The long-wavelength view of GG Tau A: rocks in the ring world

We present the first detection of GG Tau A at centimetre wavelengths, made with the Arcminute Microkelvin Imager Large Array at a frequency of 16 GHz (λ = 1.8 cm). The source is detected at >6 σrms with an integrated flux density of S16GHz = 249 ± 45 µJy. We use these new centimetre-wave data, in conjunction with additional measurements compiled from the literature, to investigate the long-wavelength tail of the dust emission from this unusual protoplanetary system. We use an MCMC-based method to determine maximum likelihood parameters for a simple parametric spectral model and consider the opacity and mass of the dust contributing to the microwave emission. We derive a dust mass of Md ~ 0.1 Msun, constrain the dimensions of the emitting region and find that the opacity index at λ > 7 mm is less than unity, implying a contribution to the dust population from grains exceeding ~4 cm in size. We suggest that this indicates coagulation within the GG Tau A system has proceeded to the point where dust grains have grown to the size of small rocks with dimensions of a few centimetres. Considering the relatively young age of the GG Tau association in combination with the low derived disc mass, we suggest that this system may provide a useful test case for rapid core accretion planet formation models

Multiscale Analysis of the Gradient of Linear Polarisation

We propose a new multiscale method to calculate the amplitude of the gradient of the linear polarisation vector using a wavelet-based formalism. We demonstrate this method using a field of the Canadian Galactic Plane Survey (CGPS) and show that the filamentary structure typically seen in gradients of linear polarisation maps depends strongly on the instrumental resolution. Our analysis reveals that different networks of filaments are present on different angular scales. The wavelet formalism allows us to calculate the power spectrum of the fluctuations seen in gradients of linear polarisation maps and to determine the scaling behaviour of this quantity. The power spectrum is found to follow a power law with gamma ~ 2.1. We identify a small drop in power between scales of 80 < l < 300 arcmin, which corresponds well to the overlap in the u-v plane between the Effelsberg 100-m telescope and the DRAO 26-m telescope data. We suggest that this drop is due to undersampling present in the 26-m telescope data. In addition, the wavelet coefficient distributions show higher skewness on smaller scales than at larger scales. The spatial distribution of the outliers in the tails of these distributions creates a coherent subset of filaments correlated across multiple scales, which trace the sharpest changes in the polarisation vector P within the field. We suggest that these structures may be associated with highly compressive shocks in the medium. The power spectrum of the field excluding these outliers shows a steeper power law with gamma ~ 2.5.Comment: 12 pages, 12 figure

Transfer learning for radio galaxy classification

In the context of radio galaxy classification, most state-of-the-art neural network algorithms have been focused on single survey data. The question of whether these trained algorithms have cross-survey identification ability or can be adapted to develop classification networks for future surveys is still unclear. One possible solution to address this issue is transfer learning, which re-uses elements of existing machine learning models for different applications. Here we present radio galaxy classification based on a 13-layer Deep Convolutional Neural Network (DCNN) using transfer learning methods between different radio surveys. We find that our machine learning models trained from a random initialization achieve accuracies comparable to those found elsewhere in the literature. When using transfer learning methods, we find that inheriting model weights pre-trained on FIRST images can boost model performance when re-training on lower resolution NVSS data, but that inheriting pre-trained model weights from NVSS and re-training on FIRST data impairs the performance of the classifier. We consider the implication of these results in the context of future radio surveys planned for next-generation radio telescopes such as ASKAP, MeerKAT, and SKA1-MID

Testing protoplanetary disc dispersal with radio emission

We consider continuum free-free radio emission from the upper atmosphere of protoplanetary discs as a probe of the ionized luminosity impinging upon the disc. Making use of previously computed hydrodynamic models of disc photoevaporation within the framework of EUV and X-ray irradiation, we use radiative transfer post-processing techniques to predict the expected free-free emission from protoplanetary discs. In general, the free-free luminosity scales roughly linearly with ionizing luminosity in both EUV and X-ray driven scenarios, where the emission dominates over the dust tail of the disc and is partial optically thin at cm wavelengths. We perform a test observation of GM Aur at 14-18 Ghz and detect an excess of radio emission above the dust tail to a very high level of confidence. The observed flux density and spectral index are consistent with free-free emission from the ionized disc in either the EUV or X-ray driven scenario. Finally, we suggest a possible route to testing the EUV and X-ray driven dispersal model of protoplanetary discs, by combining observed free-free flux densities with measurements of mass-accretion rates. On the point of disc dispersal one would expect to find a M_dot^2 scaling with free-free flux in the case of EUV driven disc dispersal or a M_dot scaling in the case of X-ray driven disc dispersal.Comment: Accepted MNRAS, 12 pages, 11 figures, (pdf generation fixed

Limits on the validity of the thin-layer model of the ionosphere for radio interferometric calibration

For a ground-based radio interferometer observing at low frequencies, the ionosphere causes propagation delays and refraction of cosmic radio waves which result in phase errors in the received signal. These phase errors can be corrected using a calibration method that assumes a two-dimensional phase screen at a fixed altitude above the surface of the Earth, known as the thin-layer model. Here we investigate the validity of the thin-layer model and provide a simple equation with which users can check when this approximation can be applied to observations for varying time of day, zenith angle, interferometer latitude, baseline length, ionospheric electron content and observing frequency.Comment: 8 pages, 10 figures, accepted MNRA

Diffuse radio emission in MACS J0025.4−-1222: the effect of a major merger on bulk separation of ICM components

Mergers of galaxy clusters are among the most energetic events in the Universe. These events have significant impact on the intra-cluster medium, depositing vast amounts of energy - often in the form of shocks - as well as heavily influencing the properties of the constituent galaxy population. Many clusters have been shown to host large-scale diffuse radio emission, known variously as radio haloes and relics. These sources arise as a result of electron (re-)acceleration in cluster-scale magnetic fields, although the processes by which this occurs are still poorly understood. We present new, deep radio observations of the high-redshift galaxy cluster MACS J0025.4$-$1222, taken with the GMRT at 325 MHz, as well as new analysis of all archival $Chandra$ X-ray observations. We aim to investigate the potential of diffuse radio emission and categorise the radio population of this cluster, which has only been covered previously by shallow radio surveys. We produce low-resolution maps of MACS J0025.4$-$1222 through a combination of uv-tapering and subtracting the compact source population. Radial surface brightness and mass profiles are derived from the $Chandra$ data. We also derive a 2D map of the ICM temperature. For the first time, two sources of diffuse radio emission are detected in MACS J0025.4$-$1222, on linear scales of several hundred kpc. Given the redshift of the cluster and the assumed cosmology, these sources appear to be consistent with established trends in power scaling relations for radio relics. The X-ray temperature map presents evidence of an asymmetric temperature profile and tentative identification of a temperature jump associated with one relic. We classify the pair of diffuse radio sources in this cluster as a pair of radio relics, given their consistency with scaling relations, location toward the cluster outskirts, and the available X-ray data.Comment: 20 pages, 15 figures, accepted for publication in A&

Early Science with the Karoo Array Telescope: a Mini-Halo Candidate in Galaxy Cluster Abell 3667

Abell 3667 is among the most well-studied galaxy clusters in the Southern Hemisphere. It is known to host two giant radio relics and a head-tail radio galaxy as the brightest cluster galaxy. Recent work has suggested the additional presence of a bridge of diffuse synchrotron emission connecting the North-Western radio relic with the cluster centre. In this work, we present full-polarization observations of Abell 3667 conducted with the Karoo Array Telescope at 1.33 and 1.82 GHz. Our results show both radio relics as well as the brightest cluster galaxy. We use ancillary higher-resolution data to subtract the emission from this galaxy, revealing a localised excess, which we tentatively identify as a radio mini-halo. This mini-halo candidate has an integrated flux density of $67.2\pm4.9$ mJy beam$^{-1}$ at 1.37 GHz, corresponding to a radio power of P$_{\rm{1.4\,GHz}}=4.28\pm0.31\times10^{23}$ W Hz$^{-1}$, consistent with established trends in mini-halo power scaling.Comment: 17 pages, 10 figures, accepted MNRA