Intrinsic correlation of galaxy shapes: implications for weak lensing measurements

Weak gravitational lensing is now established as a powerful method to measure mass fluctuations in the universe. It relies on the measurement of small coherent distortions of the images of background galaxies. Even low-level correlations in the intrinsic shapes of galaxies could however produce a significant spurious lensing signal. These correlations are also interesting in their own right, since their detection would constrain models of galaxy formation. Using     haloes found in N -body simulations, we compute the correlation functions of the intrinsic ellipticity of spiral galaxies assuming that the disc is perpendicular to the angular momentum of the dark matter halo. We also consider a simple model for elliptical galaxies, in which the shape of the dark matter halo is assumed to be the same as that of the light. For deep lensing surveys with median redshifts ∼1, we find that intrinsic correlations of ∼10⁻⁴ on angular scales     are generally below the expected lensing signal, and contribute only a small fraction of the excess signals reported on these scales. On larger scales we find limits to the intrinsic correlation function at a level ∼10⁻⁵, which gives a (model-dependent) range of separations for which the intrinsic signal is about an order of magnitude below the ellipticity correlation function expected from weak lensing. Intrinsic correlations are thus negligible on these scales for dedicated weak lensing surveys. For wider but shallower surveys such as SuperCOSMOS, APM and SDSS, we cannot exclude the possibility that intrinsic correlations could dominate the lensing signal. We discuss how such surveys could be used to calibrate the importance of this effect, as well as study spin–spin correlations of spiral galaxies.     

Galaxy density profiles and shapes – II. Selection biases in strong lensing surveys

Many current and future astronomical surveys will rely on samples of strong gravitational lens systems to draw conclusions about galaxy mass distributions. We use a new strong lensing pipeline (presented in Paper I of this series) to explore selection biases that may cause the population of strong lensing systems to differ from the general galaxy population. Our focus is on point-source lensing by early-type galaxies with two mass components (stellar and dark matter) that have a variety of density profiles and shapes motivated by observational and theoretical studies of galaxy properties. We seek not only to quantify but also to understand the physics behind selection biases related to: galaxy mass, orientation and shape; dark matter profile parameters such as inner slope and concentration; and adiabatic contraction. We study how all of these properties affect the lensing Einstein radius, total cross-section, quad/double ratio and image separation distribution, with a flexible treatment of magnification bias to mimic different survey strategies. We present our results for two families of density profiles: cusped and deprojected Sérsic models. While we use fixed lens and source redshifts for most of the analysis, we show that the results are applicable to other redshift combinations, and we also explore the physics of how our results change for very different redshifts. We find significant (factors of several) selection biases with mass; orientation, for a given galaxy shape at fixed mass; cusped dark matter profile inner slope and concentration; concentration of the stellar and dark matter deprojected Sérsic models. Interestingly, the intrinsic shape of a galaxy does not strongly influence its lensing cross-section when we average over viewing angles. Our results are an important first step towards understanding how strong lens systems relate to the general galaxy population.     

Galaxy density profiles and shapes – I. Simulation pipeline for lensing by realistic galaxy models

Studies of strong gravitational lensing in current and upcoming wide and deep photometric surveys, and of stellar kinematics from (integral-field) spectroscopy at increasing redshifts, promise to provide valuable constraints on galaxy density profiles and shapes. However, both methods are affected by various selection and modelling biases, which we aim to investigate in a consistent way. In this first paper in a series, we develop a flexible but efficient pipeline to simulate lensing by realistic galaxy models. These galaxy models have separate stellar and dark matter components, each with a range of density profiles and shapes representative of early-type, central galaxies without significant contributions from other nearby galaxies. We use Fourier methods to calculate the lensing properties of galaxies with arbitrary surface density distributions, and Monte Carlo methods to compute lensing statistics such as point-source lensing cross-sections. Incorporating a variety of magnification bias modes lets us examine different survey limitations in image resolution and flux. We rigorously test the numerical methods for systematic errors and sensitivity to basic assumptions. We also determine the minimum number of viewing angles that must be sampled in order to recover accurate orientation-averaged lensing quantities. We find that for a range of non-isothermal stellar and dark matter density profiles typical of elliptical galaxies, the combined density profile and corresponding lensing properties are surprisingly close to isothermal around the Einstein radius. The converse implication is that constraints from strong lensing and/or stellar kinematics, which are indeed consistent with isothermal models near the Einstein radius, cannot trivially be extrapolated to smaller and larger radii.     

Shakbazian Compact Groups: Mass‐to‐light ratios and mass distribution models: I. ShCG 362 and ShCG 166

We present results of photometric observations under excellent seeing conditions of Shakbazian Compact Groups. We obtained the seeing‐unconvolved surface brightness profiles of individual galaxies in the I band. We also determined the B – I color index for each galaxy, and investigated the presence of cores in the early type galaxies. We constructed models for the mass distribution of the individual galaxies. The mass‐to‐light (𝔐/L) ratios have normal values, and the conclusion that these groups have little dark matter is confirmed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Comparisons of u ‐, g ‐, r ‐, i ‐, and z ‐band luminosity distributions between galaxy members of compact groups and isolated galaxies

To investigate the environmental dependence of u ‐, g ‐, r ‐, i ‐, and z ‐band luminosities, we perform comparative studies of luminosity distributions between galaxy members of compact groups (CGs) and isolated galaxies. It is found that for the r, i, and z bands isolated galaxies have a higher proportion of faint galaxies and a lower proportion of luminous galaxies than galaxy members of CGs, but for the u band an opposite trend is observed. The correlation between the g ‐band luminosity and the environment has different trends in different luminosity regions (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Broad-band and Hα surface photometry of the central regions of nearby active galaxies – I. Observations

Broad-band V , R and I and narrow-band H α CCD surface photometry of a sample of 24 active and three normal nearby galaxies is presented, in order to study their morphological and structural properties. This first paper presents the observations, data reduction and processing techniques used; the main characteristics of the central regions of the sample galaxies are derived from broad-band and H α images; broad-band surface brightness and colour profiles are presented; based on the inspection of individual images and the examination of the radial variation of ellipticity and polar angle, the morphological class of the sample objects is revised: several objects previously classified as non-barred or unclassified are found to be barred galaxies. A high fraction (∼71 per cent) of the objects that present any kind of nuclear activity are barred. This proportion is even higher (∼87 per cent) if only Seyfert galaxies are considered. This result can reinforce the clues about the connection between bars and nuclear activity as a fuelling mechanism that can trigger and sustain nuclear activity.