Intrinsic Alignment of Galaxies and Impacts on Weak Lensing

Key Points

• Spurious (non-cosmological) correlations between galaxies, known as intrinsic alignments (IAs) have long been known to affect cosmic shear estimates. [Samuroff et al. 2018]
• The particular challenge posed by IA modelling is in large part down to the nature of the contamination: IA correlations are a real astrophysical signal, which enters much the same angular scales as cosmic shear itself. [Samuroff et al. 2018]

Reviews

• Troxel & Ishak 2015 - The intrinsic alignment of galaxies and its impact on weak gravitational lensing in an era of precision cosmology
  • Important
• Galaxy Alignments
  • Joachimi et al. 2015 - Overview
  • Kirk et al. 2015 - Observations and Impact on Cosmology
  • Kiessling et al. 2015 - Theory, Modelling & Simulation

Key Paper

• Alignment of the intrinsic shapes of physically close pairs of galaxies, known as II correlations (Intrinsic-Intrinsic)
  • Catelan et al. 2001 - Intrinsic and extrinsic galaxy alignment
    • Tidal interaction: Long-range correlations in the tidal field will thus lead to long-range ellipticity-ellipticity correlations that mimic the shear correlations arising from weak gravitational lensing.
  • Mackey et al. 2002 - Theoretical estimates of intrinsic galaxy alignment
    • Rotational torquing: We present a new model relating ellipticity to angular momentum, which can be calculated in linear theory.
• An often more pervasive effect comes from the fact that the same foreground matter experiences local gravitational interactions over short spatial scales, and also induces lensing of background galaxies. This generates correlations in shape between foreground galaxies and background sources (Gravitational-Intrinsic)
  • Hirata & Seljak 2004 - Intrinsic alignment-lensing interference as a contaminant of cosmic shear
• Total IA contamination to cosmological shear can be as high as 10% in modern surveys
  • Croft & Metzler 2000 - Weak-Lensing Surveys and the Intrinsic Correlation of Galaxy Ellipticities
  • Kirk et al. 2012 - The cosmological impact of intrinsic alignment model choice for cosmic shear
  • Krause et al. 2016 - The impact of intrinsic alignment on current and future cosmic shear surveys
• The IA signal, if correctly modelled, can in principle be used as a probe of:
  • Cosmology: Chisari & Dvokin 2013 - Cosmological information in the intrinsic alignments of luminous red galaxies
    • We make forecasts for the ability of current and future spectroscopic surveys to constrain local primordial non-Gaussianity and Baryon Acoustic Oscillations (BAO) in the cross-correlation function of intrinsic alignments and the galaxy density field
  • Primordial non-Gaussianity: Chisari et al. 2016 - Multitracing anisotropic non-Gaussianity with galaxy shapes
    • We study the use of two different shape estimators as a multitracer probe of intrinsic alignments. We show, by means of a Fisher analysis, that this technique promises a significant improvement on anisotropic non-Gaussianity constraints over a single-tracer method.
  • Galaxy formation: Schmitz et al. 2018 - Time evolution of intrinsic alignments of galaxies
    • In particular, advection of galaxies due to peculiar velocities alters the impact of IA, because galaxy positions when observed are generally different from their positions at the epoch when IA is believed to be set.
• A number of mitigation techniques have been proposed, involving:
  • Discarding physically close pairs of galaxies: Catelan et al. 2001 and Kirk et al. 2015 - Galaxy Alignments: Observations and Impact on Cosmology
  • Downweighting:
    • King & Schneider 2003 - Separating cosmic shear from intrinsic galaxy alignments: Correlation function tomography
    • Heymans & Heavens 2003 - Weak gravitational lensing: reducing the contamination by intrinsic alignments
      • We show how distance information, either spectroscopic or photometric redshifts, can be used to downweight nearby pairs in an optimized way, to reduce the errors in the shear signal arising from intrinsic alignments. Provided a conservatively large intrinsic alignment is assumed, the optimized weights will essentially remove all traces of contamination
  • Nulling: Heavens & Joachimi 2011 - Cosmic magnification: nulling intrinsic clustering
  • All of these methods depend on the existence of accurate redshift information to allow galaxies to be located relative to each other along the line of sight. Significantly, they are also ineffective in mitigating GI correlations [Samuroff et al. 2018]
• It has been noted in both simulations and data that the choice of galaxy shape estimation method can alter the magnitude of the IA signal by an overall scale-independent factor
  • Singh & Mandelbaum 2016 - Intrinsic alignments of BOSS LOWZ galaxies - II. Impact of shape measurement methods
    • 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 non-linear alignment model and hydrodynamic simulations.
  • Leonard & Mandelbaum 2018 - Measuring the scale dependence of intrinsic alignments using multiple shear estimates
• Attempts to place observational constraints on the alignment properties of galaxy samples more representative of the sort used for cosmological lensing measurements
  • Blazek et al. 2012 - Separating intrinsic alignment and galaxy-galaxy lensing
  • Tonegawa et al. 2017 - The Subaru FMOS galaxy redshift survey (FastSound). V. Intrinsic alignments of emission-line galaxies at z~1.4

Recent Measurements

• Samuroff et al. 2018 - Dark Energy Survey Year 1 Results: Constraints on Intrinsic Alignments and their Colour Dependence from Galaxy Clustering and Weak Lensing
• Intrinsic alignments in redMaPPer clusters:
  • Huang et al. 2018a - I. Central galaxy alignments and angular segregation of satellites
    • We find that central galaxies are more aligned with their member galaxy distributions in clusters that are more elongated and have higher richness, and for central galaxies with larger physical size, higher luminosity and centering probability, and redder color. Satellites with redder color, higher luminosity, located closer to the central galaxy, and with smaller ellipticity show a stronger angular segregation towards their central galaxy major axes.
  • Huang et al. 2018b - II. Radial alignment of satellites towards cluster centres
    • Satellites alignments (SA): We detect the strongest SA signal in isophotal shapes, followed by de Vaucouleurs shapes. While no net SA signal is detected using re-Gaussianization shapes across the entire sample
    • We find that the measured SA signal is strongest for satellites with the following characteristics: higher luminosity, smaller distance to the cluster centre, rounder in shape, higher bulge fraction, and distributed preferentially along the major axis directions of their centrals.
• van Uitert & Joachimi 2017 - Intrinsic alignment of redMaPPer clusters: cluster shape-matter density correlation
  • We obtain tentative evidence that the signal increases towards higher richness and lower redshift.
  • Comparing our results to the IA measurements of luminous red galaxies, we find that the IA amplitude of galaxy clusters forms a smooth extension towards a higher mass.

In Simulations

• Illustris and IllustrisTNG
  • Hilbert et al. 2017 - http://adsabs.harvard.edu/abs/2016MNRAS.457.2301S
    • The correlations considered include the matter density-intrinsic ellipticity (mI), galaxy density-intrinsic ellipticity (dI), gravitational shear-intrinsic ellipticity (GI) and intrinsic ellipticity-intrinsic ellipticity (II) correlations. We find stronger correlations for more massive and more luminous galaxies, as well as for earlier photometric types.
    • We also find that the GI contributions to the observed ellipticity correlations could be inferred directly from measurements of galaxy density-intrinsic ellipticity correlations, except on small scales, where systematic differences between mI and dI correlations are large.
• Horizon-AGN
  • Chisari et al. 2015 - Intrinsic alignments of galaxies in the Horizon-AGN cosmological hydrodynamical simulation
    • We find that spheroidal galaxies in the simulation show a tendency to be aligned radially towards overdensities in the dark matter density field and other spheroidals.
    • Disc galaxies show a tendency to be oriented tangentially around spheroidals in three dimensions.
• MassiveBlackII
  • Tenneti et al. 2017 - Impact of Baryonic Physics on Intrinsic Alignments
    • We explore the parameter space of the subgrid star formation and feedback model and find remarkable robustness of the observable statistical measures to the details of subgrid physics.
  • Tenneti et al. 2016 - Intrinsic alignments of disc and elliptical galaxies in the MassiveBlack-II and Illustris simulations
    • We find that simulated disc galaxies are more oblate in shape and more misaligned with the shape of their host dark matter subhalo when compared with ellipticals. The disc major axis is found to be oriented towards the location of nearby elliptical galaxies.