Hintze Centre for Astrophysical Surveys

Abstract:

We use the Herschel-Astrophysical Terahertz Large Area Survey (ATLAS) science demonstration data to investigate the star formation properties of radio-selected galaxies in the GAMA-9h field as a function of radio luminosity and redshift. Radio selection at the lowest radio luminosities, as expected, selects mostly starburst galaxies. At higher radio luminosities, where the population is dominated by active galactic nuclei (AGN), we find that some individual objects are associated with high far-infrared luminosities. However, the far-infrared properties of the radio-loud population are statistically indistinguishable from those of a comparison population of radio-quiet galaxies matched in redshift and K-band absolute magnitude. There is thus no evidence that the host galaxies of these largely low-luminosity (Fanaroff-Riley class I), and presumably low-excitation, AGN, as a population, have particularly unusual star formation histories. Models in which the AGN activity in higher luminosity, high-excitation radio galaxies is triggered by major mergers would predict a luminosity-dependent effect that is not seen in our data (which only span a limited range in radio luminosity) but which may well be detectable with the full Herschel-ATLAS data set. © 2010 The Authors. Journal compilation © 2010 RAS.

Abstract:

In the following paper, we present an internal shocks model, iShocks, for simulating a variety of relativistic jet scenarios; these scenarios can range from a single ejection event to an almost continuous jet, and are highly user configurable. Although the primary focus in the following paper is black hole X-ray binary jets, the model is scale and source independent and could be used for supermassive black holes in active galactic nuclei or other flows such as jets from neutron stars. Discrete packets of plasma (or 'shells') are used to simulate the jet volume. A two-shell collision gives rise to an internal shock, which acts as an electron re-energization mechanism. Using a pseudo-random distribution of the shell properties, the results show how for the first time it is possible to reproduce a flat/inverted spectrum (associated with compact radio jets) in a conical jet whilst taking the adiabatic energy losses into account. Previous models have shown that electron re-acceleration is essential in order to obtain a flat spectrum from an adiabatic conical jet: multiple internal shocks prove to be efficient in providing this re-energization. We also show how the high-frequency turnover/break in the spectrum is correlated with the jet power, νb ∝ L∼0.6W, and the flat-spectrum synchrotron flux is correlated with the total jet power, F ν ∝ L∼1.4W. Both the correlations are in agreement with previous analytical predictions. © 2009 RAS.

Abstract:

Lynds dark cloud LDN1622 represents one of the best examples of anomalous dust emission, possibly originating from small spinning dust grains. We present Cosmic Background Imager (CBI) 31-GHz data of LDN1621, a diffuse dark cloud to the north of LDN1622 in a region known as Orion East. A broken ring-like structure with diameter ≈20 arcmin of diffuse emission is detected at 31 GHz, at ≈20-30 mJy beam-1 with an angular resolution of ≈5 arcmin. The ring-like structure is highly correlated with far-infrared (FIR) emission at 12-100 μm with correlation coefficients of r ≈ 0.7-0.8, significant at ~10σ. The FIR-correlated emission at 31 GHz therefore appears to be mostly due to radiation associated with dust. Multifrequency data are used to place constraints on other components of emission that could be contributing to the 31-GHz flux. An analysis of the GB6 survey maps at 4.85 GHz yields a 3σ upper limit on free-free emission of 7.2 mJy beam-1 (30 per cent of the observed flux) at the CBI resolution. The bulk of the 31-GHz flux therefore appears to be mostly due to dust radiation. Aperture photometry, at an angular resolution of 13 arcmin and with an aperture of diameter 30 arcmin, allowed the use of IRAS maps and the Wilkinson Microwave Anisotropy Probe 5-yr W-band map at 93.5 GHz. A single modified blackbody model was fitted to the data to estimate the contribution from thermal dust, which amounts to~10 per cent at 31 GHz. In this model, an excess of 1.52 ± 0.66 Jy (2.3σ) is seen at 31 GHz. Correlations with the IRAS 100 μm gave a coupling coefficient of 18.1 ± 4.4 μK (MJy sr-1)-1, consistent with the values found for LDN1622. © 2010 The Authors. Journal compilation. © 2010 RAS.

Abstract:

In studies of accreting black holes in binary systems, empirical relations have been proposed to quantify the coupling between accretion processes and ejection mechanisms. These processes are probed, respectively, by means of X-ray and radio/optical-infrared observations. The relations predict, given certain accretion conditions, the expected energy output in the form of a jet. We investigated this coupling by studying the black hole candidate Swift J. 1753.5-0127, via multiwavelength-coordinated observations over a period of ∼4 yr. We present the results of our campaign showing that, all along the outburst, the source features a jet that is fainter than expected from the empirical correlation between the radio and the X-ray luminosities in a hard spectral state. Because the jet is so weak in this system the near-infrared emission is, unusually for this state and luminosity, dominated by thermal emission from the accretion disc. We briefly discuss the importance and the implications of a precise determination of both the slope and the normalization of the correlations, listing some possible parameters that broad-band jet models should take into account to explain the population of sources characterized by a dim jet. We also investigate whether our data can give any hint on the nature of the compact object in the system, since its mass has not been dynamically measured. © 2010 The Authors. Journal compilation © 2010 RAS.

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.