We present NOEMA CO(2–1) observations of a nearby, young, low-luminosity radio source, B2 0258+35. Our earlier CO(1–0) study had shown the presence of strong jet-ISM interaction and a massive molecular gas outflow involving 75% of the circumnuclear gas. Our follow-up CO(2–1) observations have revealed even more complex gas kinematics, where the southern radio jet is driving out molecular gas in the form of a swiftly expanding bubble, with velocities up to almost 400 km s−1. We found highly elevated CO(2–1)/CO(1–0) line ratios for the gas belonging to the bubble and also further away from the radio jets. Previous observations have shown that the active galactic nucleus (AGN) in the host galaxy, NGC 1167, is in a very low-accretion state. Thus, we attribute the high line ratios to the high gas excitation caused by the jet–ISM interaction. The radio jets, despite exhibiting a relatively low luminosity (1.3 × 1044 erg s−1), are solely responsible for the observed extreme gas kinematics. This is one of the clearest detections of an expanding cold gas bubble in such a type of source, showing that the jets are affecting both the kinematics and physicals conditions of the gas. Our study adds to the growing store of evidence that low-luminosity radio sources can also affect the kinematics and physical conditions of the cold gas, which fuels star formation, in their host galaxies to a significant extent. Hence, such sources should be considered in models seeking to quantify feedback from radio AGN.

Gas accretion by a galaxy’s central supermassive black hole (SMBH) and the resultant energetic feedback by the accreting active galactic nucleus (AGN) on the gas in and around a galaxy are two tightly intertwined but competing processes that play a crucial role in the evolution of galaxies. Observations of galaxy clusters have shown how the plasma jets emitted by an AGN heat the intracluster medium, preventing cooling of the cluster gas and thereby the infall of this gas onto the central galaxy. On the other hand, outflows of multiphase gas, driven by the jets, can cool as they rise into the intracluster medium, leading to filaments of colder gas. The fate of this cold gas is unclear, but it has been suggested that it plays a role in feeding the central SMBH. We present the results of reprocessed CO(2-1) Atacama Large Millimeter/submillimeter Array observations of the cold molecular gas in the central regions of NGC 1275, the central galaxy of the Perseus cluster and which hosts the radio-loud AGN 3C 84 (Perseus A). These data show in detail how kiloparsec-sized cold gas filaments resulting from the jet-induced cooling of cluster gas are flowing towards the galaxy centre and how they feed the circum-nuclear accretion disk (100 pc diameter) of the SMBH. Thus, cooled gas can, in this way, play a role in feeding the AGN. These results complete our view of the feedback loop of how an AGN can impact its surroundings and how the effects of this impact maintain the AGN activity. ALMA observations show the streams of molecular gas blown from the centre of a galaxy by the energy released by an active supermassive black hole are falling back onto the black hole, making sure it stays active.

We present deep kiloparsec- and parsec-scale neutral atomic hydrogen (H I) absorption observations of a very young radio source (<= 5000 years), 4C 31.04, using the Westerbork Synthesis Radio Telescope (WSRT) and the Global Very Long Baseline Interferometry (VLBI) array. Using z = 0.0598, derived from molecular gas observations, we detect, at both kpc and pc scales, a broad absorption feature (FWZI = 360 km s(-1)) centred at the systemic velocity, and narrow absorption (FWZI = 6.6 km s(-1)) redshifted by 220 km s(-1), both previously observed. Additionally, we detect a new blueshifted, broad, shallow absorption wing. At pc scales, the broad absorption at the systemic velocity is detected across the entire radio source while the shallow wing is only seen against part of the eastern lobe. The gas has higher H I column density along the eastern lobe than along the western one. The velocity dispersion of the gas is high (>= 40 km s(-1)) along the entire radio continuum, and is highest (>= 60 km s(-1)) in the region including the outflow and the radio hot spot. While we detect a velocity gradient along the western lobe and parts of the eastern lobe at PA similar to 5 degrees -10 degrees, most of the gas along the rest of the eastern lobe exhibits no signs of rotation. Earlier optical spectroscopy suggests that the optical AGN is very weak. We therefore conclude that the radio lobes of 4C 31.04 are expanding into a circumnuclear disc, partially disrupting it and making the gas highly turbulent. The distribution of gas is predominantly smooth at the spatial resolution of similar to 4 pc studied here. However, clumps of gas are also present, particularly along the eastern lobe. This lobe appears to be strongly interacting with the clouds and driving an outflow similar to 35 pc from the radio core, with a mass-outflow rate of 0.3 <= M(over dot) <= 1.4 M-circle dot year(-1). It is likely that this interaction has caused the eastern lobe to be rebrightened, giving the source an asymmetric morphology. We compare our observations with the predictions of a recent analytical model regarding the survival of atomic gas clouds in radio-jet-driven outflows and find that the existence of a subkpc-scale outflow in this case could imply inefficient mixing of the cold gas with the hot medium and high gas density, leading to very short cooling times. Overall, our study provides further evidence of the strong impact of radio jets on the cold interstellar medium (ISM) in the early stages of their evolution and supports the predictions of numerical simulations regarding jet-ISM interactions and the nature of the circumnuclear gas into which the jets expand.

We present an overview of the results obtained from the study of the resolved distribution of molecular gas around eight young (≲106yr), peaked-spectrum radio galaxies. Tracing the distribution and kinematics of the gas around these radio sources allows us to trace the interplay between the jets and the surrounding medium. For three of these sources, we present new CO(1-0) observations, obtained with the Northern Extended Millimeter Array (NOEMA) with arcsecond resolution. In two of these targets, we also detected CN lines, both in emission and absorption. Combining the new observations with already published data, we discuss the main results obtained. Although we found that a large fraction of the cold molecular gas was distributed in disc-like rotating structures, in the vast majority of the sources, high turbulence and deviations from purely quiescent gas (including outflows) were observed in the region co-spatial with the radio continuum emission. This suggests the presence of an interaction between radio plasma and cold molecular gas. In particular, we found that newly born and young radio jets, even those with low power i.e., Pjet < 1045 erg s−1), are able to drive massive outflows of cold, molecular gas. The outflows are, however, limited to the sub-kpc regions and likely short lived. On larger scales (a few kpc), we observed cases where the molecular gas appears to avoid the radio lobes and, instead, wraps around them. The results suggest the presence of an evolutionary sequence, which is consistent with previous simulations, where the type of impact of the radio plasma changes as the jet expands, going from a direct jet-cloud interaction able to drive gas outflows on sub-kpc scales to a more gentle pushing aside of the gas, increasing its turbulence and likely limiting its cooling on kpc scales. This effect can be mediated by the cocoon of shocked gas inflated by the jet–cloud interactions. Building larger samples of young and evolved radio sources for observation at a similar depth and spatial resolution to test this scenario is now needed and may be possible thanks to more data becoming available in the growing public archives.

We present new JVLA and VLBA observations tracing the HI in the central region of 3C84 (Perseus A). This radio source is hosted by the bright cluster galaxy NGC 1275 in the centre of the iconic Perseus cluster. With the JVLA, we detected broad (FWHM~500 km/s) HI absorption at arcsecond resolution (~300 pc) centred at the systemic velocity of NGC 1275 against the bright radio continuum, suggesting that the detected gas is very close to the supermassive black hole (SMBH). However, we did not detect any absorption in the higher-resolution VLBA data against the parsec-scale radio core and jet. Based on a comparison of the properties of the HI absorption with those of the molecular circum-nuclear disc (CND) known to be present in NGC 1275, we argue that the HI seen in absorption arises from HI in this fast-rotating CND, and that neutral atomic hydrogen is present as close as ~20 pc from the SMBH. The radio continuum providing the background for absorption arises from non-thermal synchrotron emission from the star formation activity in the CND, whose presence has been reported by earlier VLBA studies. We did not detect any signature that the HI gas is affected by an interaction with the radio jet. Thus, at this stage of the evolution of the source, the impact of the radio jet on the gas in NGC 1275 mainly creates cavities on much larger galaxy scales. Overall, the properties of the CND in Perseus A present strong similarities with Mrk 231, suggesting that, unlike often assumed, HI absorption can arise against the radio emission from star formation in a CND. With the JVLA, we serendipitously detected a new, faint absorbing system that is redshifted by ~2660 km/s, in addition to the already known high-velocity absorption system that is redshifted 2850 km/s with respect to NGC 1275. We identify this new system as gas that is stripped from a foreground galaxy falling into the Perseus cluster.

The energy emitted by active galactic nuclei (AGNs) may provide a self-regulating process (AGN feedback) that shapes the evolution of galaxies. This is believed to operate along two modes, namely on galactic scales by clearing the interstellar medium via outflows, and on circumgalactic scales by preventing the cooling and accretion of gas onto the host galaxy. Radio jets associated with radiatively inefficient AGNs are known to contribute to the latter mode of feedback. However, such jets could also play a role on circumnuclear and galactic scales, blurring the distinction between the two modes. We have discovered a spatially resolved, massive molecular outflow, carrying ~75% of the gas in the central region of the host galaxy of a radiatively inefficient AGN. The outflow coincides with the radio jet 540 pc offset from the core, unambiguously pointing to the jet as the driver of this phenomenon. The modest luminosity of the radio source ( L 1.4 GHz = 2.1 × 10 23 W Hz −1 ) confirms predictions of simulations that jets of low-luminosity radio sources carry enough power to drive such outflows. Including kiloparsec-scale feedback from such sources, which comprise the majority of the radio AGN population, in cosmological simulations may assist in resolving some of their limitations.

The supermassive blackholes inhabiting the centres of massive galaxies become active galactic nuclei (AGN) when the ambient matter gets accreted onto them. This results in the emission of enormous amount of energy which influences the evolution of their host galaxies. In radio-loud AGN, the energy is emitted in the form of collimated jets. These jets and the ambient interstellar medium (ISM) interact and affect each other: while the radio jets make the ISM turbulent and drive fast outflows, a dense ISM slows the growth of these radio jets. Understanding the nature of this jet-ISM interaction and its evolution with redshift is crucial to understand the role of radio AGN in galaxy evolution.In this thesis, we carried out a detailed study of the interaction between radio jets and the cold ISM: atomic (HI) and molecular, in a low-luminosity radio galaxy using the Very Large Array (VLA), European VLBI Network (EVN) and NOrthern Extended Millimeter Array (NOEMA). This resulted in the first discovery of a massive molecular-gas outflow in this AGN, driven single-handedly by radio jets. This showed that kpc-scale jet-ISM interaction in such sources is important for models of galaxy evolution. We further searched for HI in samples of radio AGN at higher redshifts and suggested that the incidence and kinematics of HI in radio AGN does not evolve up to z ~ 0.4 and provided the first evidence for the evolution of cold-gas content in radio AGN environments at z>0.7.

We report the results of joint Chandra/ACIS - NuSTAR deep observations of NGC 1167, the host galaxy of the young radio jet B2 0258+35. In the ACIS data we detect X-ray emission, extended both along and orthogonal to the jet. At the end of the SE radio jet, we find lower-energy X-ray emission that coincides with a region of CO turbulence and fast outflow motions. This suggests that the hot Interstellar Medium (ISM) may be compressed by the jet and molecular outflow, resulting in more efficient cooling. Hydrodynamic simulations of jet-ISM interaction tailored to NGC 1167 are in agreement with this conclusion and with the overall morphology and spectra of the X-ray emission. The faint hard nuclear source detected with Chandra and the stringent NuSTAR upper limits on the harder X-ray emission show that the active galactic nucleus (AGN) in NGC 1167 is in a very low-accretion state. However, the characteristics of the extended X-ray emission are more consonant to those of luminous Compton Thick AGNs, suggesting that we may be observing the remnants of a past high accretion rate episode, with sustained strong activity lasting 2 x 103 yr. We conclude that NGC1167 is presently a LINER, but was an AGN in the past, given the properties of the extended X-ray emission and their similarity with those of CT AGN extended emission.

We present a search for associated H I 21 cm absorption in a sample of 29 radio-loud active galactic nuclei (AGNs) at 0.7 < z < 1, carried out with the upgraded Giant Metrewave Radio Telescope. We detect H I 21 cm absorption against none of our target AGNs, obtaining 3σ upper limits to the optical depth of ≲1% per 50 km s−1 channel. The radio luminosity of our sources is lower than that of most AGNs searched for H I 21 cm absorption at similar redshifts in the literature, and, for all targets except two, the UV luminosity is below the threshold 1023 W Hz−1, above which the H I in the AGN environment has been suggested to be completely ionised. We stacked the H I 21 cm spectra to obtain a more stringent limit of ≈0.17% per 50 km s−1 channel on the average H I 21 cm optical depth of the sample. The sample is dominated by extended radio sources, 24 of which are extended on scales of tens of kiloparsecs. Including similar extended sources at 0.7 < z < 1.0 from the literature, and comparing with a low-z sample of extended radio sources, we find statistically significant (≈3σ) evidence that the strength of H I 21 cm absorption towards extended radio sources is weaker at 0.7 < z < 1.0 than at z < 0.25, with a lower detection rate of H I 21 cm absorption at 0.7 < z < 1.0. Redshift evolution in the physical conditions of H I is the likely cause of the weaker associated H I 21 cm absorption at high redshifts, due to either a low H I column density or a high spin temperature in high-z AGN environments.

Ranging from a few pc to hundreds of kpc in size, radio jets have, during their evolution, an impact on their gaseous environment on a large range of scales. While their effect on larger scales is well established, it is now becoming clear that they can also strongly affect the interstellar medium (ISM) inside the host galaxy. Particularly important is the initial phase ($<10^6$ yr) of the evolution of the radio jet, when they expand into the inner few kpc of the host galaxy. Here we report on results obtained for a representative group of young radio galaxies using the cold molecular gas as a tracer of jet-ISM interactions. The sensitivity and high spatial resolution of ALMA and NOEMA are ideal to study the details of this process. In many objects we find massive molecular outflows driven by the plasma jet, even in low-power radio sources. However, the observed outflows are limited to the circumnuclear regions and only a small fraction of the ISM is leaving the galaxy. Beyond this region, the impact of the jet seems to change. Fast outflows are replaced by a milder expansion driven by the expanding cocoon created by the jet-ISM interaction, resulting in dispersing and heating the ISM. These findings are in line with predictions from simulations of jets interacting with a clumpy medium and suggest a more complex view of the impact of AGN than presently implemented in cosmological simulations.

We present an H I 21 cm absorption study of a sample of 26 radio-loud active galactic nuclei (AGN) at 0.25 < z < 0.4 carried out with the Karl G. Jansky Very Large Array. Our aim was to study the rate of incidence of H I in different classes of radio AGN, the morphology and kinematics of the detected H I, and the nature of the interaction between the H I and the radio source at these redshifts. Our sample consists of 14 sources with sizes of up to tens of kpc and 12 compact sources (< a few kpc) in the radio-power range 1025.7 W Hz−1–1026.5 W Hz−1. We detect H I in five sources, corresponding to a detection rate of ∼19%. Within the error bars, this agrees with the detection rate found at lower redshifts. We find that the rest-frame UV luminosities of most of the sources in the sample, including all the detections, are below the proposed threshold above which the H I is supposed to have been ionised. An analysis of the optical emission-line spectra of the sources shows that despite their high radio powers, about one-third of the sample, including two detections, are low-ionisation sources. The radio continuum emission from the sources detected in H I is unresolved at ∼5 to 10 kpc scales in our observations, but shows extended structure on parsec scales. We analysed the H I 21 cm absorption spectra of the detections to understand the morphology and kinematics of H I. The absorption profiles are mostly complex with widths between the nulls ranging from ∼60 km s−1 to 700 km s−1. These detections also exhibit remarkably high H I column densities in the range ∼1021 cm−2–1022 cm−2 for Tspin = 100 K and unit covering factor. Our modelling of the H I 21 cm absorption profiles suggests that in two sources the gas appears to be disturbed, and in three cases, including one with disturbed H I, the majority of the absorption is consistent with it arising from an H I disc. Despite the high radio power of our sources, we do not detect fast outflows. However, the optical emission lines in these detections show the presence of significantly disturbed gas in the nuclear regions in the form of very wide and highly blueshifted emission-line components. Since some of our detections are also low-ionisation AGN, it is quite possible that this disturbance is caused by the radio jets. Overall, our findings point towards a continuation of the low-z trends in the H I detection rate and the incidence of H I in radio AGN up to z ∼ 0.4.

The neutral atomic gas content of galaxies is usually studied in the H I 21 cm emission line of hydrogen. However, as we go to higher redshifts, owing to the weak strength of the transition, we need very deep integrations to detect H I emission. The H I absorption does not suffer from this dependence on distance as long as there is a sufficiently bright radio source to provide the background continuum. However, resolved H I absorption studies of galaxies are rare. We report one such rare study of resolved H I absorption against the radio galaxy 3C 433 at z = 0.101, detected with the Very Large Array. The absorption was known from single-dish observations, but owing to the higher spatial resolution of our data, we find that the absorber is located against the southern lobe of the radio galaxy. The resolved kinematics shows that the absorber has regular kinematics with an H I mass ≲3.4 × 10 8 M ⊙ for T spin = 100 K. We also present deep optical continuum observations and H α observations from the Gran Telescopio CANARIAS (GTC), which reveal that the absorber is likely to be a faint disc galaxy in the same environment as 3C 433 , with a stellar mass of ∼10 10 M ⊙ and a star-formation rate of 0.15 M ⊙ yr −1 or less. Considering its H I mass, H I column density, stellar mass, and star-formation rate, this galaxy lies well below the main sequence of star forming galaxies. Its H I mass is lower than the galaxies studied in H I emission at z ∼ 0.1. Our GTC imaging has revealed, furthermore, interesting alignments between H α and radio synchrotron emission in the H I companion and in the host galaxy of the active galactic nucleus as well as in the circumgalactic medium in between. This suggests that the shock ionization of gas by the propagating radio source may happen across a scale spanning many tens of kpc. Overall, our work supports the potential of studying the H I content in galaxies via absorption in the case of a fortuitous alignment with an extended radio continuum source. This approach may allow us to trace galaxies with low H I masses which would otherwise be missed by deep H I emission surveys. In conjunction with the deep all-sky optical surveys, the current and forthcoming blind H I surveys with the Square Kilometre Array (SKA) pathfinder facilities will be able to detect many such systems, though they may not be able to resolve the H I absorption spatially. Phase 1 of the SKA, with its sub-arcsecond resolution and high sensitivity, will be all the more able to resolve the absorption in such systems.

Low-luminosity radio-loud active galactic nuclei (AGN) are of importance in studies concerning feedback from radio AGN since a dominant fraction of AGN belong to this class. We report high-resolution Very Large Array (VLA) and European VLBI Network (EVN) observations of HI-21cm absorption from a young, compact steep-spectrum radio source, B2 0258+35, nested in the early-type galaxy NGC 1167, which contains a 160 kpc HI disc. Our VLA and EVN HI absorption observations, modelling, and comparison with molecular gas data suggest that the cold gas in the centre of NGC 1167 is very turbulent (with a velocity dispersion of ~ 90 km/s) and that this turbulence is induced by the interaction of the jets with the interstellar medium (ISM). Furthermore, the ionised gas in the galaxy shows evidence of shock heating at a few kpc from the radio source. These findings support the results from numerical simulations of radio jets expanding into a clumpy gas disc, which predict that the radio jets in this case percolate through the gas disc and drive shocks into the ISM at distances much larger than their physical extent. These results expand the number of low-luminosity radio sources found to impact the surrounding medium, thereby highlighting the possible relevance of these AGN for feedback.

Radio jets can play multiple roles in the feedback loop by regulating the accretion of the gas, by enhancing gas turbulence, and by driving gas outflows. Numerical simulations are beginning to make detailed predictions about these processes. Using high resolution VLBI observations we test these predictions by studying how radio jets of different power and in different phases of evolution affect the properties and kinematics of the surrounding HI gas. Consistent with predictions, we find that young (or recently restarted) radio jets have stronger impact as shown by the presence of HI outflows. The outflowing medium is clumpy {with clouds of with sizes up to a few tens of pc and mass ~10^4 m_sun) already in the region close to the nucleus ($< 100$ pc), making the jet interact strongly and shock the surrounding gas. We present a case of a low-power jet where, as suggested by the simulations, the injection of energy may produce an increase in the turbulence of the medium instead of an outflow.