Intrinsic alignments of BOSS LOWZ galaxies – II. Impact of shape measurement methods

Abstract

Measurements of intrinsic alignments of galaxy shapes with the large-scale density field, and the inferred intrinsic alignments model parameters, are sensitive to the shape measurement methods used. In this paper we measure the intrinsic alignments of the Sloan Digital Sky Survey-III (SDSS-III) Baryon Oscillation Spectroscopic Survey (BOSS) LOWZ galaxies using three different shape measurement methods (reGaussianization, isophotal, and de Vaucouleurs), identifying a variation in the inferred intrinsic alignments amplitude at the 40% level between these methods, independent of the galaxy luminosity or other properties. We also carry out a suite of systematics tests on the shapes and their two-point correlation functions, identifying a pronounced contribution from additive PSF systematics in the de Vaucouleurs shapes. Since different methods measure galaxy shapes at different effective radii, the trends we identify in the intrinsic alignments amplitude are consistent with the interpretation that the outer regions of galaxy shapes are more responsive to tidal fields, resulting in isophote twisting and stronger alignments for isophotal shapes. We observe environment dependence of ellipticity, with brightest galaxies in groups being rounder on average compared to satellite and field galaxies. We also study the anisotropy in intrinsic alignments measurements introduced by projected shapes, finding effects consistent with predictions of the nonlinear alignment model and hydrodynamic simulations. The large variations seen using the different shape measurement methods have important implications for intrinsic alignments forecasting and mitigation with future surveys.

GAMA+KiDS: Alignment of galaxies in galaxy groups and its dependence on galaxy scale

“…Comparing to other observations, Singh & Mandelbaum (2016) found, using low redshift luminous red galaxies, that alignment measurements with different shape estimators resulted in different inferred intrinsic alignment amplitudes. One of the possible reasons for this is that these shape estimators are sensitive to different galaxy scales.…”

“…Such a dependence is expected due to the tidal nature of the phenomenon and has been seen in hydrodynamical simulations, where the alignment signal was stronger when the method used to define galaxy shapes up-weighted outer regions of galaxies (Velliscig et al 2015;Chisari et al 2015;Hilbert et al 2017). Observations have also hinted towards this, with Singh & Mandelbaum (2016); Huang et al (2018) looking into alignments as measured by different shape measurement methods and finding stronger alignments for methods more sensitive to the outer shape of a galaxy. In addition, Georgiou et al (2019) found that the alignment signal changes on small scales when shapes were measured from different broad band filters, with the change mostly sourced by satellite galaxies.…”

“…Multiplicative bias can also cause such a difference, as the observed alignment signal scales linearly with bias (Eq. 24 Singh & Mandelbaum 2016), and the shape measurements in Singh & Mandelbaum (2016) were not corrected for it.…”

Controlling and leveraging small-scale information in tomographic galaxy–galaxy lensing

“…So far, detections of this intrinsic alignment signal have largely been limited to bright, red galaxies (e.g. Mandelbaum et al 2006;Hirata et al 2007;Joachimi et al 2011;Blazek et al 2011;Singh & Mandelbaum 2016). It is possible that this contamination can be mitigated by removing these galaxies from the source sample, through improved photo-z methods, or with modelling approaches (e.g.…”

An EFT description of galaxy intrinsic alignments

“…before the effect of gravitational lensing on the observed shapes [see 1, 2, for reviews]. These are known to exist between luminous red galaxies at low redshift [3][4][5][6][7], and are considered a contaminant for cosmology with weak gravitational lensing [8][9][10]. Despite these alignments being measured to high significance in current galaxy surveys, there have been relatively few efforts on exploring possible models for this signal.…”