AGNs
With the exception of dwarfs, all galaxies have the potential to be active and almost certainly went through a period of intense activity at the time when the central bulge and associated black hole were being formed. But only about 10% of local galaxies are classed as active, though the percentage increases as we look to higher redshifts. Activity is a somewhat slippery concept but is most easily defined as emission resulting from accretion of matter on to the black hole at the centre of a galaxy. Observationally, this activity can manifest itself in the form of Seyfert-like behaviour, i.e., strong, broad, high ionisation emission lines and strong blue/UV continuum emission, or as radio emission powered by jet of relativistic particles which can occur either with or without associated Seyfert-like behaviour. The range of optical luminosities can be enormous ranging from LINERS (poor cousins of Seyferts) to quasars which are the most luminous objects in the Universe. The same is true for the radio emission; the most luminous radio sources are usually accompanied by strong optical activity but the converse is not true. There really do seem tobe two types of AGN, those that are radio-loud and produce jets, and those that are radio quiet. Most of the research at JBCA focuses on the radio loud objects but even amongst those there is a vast range of luminosities.Some of the key questions are:
1. What triggers a galaxy to become active? This is almost certainly related to availibility of gas to be accreted by the black hole, perhaps liberated by an interaction with another galaxy, but the details of the process remain a mystery.
2. Why are some object radio loud and others, apparently identical in other respects, radio quiet?
3. How are the radio jets produced and what regulates the maximum energy of the synchrotron electrons that are accelerated? Radio loud objects emit over the whole EM spectrum up to gamma-ray wavelengths and the study of their spectral energy distributions is an important topic.
4. How do radio jets interact with their host galaxies? On a global level it is thought that jets in the dramatic early stages of galaxy formation can regulate the growth of galaxies through feedback. In nearby galaxies such physical process can be studied in detail but at a much less grander scale.
Exciting times are ahead for AGN science: new radio facilities like e-MERLIN, EVLA, SCUBA2 and ALMA are about to come on line and there is a flood of multi-wavelength data from optical/IR/Gamma-ray surveys like SDSS, UKIDDS, Herschel GLAST. A list of current projects JBCA is involved in is available here and some are introduced below.
Not very active elliptical galaxies
(R. Battye, I. Browne)The details of how the elliptical galaxies were assembled is one of the key questions in modern astrophysics. There are two main approaches to the problem: to look back in redshift to the times when assembly was happening or to look at local galaxies for clues in their properties as to their histories. By studying of the radio emission from relatively local SDSS galaxies, we have discovered a statistically highly significant trend for the axes of the radio emission to be aligned with minor axes of the stellar distribution in elliptical galaxies. Remarkably, when the sample is split on the basis of the ratio of radio flux density to optical flux density it is found that the tendency to align is enhanced if we select only the radio weaker sources.
Alignments have been long searched for because of their potential to tell us about the shapes of galaxies and about the orientation of the accretion disk/black hole system. Our results imply that the galaxies must be oblate spheroids (squashed spheres) and the emerging radio jets must be aligned with the minor axes of the spheroids. This is itself a new result. An interesting open question is the reason for the dependence of alignment properties on radio loudness. We speculate that the division we find is related to the merger history of elliptical galaxies which in turn is related to the recently recognized division of elliptical galaxies into those that are rotationally supported and those which are not rotating. In this picture radio weaker objects are rotationally supported and kinematically well ordered. Conversely, the louder objects live in kinematically more complex galaxies.
We are following up these results -- there are opportunities here for student projects. We can extend the sample using existing SDSS data and also look for correlations with other measured parameters. The environments of the aligned and non-aligned objects are being studied because the richness of the environments tells us about the likelihood of mergers which is how ellipticals are built up. We also plan to examine in depth the radio structures of a subset of the aligned and unaligned objects to see if the differences are reflected in their radio morphologies. Does the complex kinematics of the non-rotators destroy the alignment information? We also want to make long-slit or integral field optical spectroscopic observations to study the kinematics in objects with detailed radio maps. This work is complementary to that in the SENSE project because the intrinsic radio luminosities (not the beamed luminosities) and redshifts of the samples of objects are very similar.
