Astrophysics Research at Bristol

The Coldrick Observatory

The most visible sign of the research carried out by the Astrophysics Group is the impressive 6-m radio telescope on the roof of the School of Physics, which was funded through the generosity of University of Bristol alumnus, William P. Coldrick, after whom the telescope is named. The telescope is equipped with a novel fully-digital, real-time Fast Fourier Transform Spectrometer, developed by the Group in partnership with AphaData Parallel Systems Ltd. (Edinburgh) and Beam Ltd. (Bristol). It will be used to survey our Galaxy for sources of maser emission, the (natural) radio equivalents of lasers.

Extragalactic astronomy: active galaxies

Moving further out in the Universe, the Group has a major involvement in numerous international research programmes in extra-galactic astronomy and cosmology. For instance, the image to the right shows the central two thousand light years of the nearby radio galaxy Centaurus A, as viewed in radio emission (red) and at X-ray wavelengths (blue). The core (at the lower right of the image) is at the centre of the galaxy, and probably indicates the location of a black hole a billion times the mass of the Sun. The extended, linear feature, seen in both images, is due to a jet of material shot out of the core at high speeds. The radiation from the jet comes from electrons of extremely high energies spiralling around magnetic fields. The X-ray and radio images are significantly different, as can be seen by comparing the red and blue images. By studying the differences between them we hope to understand the processes that accelerate the electrons to high energies and transport them at speeds close to the speed of light along the jet.

Extragalactic astronomy: normal galaxies

Modern technology on large ground based optical telescopes also allows us to make huge steps forward in surveying the whole Universe. We have been able to use the '2dF' multi-object spectrograph on the Anglo-Australian Telescope in New South Wales to obtain spectra for several thousand objects in the direction of the Fornax Galaxy Cluster.

Sifting through the huge data base of spectra in this survey has revealed a rare but fascinating new class of galaxies, so small as to be indistinguishable from stars if we have only their images to work with. The spectra enable us to find their redshifts and hence prove that, rather than being nearby stars, they are in fact tiny galaxies within the Fornax Cluster , 60 million light years away. Galaxies like these (and, at the opposite extreme, diffuse low surface brightness galaxies hardly detectable against the general sky brightness) can add to the mass audit of the universe, a vital ingredient in discussions of dark matter, dark energy and the possibility of accelerating universes.

Extragalactic astronomy: galaxy clusters

In addition to the thousands of galaxies that can be seen with optical telescopes, clusters of galaxies contain an atmosphere of hot ionised gas which glows brightly at X-ray wavelengths. In fact, this hot plasma contains between five and ten times the mass in all of the galaxies in the cluster, so by studying its properties using X-ray observatories, we can learn a great deal about galaxy clusters.

X-ray observations show that the gas in galaxy clusters is so hot that a great deal of material must be present within the cluster to provide a deep enough gravitational potential to prevent the gas from escaping. The amount of mass required is many times more than is observed in the gas and galaxies, providing strong evidence that around 90% of a galaxy cluster is made up of dark matter. We are studying galaxy clusters using X-ray, optical and radio observations to learn more about their formation and evolution. This is a key step towards gaining a full understanding of the nature of dark matter and the development and future evolution of the Universe and the structures within it.

Galactic astronomy

Returning nearer home, the final figure shows two small parts of an Hα survey of our Galaxy, which our Group helped to instigate at the UK Schmidt Telescope in Australia. This survey uses the light emitted by hydrogen atoms to map the gas from which stars are made and which then gets replenished when the (more massive) stars lose material at the ends of their lives, becoming supernovae or, rather less dramatically, planetary nebulae. Besides providing images of supernova remnants like the one in Vela with exquisite detail, the survey has led to the discovery of more planetary nebulae than have been found in all previous work over the last century put together! More recently we have been part of the similar IPHAS survey of the northern half of the Galactic Plane, carried out at the Isaac Newton Telescope on La Palma.

Extrasolar Planets

The group has recently expanded into the study of extrasolar planets. There are currently over 500 planets that have been discovered outside of our own solar system. The planetary systems that are observed are very different from our solar system and surprisingly diverse. Most of these observed "extrasolar planets" are similar in mass to Jupiter but have orbits that are much closer to their central star than Jupiter is to the Sun. New observations have also found rocky extrasolar planets called "super-Earths", which are just a bit more massive than the Earth. The major problem facing the scientific community with regards to these discoveries is that observations cannot trace the history of planet formation. Instead observations provide snapshots of either the early stages of a dusty gas disk orbiting a young star or the late stages after planetary systems have formed. But the evolution from a young star to a planetary system has not been observed. The challenge is to connect the early and late stages of planet formation. The approach that is being used in our group is to build a computer based simulation that numerically models various scenarios. This work will trace the histories that could evolve thus it will predict which stars are most likely to harbour Earth-like planets and determine whether our solar system is unique, or may have many counterparts throughout the Galaxy.