Hintze Centre for Astrophysical Surveys

Abstract:

We study resolution effects in numerical simulations of gas-rich (20% of the total baryonic mass) major mergers, and show that the formation of slowly-rotating elliptical galaxies requires a resolution that is beyond the present-day standards to be properly modelled. Our findings show that a high-enough resolution is required to accurately model the global properties of merger remnants and the evolution of their angular momentum. The role of wet mergers of spiral galaxies in the formation of slow-rotating ellipticals may therefore have been underestimated. © 2010 American Institute of Physics.

Abstract:

We describe the reduction of data taken with the PACS instrument on board the Herschel Space Observatory in the Science Demonstration Phase of the Herschel-ATLAS (H-ATLAS) survey, specifically data obtained for a 4 × 4 deg2 region using Herschel's fast-scan (60 arcsec s-1) parallel mode. We describe in detail a pipeline for data reduction using customized procedures within hipe from data retrieval to the production of science-quality images. We found that the standard procedure for removing cosmic ray glitches also removed parts of bright sources and so implemented an effective two-stage process to minimize these problems. The pronounced 1/f noise is removed from the timelines using 3.4- and 2.5-arcmin boxcar high-pass filters at 100 and 160 μm. Empirical measurements of the point spread function (PSF) are used to determine the encircled energy fraction as a function of aperture size. For the 100- and 160-μm bands, the effective PSFs are ~9 and ~13 arcsec (FWHM), and the 90-per cent encircled energy radii are 13 and 18 arcsec. Astrometric accuracy is good to ≤2 arcsec. The noise in the final maps is correlated between neighbouring pixels and rather higher than advertised prior to launch. For a pair of cross-scans, the 5σ point-source sensitivities are 125-165 mJy for 9-13 arcsec radius apertures at 100 μm and 150-240 mJy for 13-18 arcsec radius apertures at 160 μm. © 2010 The Authors. Journal compilation © 2010 RAS.

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.