Young active galaxies across the radio spectrum

Abstract

The lifecycle of active galaxies is an open question in modern astronomy. In particular, not all active galaxies are active radio galaxies, in possession of synchrotron-emitting jets, and it is not obvious why. This thesis focuses on these active radio galaxies, and was completed as part of the First Large Absorption Survey in HI conducted with the Australian SKA Pathfinder (ASKAP-FLASH). Early on, we realised many HI detections in ASKAP-FLASH were made towards so-called 'Peaked Spectrum' sources, active galaxies in which the synchrotron jets are still embedded in the dense, nuclear gas of their host galaxy. The thesis presents a new Bayesian framework for identifying peaked spectrum sources using pre-existing radio survey data, RadioSED. Applying RadioSED to a test field, we increase the number of known peaked spectrum sources by more than an order of magnitude in that area through careful treatment of pre-existing datasets. We investigate the multiwavelength properties of this sample, and identify that many of them are distant, making them interesting probes of the physical conditions at cosmic noon and beyond. We then take early results from the ASKAP-FLASH Pilot Surveys and use a new pipeline for mock observations, SANGRiA, to determine whether we can recover the high observed HI detection rate towards compact jets in simulations, under the assumption that it is a purely geometrical effect. Finally, this thesis presents a multiwavelength study of a different type of radio AGN detected in HI with ASKAP-FLASH, demonstrating the gains to be made by multi-wavelength follow up of HI detections, and revealing at least three intervening galaxies along that line of sight, one of which is the likely host of the HI. Overall, this thesis reinforces the key role of peaked spectrum sources in studies of active galaxy evolution, jet-gas interactions, and the changing distribution of gas across the different ages of our Universe.