Luke Holden

Astrophysics PhD student

Image credit: NASA, ESA & A. van der Hoeven.

My work focuses on precise and careful diagnostics of gas outflows driven by Active Galactic Nuclei (AGN), which are accreting supermassive black holes (SMBHs) at the centres of galaxies. Such outflows are thought to possibly play an important part of how AGN may impact their host galaxies, a process known as `AGN feedback'. However, despite this, the true impact of AGN-driven outflows is uncertain. In large part, this is because their physical properties and acceleration mechanisms are not well constrained.

To this end, I use observational techniques such as optical/near-infrared spectroscopy and sub-mm/radio interferometry to investigate the true nature of AGN-driven outflows. This involves detailed, high-spatial resolution single-object studies of nearby (z<0.1) active galaxies which make use of robust and novel outflow diagnostics. For example, perhaps the largest source of uncertainty when calculating outflow kinetic powers for the warm ionised gas phase (often traced by optical observations) is the electron density of the outflowing gas: commonly-used density diagnostics are only sensitive up to moderate gas densities, and so outflow kinetic powers may be overestimated. In the first paper of my PhD, we used a technique involving the `transauroral' [OII] and [SII] lines - which are sensitive to a high range of densities - to derive the density of spatially-resolved outflowing gas for the first time in an active galaxy (IC 5063). We also presented evidence that the transauroral lines are likely tracing the same gas as other key diagnostic lines such as [OIII]λλ4959,5007 and Hβ. In a subsequent study of the prototypical Seyfert galaxies NGC 1068 and NGC 4151, we derived high electron densities using the transauroral lines with spatially-resolved observations for the first time, further reinforcing their utility.

Another important consideration is that AGN outflows consist of several gas phases, ranging from hot (T>10,000K) and ionised to cold (T~100K) and molecular: in order to properly account for the impact of outflows, all of these phases must be considered. Therefore, I am interested in the relative powers and masses of these outflow phases, as well as the physical link between them. Using VLT/Xshooter optical and near-infrared spectroscopic spatially-resolved observations of the nearby galaxy IC 5063, we found that most of the outflowing mass and power was tied-up in the cooler gas. Furthermore, we proposed that the different phases represent a post-shock cooling sequence that occurs after the gas has been accelerated by a radio jet from the central AGN. I am also currently leading a project to investigate and quantify the masses and properties of cold molecular outflows in Ultra Luminous Infrared Galaxies (ULIRGs) - where such outlfows are thought to be particularly prominent - using high spatial resolution ALMA data.

On a similar note, the acceleration mechanisms of AGN-driven outflows in different objects is another key focus of my work. Determining the true outflow acceleration mechanism in a given object is crucial for several reasons, principally that it may determine the ionisation state of the gas, meaning methods such as density and temperature diagnostics need to be carefully chosen. A good example of the need for caution when investigating outflow acceleration mechanisms is, again, IC 5063. We found that, despite the outflows being almost certainly jet-accelerated (on account of spatial association between the radio structure and gas kinematics), the gas appeared to be photoionised by the central AGN rather than shocks from the radio jet. However, looking at near-infrared gas, we found that the warm molecular outflow phase was likely shock-ionised, consistent with jet-acceleration. The key point here is that ionisation mechanisms do not always infer information about acceleration mechanisms, and the relationship between the two can be complex.

Currently, I am focused on expanding on these three key aspects of AGN-driven outflow research through further detailed studies of local AGN of different types.

As well as my work on AGN, I have previously investigated the evolution of light pollution at the Roque de Los Muchachos observatory, as well as the variation of atmospheric seeing there. Presentation slides for seminars that I have given regarding this work can be found here (light pollution) and here (seeing).