Cosmology

Abstract:

We describe the objectives, design and predicted performance of Clover, a fully-funded, UK-led experiment to measure the B-mode polarisation of the Cosmic Microwave Background (CMB). Three individual telescopes will operate at 97, 150 and 225 GHz, each populated by up to 256 horns. The detectors, TES bolometers, are limited by unavoidable photon noise, and coupled to an optical design which gives very low systematic errors, particularly in cross-polarisation. The telescopes will sit on three-axis mounts on a site in the Atacama Desert. The angular resolution of around 8 ́ and sky coverage of around 1000 deg2 provide multipole coverage of 20<ℓ<1000. Combined with the high sensitivity, this should allow the B-mode signal to be measured (or constrained) down to a level corresponding to a tensor-to-scalar ratio of r = 0.01, providing the emission from polarised foregrounds can be subtracted. This in turn will allow constraints to be placed on the energy scale of inflation, providing an unprecedented insight into the early history of the Universe.

Abstract:

We have studied ∼2100 early-type galaxies in the SDSS DR3 which have been detected by the GALEX Medium Imaging Survey (MIS), in the redshift range O < z < 0.1.1. Combining GALEXUV photometry with corollary optical data from the SDSS, we find that, at a 95% confidence level, at least ∼30% of galaxies in this sample have UV to optical colors consistent with some recent star formation within the last Gyr. In particular, galaxies with an NUV - r color less than 5.5 are very likely to have experienced such recent star formation, taking into account the possibility of a contribution to NUV flux from the UV upturn phenomenon. We find quantitative agreement between the observations and the predictions of a semianalytical ACDM hierarchical merger model and deduce that early-type galaxies in the redshift range 0 < z < 0.11 have ∼ 1 % -3 % of their stellar mass in stars less than 1 Gyr old. The average age of this recently formed population is ∼300-500 Myr. We also find that "monolithically" evolving galaxies, where recent star formation can be driven solely by recycled gas from stellar mass loss, cannot exhibit the blue colors (NUV - r < 5.5) seen in a significant fraction (∼30%) of our observed sample. © 2007. The American Astronomical Society. All rights reserved.

Abstract:

Context. X-ray surveys of Active Galactic Nuclei (AGN) indicate "cosmic downsizing", with the comoving number density of high-luminosity objects peaking at higher redshifts (z ∼ 2) than low-luminosity AGN (z < 1). Aims. We test whether downsizing is caused by activity shifting towards low-mass black holes accreting at near-Eddington rates, or by a change in the average rate of accretion onto supermassive black holes. We estimate the black hole masses and Eddington ratios of an X-ray selected sample of AGN in the Chandra Deep Field South at z < 1, probing the epoch where AGN cosmic downsizing has been reported. Methods. Black hole masses are estimated both from host galaxy stellar masses, which are estimated from fitting to published optical and near-infrared photometry, and from near-infrared luminosities, applying established correlations between black hole mass and host galaxy properties. Both methods give consistent results. Comparison and calibration of possible redshift-dependent effects is also made using published faint host galaxy velocity dispersion measurements. Results. The Eddington ratios in our sample span the range ∼10-5-1, with median log(Lbol/LEdd) = -2.87, and with typical black hole masses MBH ∼ 108 M⊙. The broad distribution of Eddington ratios is consistent with that expected for AGN samples at low and moderate luminosity. We find no evidence that the CDF-S AGN population is dominated by low-mass black holes accreting at near-Eddington ratios and the results suggest that diminishing accretion rates onto average-sized black holes are responsible for the reported AGN downsizing at redshifts below unity. © ESO 2007.

Abstract:

Context. The variable X-ray spectra of AGN systematically show steep power-law high states and hard-spectrum low states. The hard, low state has previously been found to be a component with only weak variability. The origin of this component and the relative importance of effects such as absorption and relativistic blurring are currently not clear. Aims. In a follow-up of previous principal components analysis we aim to determine the relative importance of scattering and absorption effects on the time-varying X-ray spectrum of the narrow-line Seyfert 1 galaxy Mrk 766. Methods. Time-resolved spectroscopy, slicing XMM and Suzaku data down to 25 ks elements is used to investigate whether absorption or scattering components dominate the spectral variations in Mrk 766. Results. Time-resolved spectroscopy confirms that spectral variability in Mrk 766 can be explained by either of two interpretations of principal components analysis. Detailed investigation confirm rapid changes in the relative strengths of scattered and direct emission or rapid changes in absorber covering fraction provide good explanations of most of the spectral variability. However, a strong correlation between the 6.97 keV absorption line and primary continuum together with rapid opacity changes show that variations in a complex and multi-layered absorber, most likely a disk wind, are the dominant source of spectral variability in Mrk 766. © ESO 2007.