Prof. Matt Jarvis

Abstract:

We describe a 325-MHz survey, undertaken with the Giant Metrewave Radio Telescope (GMRT), which covers a large part of the three equatorial fields at 9, 12 and 14.5 h of right ascension from the Herschel-Astrophysical Terahertz Large Area Survey (H-ATLAS) in the area also covered by the Galaxy And Mass Assembly survey (GAMA). The full dataset, after some observed pointings were removed during the data reduction process, comprises 212 GMRT pointings covering ~90 deg^2 of sky. We have imaged and catalogued the data using a pipeline that automates the process of flagging, calibration, self-calibration and source detection for each of the survey pointings. The resulting images have resolutions of between 14 and 24 arcsec and minimum rms noise (away from bright sources) of ~1 mJy/beam, and the catalogue contains 5263 sources brighter than 5 sigma. We investigate the spectral indices of GMRT sources which are also detected at 1.4 GHz and find them to agree broadly with previously published results; there is no evidence for any flattening of the radio spectral index below S_1.4=10 mJy. This work adds to the large amount of available optical and infrared data in the H-ATLAS equatorial fields and will facilitate further study of the low-frequency radio properties of star formation and AGN activity in galaxies out to z~1.

Abstract:

We compare the environmental and star formation properties of far-infrared detected and non-far-infrared detected galaxies out to z ~ 0.5. Using optical spectroscopy and photometryfrom the Galaxy And Mass Assembly (GAMA) and Sloan Digital Sky Survey, with farin frared observations from the Herschel Astrophysical Terahertz Large Area Survey (ATLAS)Science Demonstration Phase, we apply the technique of Voronoi tessellations to analyse the environmental densities of individual galaxies. Applying statistical analyses to colour, r-band magnitude and redshift-matched samples, we show that there is a significant differenceat the 3.5σ level between the normalized environmental densities of these two populations. This is such that infrared emission (a tracer of star formation activity) favours underden seregions compared to those inhabited by exclusively optically observed galaxies selected to beof the same r-band magnitude, colour and redshift. Thus, more highly star-forming galaxiesare found to reside in the most underdense environments, confirming previous studies thathave proposed such a correlation. However, the degeneracy between redshift and far-infraredluminosity in our flux-density-limited sample means that we are unable to make a strongerstatement in this respect. We then apply our method to synthetic light cones generated fromsemi-analytic models, finding that over the whole redshift distribution the same correlations between star formation rate and environmental density are found. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Abstract:

LOFAR, the Low Frequency Radio Array, is a new pan-European radio telescope that is almost fully operational. One of its main drivers is to make deep images of the low frequency radio sky. To be able to do this a number of challenges need to be addressed. These include the high data rates, removal of radio frequency interference, calibration of the beams and correcting for the corrupting influence of the ionosphere. One of the key science goals is to study merger shocks, particle acceleration mechanisms and the structure of magnetic fields in nearby and distant merging clusters. Recent studies with the GMRT and WSRT radio telescopes of the "Sausage" and the "Toothbrush" clusters have given a very good demonstration of the power of radio observations to study merging clusters. Recently we discovered that both clusters contain relic and halo sources, large diffuse regions of radio emission not associated with individual galaxies. The 2 Mpc northern relic in the Sausage cluster displays highly aligned magnetic fields and and exhibits a strong spectral index gradient that is a consequence of cooling of the synchrotron emitting particles in the post-shock region. We have argued that these observations provide strong evidence that shocks in merging clusters are capable of accelerating particles. For the Toothbrush cluster we observe a puzzling linear relic that extends over 2 Mpc. The proposed scenario is that a triplemerger can lead to such a structure. With LOFAR's sensitivity it will not only be possible to trace much weaker shocks, but also to study those shocks due to merging clusters up to redshifts of at least one. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract:

The Galaxy And Mass Assembly (GAMA) survey is a multiwavelength photometric and spectroscopic survey, using the AAOmega spectrograph on the Anglo-Australian Telescope to obtain spectra for up to ~300 000 galaxies over 280 deg2, to a limiting magnitude of rpet < 19.8 mag. The target galaxies are distributed over 0 < z ≲ 0.5 with a median redshift of z ≈ 0.2, although the redshift distribution includes a small number of systems, primarily quasars, at higher redshifts, up to and beyond z = 1. The redshift accuracy ranges from σv ≈ 50 km s-1 to σv ≈ 100 km s-1 depending on the signal-to-noise ratio of the spectrum. Here we describe the GAMA spectroscopic reduction and analysis pipeline. We present the steps involved in taking the raw two-dimensional spectroscopic images through to flux-calibrated one-dimensional spectra. The resulting GAMA spectra cover an observed wavelength range of 3750 λ 8850Å at a resolution of R ≈ 1300. The final flux calibration is typically accurate to 10-20 per cent, although the reliability is worse at the extreme wavelength ends, and poorer in the blue than the red. We present details of the measurement of emission and absorption features in the GAMA spectra. These measurements are characterized through a variety of quality control analyses detailing the robustness and reliability of the measurements. We illustrate the quality of the measurements with a brief exploration of elementary emission line properties of the galaxies in the GAMA sample. We demonstrate the luminosity dependence of the Balmer decrement, consistent with previously published results, and explore further how Balmer decrement varies with galaxy mass and redshift. We also investigate the mass and redshift dependencies of the [NII]/Hα versus [OIII]/Hβ spectral diagnostic diagram, commonly used to discriminate between star forming and nuclear activity in galaxies. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Abstract:

The Herschel-Astrophysical Terahertz Large Area Survey (H-ATLAS) provides an unprecedented opportunity to search for blazars at sub-mm wavelengths. We cross-matched the Faint Images of the Radio Sky at Twenty-cm (FIRST) radio source catalogue with the 11 655 sources brighter than 35 mJy at 500 μm in the ∼135 deg2 of the sky covered by the H-ATLAS equatorial fields at 9h and 15h, plus half of the field at 12h. We found that 379 of the H-ATLAS sources have a FIRST counterpart within 10 arcsec, including eight catalogued blazars (plus one known blazar that was found at the edge of one of the H-ATLAS maps). To search for additional blazar candidates we have devised new diagnostic diagrams and found that known blazars occupy a region of the log(S500μm/S350μm) versus log(S500μm/S1.4 GHz) plane separated from that of sub-mm sources with radio emission powered by star formation, but shared with radio galaxies and steep-spectrum radio quasars. Using this diagnostic we have selected 12 further possible candidates that turn out to be scattered in the (r-z) versus (u-r) plane or in the Wide-Field Infrared Survey Explorer colour-colour diagram, where known blazars are concentrated in well defined strips. This suggests that the majority of them are not blazars. Based on an inspection of all the available photometric data, including unpublished VISTA Kilo-degree Infrared Galaxy survey photometry and new radio observations, we found that the spectral energy distributions (SEDs) of only one out of the 12 newly selected sources are compatible with being synchrotron dominated at least up to 500 μm, i.e. with being a blazar. Another object may consist of a faint blazar nucleus inside a bright star-forming galaxy. The possibility that some blazar hosts are endowed with active star formation is supported by our analysis of the SEDs of Planck Early Release Compact Source Catalogue blazars detected at both 545 and 857 GHz. The estimated rest-frame synchrotron peak frequencies of H-ATLAS blazars are in the range 11.5 ≤ log (νpeak, Hz) ≤ 13.7, implying that these objects are low synchrotron peak. Six of them also show evidence of an ultraviolet excess that can be attributed to emission from the accretion disc. Allowing for the possibility of misidentifications and of contamination of the 500 μm flux density by the dusty torus or by the host galaxy, we estimate that there are seven or eight pure synchrotron sources brighter than S500μm = 35 mJy over the studied area, a result that sets important constraints on blazar evolutionary models. © 2013 The Author. Published by Oxford University Press on behalf of the Royal Astronomical Society.