Liliya L. R. Williams

Extragalactic Radio Sources and the WMAP Cold Spot

We detect a dip of 20%-45% in the surface brightness and number counts of NRAO VLA Sky Survey (NVSS) sources smoothed to a few degrees at the location of the WMAP cold spot. The dip has structure on scales of ~1° to 10°. Together with independent all-sky wavelet analyses, our results suggest that the dip in extragalactic brightness and number counts and the WMAP cold spot are physically related, i.e., that the coincidence is neither a statistical anomaly nor a WMAP foreground-correction problem. If the cold spot does originate from structures at modest redshifts, as we suggest, then there is no remaining need for non-Gaussian processes at the last scattering surface of the cosmic microwave background (CMB) to explain the cold spot. The late integrated Sachs-Wolfe effect, already seen statistically for NVSS source counts, can now be seen to operate on a single region. To create the magnitude and angular size of the WMAP cold spot requires a ~140 Mpc radius completely empty void at z ≤ 1 along this line of sight. This is far outside the current expectations of the concordance cosmology, and adds to the anomalies seen in the CMB.

The behaviour of dark matter associated with four bright cluster galaxies in the 10 kpc core of Abell 3827

Galaxy cluster Abell 3827 hosts the stellar remnants of four almost equally bright elliptical galaxies within a core of radius 10 kpc. Such corrugation of the stellar distribution is very rare, and suggests recent formation by several simultaneous mergers. We map the distribution of associated dark matter, using new Hubble Space Telescope imaging and Very Large Telescope/Multi-Unit Spectroscopic Explorer integral field spectroscopy of a gravitationally lensed system threaded through the cluster core. We find that each of the central galaxies retains a dark matter halo, but that (at least) one of these is spatially offset from its stars. The best-constrained offset is 1.62(-0.49)(+0.47) kpc, where the 68 per cent confidence limit includes both statistical error and systematic biases in mass modelling. Such offsets are not seen in field galaxies, but are predicted during the long infall to a cluster, if dark matter self-interactions generate an extra drag force. With such a small physical separation, it is difficult to definitively rule out astrophysical effects operating exclusively in dense cluster core environments - but if interpreted solely as evidence for self-interacting dark matter, this offset implies a cross-section sigma(DM)/(m) similar to (1.7 +/- 0.7) x 10(-4) cm(2) g(-1) x (t(infall)/10(9) yr)(-2), where t(infall) is the infall duration.

Investigating the Origins of Dark Matter Halo Density Profiles

Although high-resolution N-body simulations make robust empirical predictions of the density distribution within cold dark matter halos, these studies have yielded little physical insight into the origins of the distribution. We therefore attempt to investigate the problem using analytic and semianalytic approaches. Simple analytic considerations suggest that the inner slope of the central cusps in dark matter halos cannot be steeper than ? = 2 (where ? r-?), with ? = 1.5-1.7 being a more realistic upper limit. Moreover, our analysis suggests that any number of effects, whether real (e.g., angular momentum imparted by tidal torques and secondary perturbations) or artificial (e.g., two-body interactions, the accuracy of the numerical integrator, round-off errors) will result in shallower slopes. We also find that the halos should exhibit a well-defined relationship between rperi/rapo and j?/jr. We derive this relationship analytically and speculate that it may be universal. Using a semianalytic scheme based on Ryden & Gunn, we further explore the relationship between the specific angular momentum distribution in a halo and its density profile. For present purposes, we restrict ourselves to halos that form primarily via the nearly smooth accretion of matter, and consider only the specific angular momentum generated by secondary perturbations associated with the cold dark matter spectrum of density fluctuations. Compared to those formed in N-body simulations, our semianalytic halos are more extended, have flatter rotation curves, and have a higher specific angular momentum, even though we have not yet taken into account the effects of tidal torques. Whether the density profile of numerical halos is indeed the result of loss in angular momentum outside the central region, and whether this loss is a feature of hierarchical merging and major mergers in particular, is under investigation.

Non-parametric reconstruction of the galaxy lens in PG 1115 + 080

We describe a new, non-parametric, method for reconstructing lensing mass distributions in multiple-image systems, and apply it to PG1115, for which time delays have recently been measured. It turns out that the image positions and the ratio of time delays between different pairs of images constrain the mass distribution in a linear fashion. Since observational errors on image positions and time delay ratios are constantly improving, we use these data as a rigid constraint in our modelling. In addition, we require the projected mass distributions to be inversion-symmetric and to have inward-pointing density gradients. With these realistic yet non-restrictive conditions it is very easy to produce mass distributions that fit the data precisely. We then present models, for

Pixelated Lenses and H0 from Time-Delay Quasars

H_0=42

Stellar and total mass in early-type lensing galaxies

, 63 and 84 \kmsmpc, that in each case minimize mass-to-light variations while strictly obeying the lensing constraints. (Only a very rough light distribution is available at present.) All three values of

A Portable Modeler of Lensed Quasars

H_0

The Hubble Time Inferred from 10 Time Delay Lenses

are consistent with the lensing data, but require quite different morphologies for the lensing galaxy. If

The core structure of galaxy clusters from gravitational lensing

H_0

Pixelated Lenses and H_0 from Time-delay QSOs

is low, the main lensing galaxy could be an early type. If