Prying eyes on radio skies: pushing the high-resolution low-frequency frontier with LOFAR

In this thesis we push the boundaries of high-resolution imaging at radio frequencies between 100 and 200 MHz. Using the International LOFAR Telescope that spans across Europe we achieve sub-arcsecond angular resolution. We demonstrate mapping of the full field of view of the telescope at its native resolution and use the resulting images to study active galactic nuclei in the Lockman Hole field.Large scale structure and cosmolog

Nuclear regions as seen with LOFAR international baselines: A high-resolution study of the recurrent activity

Context. Radio galaxies dominate the sky at radio wavelengths and represent an essential piece in the galaxy evolution puzzle. High-resolution studies focussed on statistical samples of radio galaxies are expected to shed light on the triggering mechanisms of the active galactic nucleus in their centre, alternating between the phases of activity and quiescence.Aims. For this work, we zoomed in on the sub-arcsec radio structures in the central regions of the 35 radio galaxies in the area covering 6.6 deg2 of the Lockman Hole region. The sources studied here were previously classified as active, remnant, and candidate restarted radio galaxies based on the LOw Frequency ARray (LOFAR) observations at 150 MHz. We examined the morphologies and studied the spectral properties of their central regions to explore their evolutionary stages and to revise the morphological and spectral criteria used to select the initial sample.Methods. We used the newly available LOFAR 150 MHz image obtained using international baselines, yielding a resolution of 0.38″ × 0.30″, making this the first systematic study of the nuclear regions at such a high resolution and low frequency. We used publicly available images from the Faint Images of the Radio Sky at Twenty-cm survey at 1.4 GHz and the Karl G. Jansky Very Large Array (VLA) Sky Survey at 3 GHz to achieve our goals. In addition, for one of the restarted candidates, we present new dedicated observations with the VLA at 3 GHz.Results. We characterised the central regions of the radio galaxies in our sample and found various morphologies, some even mimicking well-known double-double radio galaxies but on a smaller scale, that is, a few tens of kiloparsecs for the size of the restarted activity. We also see the beginnings of active jets or distinct detections unrelated to the large-scale structure. Furthermore, we found a variety of radio spectra characterising the sources in our sample, such as flat, steep, or peaked in the frequency range between 150 MHz and 3 GHz, indicative of the different life-cycle phases of the sources in our sample. Based on these analyses, we confirm five out of six previously considered restarted candidates and identify three more restarted candidates from the active sample. As the number of restarted candidates still exceeds that of remnant candidates, this is consistent with previous results suggesting that the restarted phase can occur after a relatively short remnant phase (i.e. a few tens of millions of years)

Measuring cavity powers of active galactic nuclei in clusters using a hybrid X-ray–radio method. A new window on feedback opened by subarcsecond LOFAR-VLBI observations

Measurements of the quantity of radio-mode feedback injected by an active galactic nucleus into the cluster environment have mostly relied on X-ray observations, which reveal cavities in the intracluster medium excavated by the radio lobes. However, the sensitivity required to accurately constrain the dimensions of these cavities has proven to be a major limiting factor and is the main bottleneck on high-redshift measurements. We describe a hybrid method based on a combination of X-ray and radio observations, which aims to enhance our ability to study radio-mode feedback. In this paper, we present one of the first samples of galaxy clusters observed with the International LOFAR Telescope (ILT) at 144 MHz and use this sample to test the hybrid method at lower frequencies than before. By comparing our measurements with results found in literature based on the traditional method using only X-ray observations, we find that the hybrid method provides consistent results to the traditional method. In addition, we find that the correlation between the traditional method and the hybrid method improves as the X-ray cavities are more clearly defined. This suggests that using radio lobes as proxies for cavities may help to circumvent systematic uncertainties in the cavity volume measurements. Encouraged by the high volume of unique ILT observations successfully processed, this hybrid method enables radio-mode feedback to be studied at high redshifts for the first time even for large samples of clusters

Spectral analysis of spatially resolved 3C295 (sub-arcsecond resolution) with the International LOFAR Telescope

3C295 is a bright, compact steep spectrum source with a well-studied integrated radio spectral energy distribution (SED) from 132 MHz to 15 GHz. However, spatially resolved spectral studies have been limited due to a lack of high resolution images at low radio frequencies. These frequencies are crucial for measuring absorption processes, and anchoring the overall spectral modelling of the radio SED. In this paper, we use International LOw-Frequency ARray (LOFAR) Telescope (ILT) observations of 3C295 to study its spatially resolved spectral properties with sub-arcsecond resolution at 132 MHz. Combining our new 132 MHz observation with archival data at 1.6, 4.8, and 15 GHz, we are able to carry out a resolved radio spectral analysis. The spectral properties of the hotspots provides evidence for low frequency flattening. In contrast, the spectral shape across the lobes is consistent with a Jaffe-Perola spectral ageing model. Using the integrated spectral information for each component, we then fitted low-frequency absorption models to the hotspots, finding that both free-free absorption and synchrotron self-absorption models provide a better fit to the data than a standard power law. Although we can say there is low-frequency absorption present in the hotspots of 3C295, future observations with the Low Band Antenna of the ILT at 55 MHz may allow us to distinguish the type of absorption

Measuring cavity powers of active galactic nuclei in clusters using a hybrid X-ray-radio method.: A new window on feedback opened by subarcsecond LOFAR-VLBI observations

Large scale structure and cosmologyGalaxie

The resolved jet of 3C 273 at 150 MHz: sub-arcsecond imaging with the LOFAR international baselines

Galaxie

High-resolution international LOFAR observations of 4C 43.15: spectral ages and injection indices in a high-z radio galaxy

Galaxie

Unmasking the history of 3C 293 with LOFAR sub-arcsecond imaging

Galaxie

Deep sub-arcsecond wide-field imaging of the Lockman Hole field at 144 MHz

Instrumentatio