] and Blazek et al [16] for further discussion of the treatment of non-linear density evolution in the NLA and similar models. tially account for potential non-zero alignments of blue and mixed-population source galaxies, a more sophisticated treatment (e.g.…”

Intrinsic alignment of redMaPPer clusters: cluster shape–matter density correlation

“…Croft & Metzler 2000;Hopkins et al 2005) and in hydrodynamical simulations (Codis et al 2015;Velliscig et al 2015;Chisari et al 2015Chisari et al , 2016Tenneti et al 2016;Hilbert et al 2016). The picture that is emerging is that massive, red galaxies are pointing towards matter overdensities, which can be reasonably well described by the tidal alignment model (Catelan et al 2001;Hirata & Seljak 2004;Bridle & King 2007;Blazek et al 2011Blazek et al , 2015 at large scales. For blue galaxies, IA has not yet been clearly detected (Hirata et al 2007;Mandelbaum et al 2011).…”

Alignment of galaxies relative to their local environment in SDSS-DR8

“…Moreover, knowledge of intrinsic alignment properties of galaxies can be used to locate filaments of galaxies (Pimbblet 2005a;Rong et al 2016). Modelling of this alignment is needed in weak-lensing studies to quantify the intrinsic alignment bias (Joachimi et al 2011;Blazek et al 2015;Codis et al 2015a); see also Joachimi et al (2015), Kiessling et al (2015), and Kirk et al (2015) for an excellent overview of galaxy alignments.…”