Cosmology

Abstract:

One of the most fundamental problems in modern astrophysics concerns the formation of galaxies and clusters of galaxies. The Dutch-European radio telescope LOFAR will open up the last unexplored window of the electromagnetic spectrum for astrophysical studies and make important contributions to our knowledge of the structure formation in the universe. LOFAR's world-class observational capabilities will be used to survey the entire Northern low-frequency sky at a number of key frequencies. Studies of the most distant radio galaxies, clusters of galaxies and the cosmic star formation history and the exploration of new parameter space for serendipitous discovery were the four key topics that drove the areas, depths and frequency coverage of the proposed surveys. In addition to the key topics, the LOFAR surveys will provide a wealth of unique data for a huge number of additional important topics, including: detailed studies of AGN, and AGN physics, AGN evolution and black hole accretion history, nearby galaxies, strong gravitational lenses, cosmological parameters and large-scale structure formation, and Galactic radio sources. In this contribution we will first briefly discuss the scientific topics that have driven the design of the surveys. Subsequently we will present the design of the surveys. We will then briefly report on commissioning work carried out to prepare the instrument and the software pipelines for carrying out these surveys. At the end we will elaborate on LOFAR studies on clusters and show some first LOFAR results related to the nearby rich cluster Abell 2256. With at the time of writing only 15 out of the planned 36 Dutch stations working and several aspects of the calibration pipleline not fully functional, the obtained 135 MHz image already is among the deepest ever produced at low frequencies. The central halo of A2256 is well detected, illustrating the potential of LOFAR to map diffuse steep spectrum radio emission.

Abstract:

We perform multidimensional radiative transfer simulations to compute spectra for a hydrodynamical simulation of a line-driven accretion disc wind from an active galactic nucleus. The synthetic spectra confirm expectations from parametrized models that a disc wind can imprint a wide variety of spectroscopic signatures including narrow absorption lines, broad emission lines and a Compton hump. The formation of these features is complex with contributions originating from many of the different structures present in the hydrodynamical simulation. In particular, spectral features are shaped both by gas in a successfully launched outflow and in complex flows where material is lifted out of the disc plane but ultimately falls back. We also confirm that the strong Fe Kα line can develop a weak, red-skewed line wing as a result of Compton scattering in the outflow. In addition, we demonstrate that X-ray radiation scattered and reprocessed in the flow has a pivotal part in both the spectrum formation and determining the ionization conditions in the wind. We find that scattered radiation is rather effective in ionizing gas which is shielded from direct irradiation from the central source. This effect likely makes the successful launching of a massive disc wind somewhat more challenging and should be considered in future wind simulations. © 2010 The Authors. Journal compilation © 2010 RAS.

Abstract:

Highly ionized fast accretion disc winds have been suggested as an explanation for a variety of observed absorption and emission features in the X-ray spectra of active galactic nuclei. Simple estimates have suggested that these flows may be massive enough to carry away a significant fraction of the accretion energy and could be involved in creating the link between supermassive black holes and their host galaxies. However, testing these hypotheses, and quantifying the outflow signatures, requires high-quality theoretical spectra for comparison with observations. Here, we describe extensions of our Monte Carlo radiative transfer code that allow us to generate realistic theoretical spectra for a much wider variety of disc wind models than that was possible in our previous work. In particular, we have expanded the range of atomic physics simulated by the code so that L- and M-shell ions can now be included. We have also substantially improved our treatment of both ionization and radiative heating such that we are now able to compute spectra for outflows containing far more diverse plasma conditions. We present example calculations that illustrate the variety of spectral features predicted by parametrized outflow models and demonstrate their applicability to the interpretation of data by comparison with observations of the bright quasar PG1211+143. We find that the major features in the observed 2-10 keV spectrum of this object can be well reproduced by our spectra, confirming that it likely hosts a massive outflow. © 2010 The Authors. Journal compilation © 2010 RAS.

Abstract:

The absolute luminosity of the Fe Kα emission line from matter illuminated by X-rays in astrophysical sources is non-trivial to calculate except when the line-emitting medium is optically thin to absorption and scattering. We characterize the Fe Kα line flux using a dimensionless efficiency, defined as the fraction of continuum photons above the Fe K shell absorption edge threshold energy that appear in the line. The optically thin approximation begins to break down even for column densities as small as 2 × 1022 cm-2. We show how to obtain reliable estimates of the Fe Kα line efficiency in the case of cold, neutral matter, even for the Compton-thick regime. We find that, regardless of geometry and covering factor, the largest Fe Kα line efficiency is attained well before the medium becomes Compton-thick. For cosmic elemental abundances it is difficult to achieve an efficiency higher than a few per cent under the most favourable conditions and lines of sight. For a given geometry, Compton-thick lines-of-sight may have Fe Kα line efficiencies that are orders of magnitude less than the maximum possible for that geometry. Configurations that allow unobscured views of a Compton-thick reflecting surface are capable of yielding the highest efficiencies. Our results can be used to estimate the predicted flux of the narrow Fe Kα line at ∼6.4 keV from absorption models in active galactic nucleus (AGN). In particular we show that contrary to a recent claim in the literature, absorption-dominated models for the relativistic Fe Kα emission line in MCG-6-30-15 do not overpredict the narrow Fe Kα line for any column density or covering factor. © 2009 RAS.

Abstract:

We analyse recently acquired near-infrared Hubble Space Telescope imaging of the Great Observatories Origins Deep Survey (GOODS)-South field to search for star-forming galaxies at z≈ 7.0. By comparing Wide Field Camera 3 (WFC3) 0.98 μm Y-band images with Advanced Camera for Surveys (ACS)z-band (0.85 μm) images, we identify objects with colours consistent with Lyman-break galaxies at z≃ 6.4-7.4. This new data cover an area five times larger than that previously reported in the WFC3 imaging of the Hubble Ultra Deep Field and affords a valuable constraint on the bright end of the luminosity function. Using additional imaging of the region in the ACS B,V and i bands from GOODS v2.0 and the WFC. 3J band, we attempt to remove any low-redshift interlopers. Our selection criteria yields six candidates brighter than YAB= 27.0, of which all except one are detected in the ACS z-band imaging and are thus unlikely to be transients. Assuming all six candidates are atz≈ 7, this implies a surface density of objects brighter than YAB= 27.0 of 0.30 ± 0.12 arcmin-2, a value significantly smaller than the prediction from z≈ 6 luminosity function. This suggests continued evolution of the bright end of the luminosity function betweenz= 6 and 7, with number densities lower at higher redshift. © 2010 The Authors. Journal compilation © 2010 RAS.