Philip Best


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Research Activities

My research covers a wide range of extragalactic observational astronomy, including:
Active Galactic Nuclei (AGN), and AGN feedback:
My interests lie in understanding the AGN phenomenon, and its connection to the formation and evolution of normal galaxies. This interest dates back to my PhD research, when I used the Hubble Space Telescope to study the interactions between powerful distant radio sources and both their host galaxies and the surrounding gas, showing that the interactions are particularly strong in young (small) radio galaxies. More recently, I have constructed large samples of radio-loud AGN in the nearby Universe by combining wide-area radio surveys with the Sloan Digitised Sky Survey. I have used these to carry out detailed statistical studies of AGN host galaxies, from which I have been able to show that energetic feedback from radio-loud AGN can control the rate of growth of massive elliptical galaxies in the local Universe. I am extending this analysis to higher redshifts to track the evolution of radio-AGN feedback across cosmic time.

Black hole accretion modes and jet-launching:
My AGN studies show that the fuelling of AGN can occur in two different modes: radiatively-efficient accretion at high accretion rates, through a "standard" geometrically thin accretion disk, and radiatively inefficient accretion at low accretion rates through a geometrically thick accretion flow. The latter are highly efficient at launching powerful radio jets. I am studying the connection between AGN fuelling and jet-launching to understand black hole accretion on small scales, as well as the large-scale feedback effects on the host galaxy.
Cosmic Star Formation
I have had a long-standing interest in studying star formation across cosmic time. Together with Ian Smail, I co-led the UKIRT large Campaign Project HiZELS. This project used the wide-field near-infrared imaging capabilities of the WFCAM telescope, together with narrow-band filters, to select emission line galaxies across all of cosmic time. Emission-line surveys are a particularly powerful method for tracing the evolution of cosmic star formation: galaxies are identified on the strength of their emission line and thus crudely represent a star-formation rate-selected sample, which must lie in a narrow range in redshift. Using a number of narrow-band filters we have used a single technique to target H-alpha emitters across a wide range of redshifts, yielding large and representative samples at each epoch and with a uniform selection function. Using these samples we have traced the evolution in the star-formation rate density across the epoch of peak activity in galaxies, as well as its distribution amongst the galaxy population. The project has now evolved into a phase of extensive follow-up studies of sub-samples of the HiZELS sources to understand in detail the evolving properties of star-forming galaxies. I am now using samples of radio sources from deep extragalactic surveys to continue and extend these studies, forming an ideal complement.
LOFAR is a next-generation radio telescope, constructed in the Netherlands and around Europe, which commenced operations in 2012. It has opened up a completely new low-frequency radio regime, as well as offering the deepest large-sky area radio surveys prior to the Square Kilometer Array. I am PI of LOFAR-UK, which is a consortium of over 20 Universities coordinating the UK's efforts in LOFAR, and am the Chair of the LOFAR International Telescope Board. Scientifically, I am a core member of the LOFAR Surveys Key Science Project carrying out deep Extragalactic Surveys with LOFAR - with the first survery data release in early 2019. I lead responsibility for deep blank fields and for studies of the evolution of star-forming galaxies and AGN.
Looking further to the future, I am involved with efforts to build the Square Kilometer Array. I am the UK's Science Director for SKA, and represent the UK on the SKAO Board of Directors.
Other interests:
I also work on many other topics relating to cosmology and to galaxy formation and evolution. I have a long-standing interest in groups and clusters of galaxies, and the effect of environment on both star-formation and AGN activity.