arXiv subjects

Lindsay J. King

Publications and source records attributed to Lindsay J. King.

23 records · Page 2Linked to original sources

arXiv2008

The Hubble constant from galaxy lenses: impacts of triaxiality and model degeneracies

The Hubble constant can be constrained using the time delays between multiple images of gravitationally lensed sources. In some notable cases, typical lensing analyses assuming isothermal galaxy density profiles produce low values for the Hubble constant, inconsistent with the result of the HST Key Project (72 +- 8 km/s/Mpc). Possible systematics in the values of the Hubble constant derived from galaxy lensing systems can result from a number of factors, e.g. neglect of environmental effects, assumption of isothermality, or contamination by line-of-sight structures. One additional potentially important factor is the triaxial structure of the lensing galaxy halo; most lens models account for halo shape simply by perturbing the projected spherical lensing potential, an approximation that is often necessary but that is inadequate at the levels of triaxiality predicted in the CDM paradigm. To quantify the potential error introduced by this assumption in estimates of the Hubble parameter, we strongly lens a distant galaxy through a sample of triaxial softened isothermal halos and use an MCMC method to constrain the lensing halo profile and the Hubble parameter from the resulting multiple image systems. We explore the major degeneracies between the Hubble parameter and several parameters of the lensing model, finding that without a way to accurately break these degeneracies accurate estimates of the Hubble parameter are not possible. Crucially, we find that triaxiality does not significantly bias estimates of the Hubble constant, and offer an analytic explanation for this behaviour in the case of isothermal profiles. Neglected triaxial halo shape cannot contribute to the low Hubble constant values derived in a number of galaxy lens systems.

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arXiv2007

Observing cosmic string loops with gravitational lensing surveys

We show that the existence of cosmic strings can be strongly constrained by the next generation of gravitational lensing surveys at radio frequencies. We focus on cosmic string loops, which simulations suggest would be far more numerous than long (horizon-sized) strings. Using simple models of the loop population and minimal assumptions about the lensing cross section per loop, we estimate the optical depth to lensing and show that extant radio surveys such as CLASS have already ruled out a portion of the cosmic string model parameter space. Future radio interferometers, such as LOFAR and especially SKA, may constrain $Gμ/c^2 < 10^{-9}$ in some regions of parameter space, outperforming current constraints from pulsar timing and the CMB by up to two orders of magnitude. This method relies on direct detections of cosmic strings, and so is less sensitive to the theoretical uncertainties in string network evolution that weaken other constraints.

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arXiv2007

Steepened inner density profiles of group galaxies via interactions: An N-body analysis

We continue to see a range of values for the Hubble constant obtained from gravitationally lensed multiple image time delays when assuming an isothermal lens despite a robust value from the HST key project (72 +- 8 km s^-1 Mpc^-1. One explanation is that there is a variation in Hubble constant values due to a fundamental heterogeneity in lens galaxies present in groups. Our goal is to see if a variety of group interactions between the most massive group members can result in significant changes in the galaxy density profiles over the scale probed by strong lensing (<15 kpc). While stripping of the outer parts of the halo can be expected, the impact on inner regions where the luminous component is important is less clear in the context of lensing, though still crucial, as a steepened density profile within this inner region allows these lens systems to be consistent with current HST/WMAP estimates on H_0. We employ the particle-mesh code SUPERBOX to carry out the group interaction simulations. We simulate interactions between group members, comparing the density profile for the satellite before and after interaction for the mass range of 10^11 to 10^13 M_sun. Our investigations show a significant steepening of the density profile in the region of < 5-20 kpc, i.e. that which dominates strong lensing in lens galaxies. Additionally, the steepening in the inner region is transient in nature, with consecutive interactions returning the profile to an isothermal state within a timeframe of ~ 0.5 - 2.0 Gyr. This factor may help explain why lens galaxies that produce lower values of H_0 (i.e. those with possibly steeper profiles) are far fewer in number than those which agree with both the HST key project value for H_0 and isothermality, since one would have to observe the lens galaxy during this transient steepened phase.

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arXiv2006

A simple analysis of halo density profiles using gravitational lensing time delays

Gravitational lensing time delays depend upon the Hubble constant and the density distribution of the lensing galaxies. This allows one to either model the lens and estimate the Hubble constant, or to use a prior on the Hubble constant from other studies and investigate what the preferred density distribution is. Some studies have required compact dark matter halos (constant M/L ratio) in order to reconcile gravitational lenses with the HST/WMAP value of the Hubble constant (72 +/- 8 km/s /Mpc and 72 +/- 5 km/s /Mpc, respectively). This is in direct contradiction with X-ray, stellar dynamical, and weak lensing studies, which all point towards extended halos and isothermal density profiles. In this work, we examine an up-to-date sample of 13 lensing galaxies resulting in a data set consisting of 21 time delays. We select systems in which there is a single primary lensing galaxy (e.g. excluding systems undergoing mergers). Analysis is performed using analytic models based upon a powerlaw density profile (rho \propto r^-n) of which the isothermal profile is a special case (n = 2). This yields a value of n = 2.11+/-0.12 (3sigma) for the mean profile when modeling with a prior on the Hubble constant, which is only consistent with isothermality within 3 sigma. Note that this is a formal error from our calculations, and does not include the impact of sample selection or simplifications in the lens modeling. We conclude that time delays are a useful probe of density profiles, in particular as a function of the environment in which the lens resides, when combined with a prior on the Hubble constant.

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arXiv2004

The covariance of cosmic shear correlation functions and cosmological parameter estimates using redshift information

Cosmological weak lensing by the large scale structure of the Universe, cosmic shear, is coming of age as a powerful probe of the parameters describing the cosmological model and matter power spectrum. It complements CMB studies, by breaking degeneracies and providing a cross-check. An important measure of the cosmic shear signal are the shear correlation functions; these can be directly calculated from data, and compared with theoretical expectations for different cosmological models and matter power spectra. We present a Monte Carlo method to quickly simulate mock cosmic shear surveys. One application of this method is in the determination of the full covariance matrix for the correlation functions; this includes redshift binning and is applicable to arbitrary survey geometries. Terms arising from shot noise and cosmic variance (dominant on small and large scales respectively) are accounted for naturally. As an illustration of the use of such covariance matrices, we consider to what degree confidence regions on parameters are tightened when redshift binning is employed. The parameters considered are those commonly discussed in cosmic shear analyses - the matter density parameter, dark energy density parameter (classical cosmological constant), power spectrum normalisation and shape parameter. We incorporate our covariance matrices into a likelihood treatment, and also use the Fisher formalism to explore a larger region of parameter space (abridged).

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