Quasars revealing the powerhouses of the distant Universe

Quasars - revealing the powerhouses of the distant Universe Rhaana Starling (rlcs@mssl. ucl. ac. uk) Supervisor: Dr. E. M. Puchnarewicz Department: Space and Climate Physics, MSSL Quasars are the highly luminous centres of galaxies, thought to be powered by accretion onto a supermassive black hole. These objects are immensely powerful, often completely outshining the billions of stars in their host galaxy. They emit at wavelengths spanning the entire electromagnetic spectrum and many quasars are extremely variable on timescales ranging from hours to years. These incredible objects show such a wide range of characteristics, many of which lack convincing explanations. As quasars are so luminous they can be seen out to great distances. This means we are seeing things as they were billions of years ago which provides a rare probe of conditions in the early Universe. It is also important to understand the physical processes which contribute to a quasars’ emission in each waveband why enormous changes in output occur on several timescales. A supermassive black hole feeding on a surrounding disk of accreting matter is widely accepted as the central engine of a quasar. Accretion disks are found elsewhere in the Universe, for example in accreting binary star systems, and less massive black holes may exist in many (possibly all) galaxies including our own Milky Way. The study of quasars is not only to unravel their mysteries, but also to help put together the bigger picture of the dynamics and evolution of the Universe. A multiwavelength approach is needed to discover a quasars’ behaviour as a whole and uncover its structure. This means obtaining data from the long wavelength radio regime through to high energy gamma rays. From each observation we can extract: Lightcurves – a plot of the light intensity over the duration of the observation. Multiple observations spanning months to years can be combined to find long-term trends. Images – some quasars have jets (top left; Chandra X-ray telescope) or the host galaxies may be visible as seen in these Hubble images. Spectra – the light dispersed to show its intensity at each wavelength, much like splitting white light (optical) into the colours of the rainbow using a prism Lightcurves provide evidence for variability and spectra reveal which chemical elements are present in the emitting material, their abundances and the materials’ density and velocity. The characteristic shape of the spectrum may help determine the physical processes taking place. A unified model describing a possible structure for all quasars has been proposed but no two objects are identical and many cannot be explained by this model. It is nicely summarized by the adjacent diagram, where the innermost layer is the accretion disk, surrounded by fast-moving clouds enshrouded by a dusty ring (shown in orange) with a low-velocity cloud region furthest from the central black hole. NASA/CXC I am looking at two very unusual objects, IRAS 13349+2438 and REJ 2248 -511, both atypical in different ways, aiming to V determine their emission mechanisms and physical geometry. To do this I have infrared and optical observations I obtained at the South African Astronomical Observatory plus some archival information at other wavelengths. I also have high resolution X-ray data from the latest Xray telescope to be launched, XMM-Newton (left; artists impression). It also has an optical telescope on-board so simultaneous optical and X-ray data are available for each observation; this is very important since many quasars are extremely variable. The particular objects I am studying have remained elusive since their discoveries many years ago. This is perhaps not surprising since so many of the fundamental physical processes underpinning quasar behaviour are yet to be fully understood. However, with better quality data now available these powerhouses of the distant Universe may soon be revealed.