Cosmology

Abstract:

potentials causes a systematic effect on measurements of cosmic shear statistics, known as the source-lens clustering (SLC) effect. The SLC effect on the skewness of lensing convergence, S3, is examined using a non-linear semi-analytic approach and is checked against numerical simulations. The semi-analytic calculations have been performed in a wide variety of generic models for the redshift distribution of source galaxies and power-law models for the bias parameter between the galaxy and dark matter distributions. The semi-analytic predictions are tested successfully against numerical simulations. We find the relative amplitude of the SLC effect on S3 to be of the order of 5 -40 per cent. It depends significantly on the redshift distribution of sources and on the way in which the bias parameter evolves. We discuss possible measurement strategies to minimize the SLC effects.

Abstract:

We have optically identified a sample of 56 featureless continuum objects without significant proper motion from the 2dF QSO Redshift Survey (2QZ). The steep number--magnitude relation of the sample, $n(\bj) \propto 10^{0.7\bj}$, is similar to that derived for QSOs in the 2QZ and inconsistent with any population of Galactic objects. Follow up high resolution, high signal-to-noise, spectroscopy of five randomly selected objects confirms the featureless nature of these sources. Assuming the objects in the sample to be largely featureless AGN, and using the QSO evolution model derived for the 2QZ, we predict the median redshift of the sample to be $z=1.1$. This model also reproduces the observed number-magnitude relation of the sample using a renormalisation of the QSO luminosity function, $\Phi^* = \Phi^*_{\rm \sc qso}/66 \simeq 1.65 \times 10^{-8} $mag$^{-1}$Mpc$^{-3}$. Only $\sim$20 per cent of the objects have a radio flux density of $S_{1.4}>3 $mJy, and further VLA observations at 8.4 GHz place a $5\sigma$ limit of $S_{8.4} < 0.2$mJy on the bulk of the sample. We postulate that these objects could form a population of radio-weak AGN with weak or absent emission lines, whose optical spectra are indistinguishable from those of BL Lac objects.

Abstract:

We present a morphological analysis of 17 X-ray-selected clusters at z ∼ 0.25, imaged uniformly with the Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2). Eight of these clusters comprise a subsample selected for their low X-ray luminosities (≲1044 ergs s -1), called the low-Lx sample. The remaining nine clusters comprise a high-Lx subsample with Lx > 10 45 ergs s-1. The two subsamples differ in their mean X-ray luminosity by a factor of 30 and span a range of more than 300. The clusters cover a relatively small range in redshift (z = 0.17-0.3, σ z/Z ∼ 0.15), and the data are homogeneous in terms of depth, resolution (0″.17 = 1 h50-1 kpc at z = 0.25), and rest wavelength observed, minimizing differential corrections from cluster to cluster. We fit the two-dimensional surface brightness profiles of galaxies down to very faint absolute magnitudes, M702 ≤, -18.2 + 5 log h50 (roughly 0.01L*R) with parametric models, and quantify their morphologies using the fractional bulge luminosity (B/T). Within a single WFPC2 image, covering a field of ∼ 3′ (1h50-1 Mpc at z = 0.25) in the cluster center, we find that the low-Lx clusters are dominated by galaxies with low B/T (∼ 0), while the high-Lx clusters are dominated by galaxies with intermediate B/T (∼ 0.4). We test whether this difference could arise from a universal morphology-density relation due to differences in the typical galaxy densities in the two samples. We find that small differences in the B/T distributions of the two samples persist with marginal statistical significance (98% confidence based on a binned Χ2 test) even when we restrict the comparison to galaxies in environments with similar projected local galaxy densities. A related difference (also of low statistical significance) is seen between the bulge-luminosity functions of the two cluster samples, while no difference is seen between the disk luminosity functions. From the correlations between these quantities, we argue that the global environment affects the population of bulges, over and above trends seen with local density. On the basis of this result, we conclude that the destruction of disks through ram pressure stripping or harassment is not solely responsible for the morphology-density relation and that bulge formation is less efficient in low-mass clusters, perhaps reflecting a less rich merger history.