Multi-wavelength studies of AGN feedback and non-thermal emission in galaxy clusters and groups

Abstract

Galaxy clusters and groups are the latest, more massive structures to have formed in our Universe. As such, their birth and evolution provides key information to understand structure formation and constrain cosmology. In the last two decades, it has become clear that galaxy clusters deviate from the predictions of models which incorporate only gravitational processes. Scaling relations and observational constrains demonstrate that non-gravitational heating is required to explain why, for example, the hot plasma permeating clusters and groups is not cooling to low temperatures. Increasing evidence points indeed to Active Galactic Nuclei (AGN) feedback as an explanation for these features: the central Black Hole (BH) feeds from gas which is cooling down from an hotter phase, and in turn produces shocks and induce heating which quench the radiative losses of the hot plasma.

There is still much to understand about AGN feedback. It is currently not clear whether the relative position of the AGN and of the cluster cooling region can have relevant effects on this duty cycle. It is possible that, if the AGN lies outside of (or is offset from) the cooling region, the BH might not be able to feed from cold gas, and the AGN not able to heat its surroundings efficiently enough to quench radiative losses. Furthermore, while it has become relatively easy to detect feedback features (e.g. shocks, bubbles) in massive clusters, investigations of AGN feedback in the lower mass range of galaxy groups are still lacking, mainly because of their low surface brightness and smaller dimension. It is also essential to understand the role of feedback when it comes to the properties of AGN optical hosts, and to model feedback prescriptions in our simulations in a way in which they can reproduce the features we observe in clusters and groups. Finally, feedback can also have a relevant role on setting the necessary conditions to power the extended radio emission produced by the acceleration of Cosmic Rays (CR) on cluster scales, which is being frequently observed in disturbed systems by low-frequency interferometers such as LOFAR, the LOw Frequency ARray.

During my PhD, I have studied clusters which show offsets between the peak of the cooling and the AGN, as well as investigated feedback in the low-mass regime of galaxy groups. In doing that, I have exploited observations by multiple instruments, in different bands of the electromagnetic spectrum, as well as new-generation surveys provided by eROSITA (X-ray) and LOFAR (radio), which together provide large sample of systems with an unprecedented wealth of multi-wavelength data available. I have learnt and developed tools to calibrate and analyse data, mainly at X-ray and radio frequencies, and combined them to get a new, unexplored perspective of how AGN feedback acts in clusters and groups. I have exploited statistical methods and simulations to study the interplay between the hot plasma permeating clusters and the non-thermal emission by central radio galaxies. Finally, we have developed a pipeline to properly calibrate LOFAR ultra low-frequency (54 MHz) observations, applying it to one of the most interesting galaxy clusters in the HETDEX sky field, Abell 1550 (A1550).

We have found evidence that clusters and groups follow a similar correlation between X-ray luminosity of the hot plasma and radio power of the central AGN. The correlation becomes tighter when we compare the total energy output from the AGN (which the radio power is a proxy of) to the X-ray luminosity, and apparently holds for both relaxed and disturbed systems. We showed that galaxies close to the cluster/group center are more likely to host a radio-loud AGN, since they can easily tap into the cold gas reservoir, while outer galaxies might rely on more episodic triggers, such as mergers. We found no apparent correlation between the extent of central radio galaxy and the density of the host group/cluster, suggesting that other factors, such as the age and radio power of the source, might be more dominant. In systems with limited offsets ($<50$ kpc) between AGN and cooling peak, we determined that the duty cycle is not broken, and that gas sloshing can induce further offsets with the warm gas which is cooling out from the hot phase. Finally, we have analysed one of the first ultra low-frequency observations provided by LOFAR, studying the disturbed galaxy cluster A1550 and finding multiple diffuse emission sources with surprisingly heterogeneous properties, which are likely produced by different cosmic-ray acceleration mechanisms. I have gathered all my findings into 5 first-author, peer-reviewed publications, which I present here.

In summary, my thesis provides new insights into the current knowledge of AGN feedback in clusters and groups, and explores the potentiality of LOFAR ultra low-frequency observations to detect and investigate diffuse, non-thermal emission in galaxy clusters.