Ionised gas outflows over the radio AGN life cycle

Feedback from AGN is known to affect the host galaxy's evolution. In radio AGN, one manifestation of feedback is seen in gas outflows. However, it is still not well understood whether the effect of feedback evolves with the radio AGN life cycle. In this study, we investigate this link using the radio spectral shape as a proxy for the evolutionary stage of the AGN. We used [OIII] emission line spectra to trace the presence of outflows on the ionised gas. Using a sample of uniformly selected 129 radio AGN with $L_\textrm{1.4GHz}\approx10^{23}-10^{26}$ W Hz$^{-1}$, and a mean stacking analysis of the [OIII] profile, we conclude that the ionised gas outflow is linked to the radio spectral shape, and it evolves with the evolution of the radio source. We find that sources with a peak in their radio spectra (optically thick), on average, drive a broad outflow ($FWHM\approx1330\pm418$ km s$^{-1}$) with a velocity $v_\textrm{out}\approx 240$ km s$^{-1}$. However, we detect no outflow in the stacked [OIII] profile of sources without a peak in their radio spectrum. In addition, we find that individual outflow detections are kinematically more extreme in peaked than non-peaked sources. We conclude that radio jets are most effective at driving gas outflows when young, and the outflow is typically short lived. Our stacking analysis shows no significant dependence of the presence of ionised gas outflows on the radio morphology, 1.4 GHz luminosity, optical luminosity and Eddington ratio of these sources. We also identify candidate restarted AGN in our sample, whose [OIII] profiles suggest that they have more disturbed gas kinematics than their evolved counterparts, although the evidence for this is tentative. Our findings support the picture where the impact of AGN feedback changes as the source evolves, and young radio jets interact with the ambient medium, clearing a channel of gas as they expand.Comment: 20 pages, 12 figures. Accepted for publication in Astronomy and Astrophysic

Seeing the forest and the trees: a radio investigation of the ULIRG Mrk 273

Galaxy mergers have been observed to trigger nuclear activity by feeding gas to the central supermassive black hole. One such class of objects are Ultra Luminous InfraRed Galaxies (ULIRGs), which are mostly late stage major mergers of gas-rich galaxies. Recently, large-scale (∼100 kpc) radio continuum emission has been detected in a select number of ULIRGs, all of which also harbour powerful Active Galactic Nuclei (AGN). This hints at the presence of large-scale radio emission being evidence for nuclear activity. Exploring the origin of this radio emission and its link to nuclear activity requires high sensitivity multi-frequency data. We present such an analysis of the ULIRG Mrk 273. Using the International LOFAR telescope (ILT), we detected spectacular large-scale arcs in this system. This detection includes, for the first time, a giant ∼190 kpc arc in the north. We propose these arcs are fuelled by a low power radio AGN triggered by the merger. We also identified a bright ∼45 kpc radio ridge, which is likely related to the ionised gas nebula in that region. We combined this with high sensitivity data from APERture Tile In Focus (Apertif) and archival data from the Very Large Array (VLA) to explore the spectral properties. The ILT simultaneously allowed us to probe the nucleus at a resolution of ∼0.3″, where we detected three components, and, for the first time, diffuse emission around these components. Combining this with archival high frequency VLA images of the nucleus allowed us to detect absorption in one component, and a steep spectrum radio AGN in another. We then extrapolate from this case study to the importance of investigating the presence of radio emission in more ULIRGs and what it can tell us about the link between mergers and the presence of radio activity

From birth to maturity: tracing the evolution and impact of radio galaxies

We now understand that a supermassive black hole lurks at the centre of nearly every galaxy in the Universe. When the gas surrounding these supermassive black holes starts swirling around them, heating up to very high temperatures before eventually falling in, it leads to a phenomenon called active galactic nuclei (AGN). This is one of the most energetic phenomena in the Universe. In some cases, jets of fast-moving plasma particles are also produced, which are brightest at radio frequencies. These sources are called radio galaxies and are the focus of this thesis. In this thesis, we have studied the lives of radio galaxies and how they interact with the gas around them. We have combined data at radio frequencies from different observatories around the world to understand the evolution of radio galaxies. We investigated the presence of restarted activity in radio galaxies, that is, galaxies where a new phase of activity has started before the older phase has completely faded away. We have also identified a radio galaxy in a merger of galaxies, allowing us to understand how mergers trigger nuclear activity. This has provided new insight into the life cycle of radio galaxies. We also tracked how the impact of radio galaxies on the gas that surrounds the black hole, changed as the AGN evolved from younger to older phases. Our studies have suggested that radio galaxies have the strongest impact on the surrounding gas when they are young, and as they evolve, this impact is reduced

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

Active galactic nuclei show episodic activity, which can be evident in galaxies that exhibit restarted radio jets. These restarted jets can interact with their environment, leaving signatures on the radio spectral energy distribution. Tracing these signatures is a powerful way to explore the life of radio galaxies. This requires resolved spectral index measurements over a broad frequency range including low frequencies. We present such a study for the radio galaxy 3C 293, which has long been thought to be a restarted galaxy on the basis of its radio morphology. Using the International LOFAR telescope (ILT) we probed spatial scales as fine as ∼0.2″ at 144 MHz, and to constrain the spectrum we combined these data with Multi-Element Radio Linked Interferometer Network and Very Large Array archival data at frequencies up to 8.4 GHz that have a comparable resolution. In the inner lobes (∼2 kpc), we detect the presence of a spectral turnover that peaks at ∼225 MHz and is most likely caused by free-free absorption from the rich surrounding medium. We confirm that these inner lobes are part of a jet-dominated young radio source (spectral age ≤ 0.17 Myr), which is strongly interacting with the rich interstellar medium of the host galaxy. The diffuse emission surrounding these lobes on scales of up to ∼4.5 kpc shows steeper spectral indices (Δα ∼ 0.2-0.5, S ∝ ν-α) and a spectral age of ≤ 0.27 Myr. The outer lobes (extending up to ∼100 kpc) have a spectral index of α ∼ 0.6-0.8 from 144-4850 MHz with a remarkably uniform spatial distribution and only mild spectral curvature (Δα ≤ 0.2). We propose that intermittent fuelling and jet flow disruptions are powering the mechanisms that keep the spectral index in the outer lobes from steepening and maintain the spatial uniformity of the spectral index. Overall, it appears that 3C 293 has gone through multiple (two to three) epochs of activity. This study adds 3C 293 to the new sub-group of restarted galaxies with short interruption time periods. This is the first time a spatially resolved study has been performed that simultaneously studies a young source as well as the older outer lobes at such low frequencies. This illustrates the potential of the International LOFAR telescope to expand such studies to a larger sample of radio galaxies

3C293 high and low resolution maps

VizieR online Data Catalogue associated with article published in journal Astronomy & Astrophysics with title 'Unmasking the history of 3C 293 with LOFAR sub-arcsecond imaging.' (bibcode: 2022A&A...658A...6K

Mrk273 high and low resolution maps

VizieR online Data Catalogue associated with article published in journal Astronomy & Astrophysics with title 'Seeing the forest and the trees: A radio investigation of the ULIRG Mrk 273.' (bibcode: 2022A&A...664A..25K

LOFAR Two-metre Sky Survey (LoTSS) DR2

VizieR online Data Catalogue associated with article published in journal Astronomy & Astrophysics with title 'The LOFAR Two-metre Sky Survey (LoTSS). V. Second data release.' (bibcode: 2022A&A...659A...1S