Active Galaxies and Black Holes

Active Galaxies

We study active galactic nuclei (AGN), particularly radio galaxies, using radio, infra-red, optical, and X-ray techniques. Interests include the environments and dynamics of radio sources, unified models for both high-power and low-power objects, observation and modelling of jets and the X-ray/radio relationship in radio galaxies and quasars.

Active galaxies can also be used as cosmological probes. Powerful radio sources are markers of massive structures (clusters of galaxies); by observing them at high redshift we can find massive structures in the early universe, which allows us to test models of structure formation, a key goal of cosmology. Once these structures are found, multi-waveband observations are being used to determine key parameters such as mass, dynamical state and baryon content. Radio observations (e.g. of the Sunyaev-Zel'dovich effect), optical (peculiar velocities, weak lensing) and X-ray observations (measurement of the hot intracluster plasma properties) all contribute to these studies.

Many of the observable properties of active galaxies are related to plasma processes in the sources, and these relate to terrestrial fusion plasma physics. Work in the group on particle acceleration and radiation has led to collaborations with the UKAEA fusion physics research group at Culham, and fruitful discussions with long-term professorial visitor A. Thyagaraja.

Under certain circumstances, the process of accreting material onto a black hole can produce large amounts of radio emission. These are a class of active galaxy called radio galaxies, that can have radio jets and lobes extending over thousands of light years. Radio telescopes give a unique view of these powerful objects. See Fig 2 for an example of a radio galaxy observed with MeerKAT.

Across our astrophysics group we use a wide array of instruments to study active galaxies and black holes such as the VLA, the VLBA, eMERLIN, the ATCA, LOFAR, the HST, Euclid, XMM and Chandra (see our Observational Astrophysics page for more).

Black Holes

Black holes have a profound impact on their environments. As gas falls towards a super-massive black hole its gravitational potential energy can be converted to heat and light, in some instances producing the most powerful sustained energy sources in the Universe. These Active Galactic Nuclei (AGN) can outshine all of the starlight in an entire galaxy, and sometimes produce relativistic outflows known as jets. The feedback between inflowing gas and outflowing jets and radiation plays an important role in regulating structure formation and evolution in the Universe.

We study the environment immediately surrounding black holes using space-based X-ray observatories (including Chandra, XMM-Newton and NuSTAR), in addition to observations at other wavelengths. X-ray spectroscopy of gas deep in the potential well of the black hole, where the effects of strong gravity are important, allows us to probe the properties of the black hole itself. We numerically model the appearance of these systems in X-rays. While current X-ray observatories cannot directly image the accretion disk, X-ray spectra can be used to learn about the spacetime (e.g., is the black hole spinning?) and accretion disk close to the black hole (e.g., its density, iron abundance, emissivity profile and ionization state).

X-rays from accretion flows are highly variable, providing valuable additional information about the system that allows some fundamental degeneracies in spectroscopic studies to be broken. We make significant use of the University of Bristol supercomputers, including its array of GPUs, to model the variability of X-rays from AGN (see our High-performance scientific computing and AI page for more). In particular, we model the disk response to variable X-ray sources located above the disk (e.g., perhaps in a corona, or the base of an outflowing jet). The simplest of these models would represent a flare from an X-ray point source somewhere above the black hole which produces a time-varying illumination pattern that sweeps across the disk that is distorted as a result of the strong gravitational field and the Doppler shift of the disk material. These models can be fit to X-ray data to learn about the changing geometry of the X-ray continuum source.