Radio surveys as a key to understanding AGN feedback

A super-massive black hole exists at the centre of almost every massive galaxy. The gravitational potential of these black holes can power highly energetic phenomena which are observed on sub-galaxy to galaxy cluster size scales, across the entire electromagnetic spectrum; when we observe these phenomena we know these are active galactic nuclei (AGN). The basic picture of AGN that has emerged is of a black hole surrounded by an accretion disk, with a corona of hot gas, and the entire region is circled by a dusty torus. In some cases, AGN launch relativistic jets that extend far beyond the galaxy itself. But no matter how much energy an AGN outputs, the black hole powering it still remains a tiny part (both in physical size and total mass) of its host galaxy. Yet we see observational evidence for tight correlations between black hole properties and the general properties of galaxies, and we know from cosmological simulations of the Universe that some energy from AGN must help regulate galaxy growth; without it we are unable to reproduce observed galaxy populations. This suggests feedback between AGN and their host galaxies, but the huge range of size scales coupled with a wide range of energy transport options means the true nature of this feedback remains unknown.

The above image shows a comparison of the instantaneous LOFAR and e-MERLIN fields of view; both telescopes can achieve 0.3" resolution but the LOFAR field of view is 20 times larger. Source positions are marked in gray. The cutout images surrounding the central image are all pipeline-generated using LOFAR data to post process this single 8-hour observation. We have achieved 0.3" resolution and ~90 uJy/bm noise at 150 MHz.

AGN can produce radio emission in multiple ways. They can host relativistic jets which extend far beyond the host galaxy and interact with their environment. Large scale jets like these were thought to be rare, although recently LOFAR has revealed they may be more prevalent than we think, and they are not well modelled in simulations. While the majority of AGN don't have these kinds of jets, they can still produce fainter, small-scale radio emission in other ways. We know there will be some contribution from star formation, as supernova remnants leftover from the deaths of massive stars are ideal locations for cosmic rays to accelerate and produce synchrotron emission. But there is a portion of the faint radio emission from AGN that cannot be accounted for by star formation: is it small scale jets? Thermal radio emission associated with disk winds? Something we haven't thought about yet?

At Durham we specialise in high-resolution radio imaging using the LOw Frequency ARray (LOFAR), at frequencies below 200 MHz. We can exploit LOFAR's wide field of view to image the entire Northern sky at sub-arcsecond resolution (see image) over the next few years, studying radio emission from AGN in great detail. The spatial information will help us understand the life cycles of radio jets, and disentangle AGN-related nuclear emission from star formation. In particular, we are investigating the radio properties of broad absorption line quasars (BALQSOs), which have a higher radio detection fraction than their "normal" quasar counterparts, although very few show evidence for large-scale radio jets. The broad, blue shifted absorption lines in BALQSOs imply strong outflows, which can be major contributors to AGN feedback. Understanding how the radio emission in BALQSOs is related to the AGN will be critical for understanding the overall population of AGN.

Staff involved with this topic at Durham include Leah Morabito and Dave Alexander.