RADIO IN RED

SPECIAL ISSUE 1999

Course: High-redshift radio galaxies

Prof. Steve Rawlings
University of Oxford
UNITED KINGDOM

"The HDF programmes have been phenomenally successful. They have produced outstanding new scientific results, and because of the way in which the data were obtained, analysed and distributed, they have changed attitudes as to how large projects should be run. There have, however, been other recent programmes of at least comparable significance; exciting as the HDF work has proved, it is essential that it is viewed as a part of a much larger observational effort. Steidel and collaborators have now found almost one thousand galaxies at redshifts around three, and the discovery that they are strongly clustered illustrates the danger of relying entirely on deep observations of small patches of sky; it also proves there’s still plenty of mileage in the classical astronomical recipe of making a wide-field imaging survey with a ‘small’ telescope, and following up spectroscopically with a big one. Another key development has been the SCUBA camera on the JCMT, a technological breakthrough which has led to the detection of dusty, distant starbursts — the sites of cosmologically-significant star formation; this illustrates the importance of observing across, and perhaps eventually outside, the electromagnetic spectrum. There are lessons here for the future. One has only to look at a simulated NGST image to be daunted by its scientific promise, but I can guarantee it will not tell us everything we need to know about the young Universe, and what it does tell us may be misleading unless it is viewed holistically. The need to develop other observational capabilities is already clear: to find and study objects at redshifts above ten, for example, astronomers will need a sensitive spectroscopic capability in the millimetre waveband. Also, putting aside the mega-buck projects, and taking as read the need for supporting survey work on smaller telescopes, we may be surprised by the impact of cheap, niche telescopes. One fascinating example concerns the Sunyaev-Zeldovich effect which provides arguably the cleanest way of studying the evolution of galaxy clusters out to high redshifts. Because SZ effects are on large angular scales, the instrument astronomers need to build is an array of small, and therefore cheap, dishes costing at least three orders of magnitude less than the NGST."

"It is truly amazing how things have changed since the `good’ old days, only five years ago, when quasars and radio galaxies were the only known objects at redshifts significantly above one. It is a sobering (but surprisingly liberating) experience to see one’s primary research area become so unfashionable, so rapidly. Amongst those of us who decided to continue working on high-redshift active galaxies, a major rethink about their cosmological role was clearly the order of the day. The outcome of this rethink leads me to suspect that high-redshift radio galaxies may swiftly come back into fashion. As cosmological probes, radio sources have one key advantage over, for example, quasars: we can measure their ages rather robustly, and these ages are short, around a million or so years at high redshift. The ability to measure ages means that we can convert space density evolution into the evolution in AGN trigger rate, and the inevitable youthfulness of high-redshift radio galaxies means we can map the cosmic evolution of this trigger rate with very fine time resolution. Observations of the structure, radio spectral index and polarisation of high-redshift radio lobes provide a wealth of information on high-redshift gaseous environments which we are only just beginning to use effectively. The role of radio sources in cosmology may, however, be more fundamental than just their use as probes of the young Universe. Powerful radio sources output roughly as much energy in bulk kinetic outflows, their jets, as is radiated by the most powerful quasars, and plausibly these outflows persist much longer than the object remains a bright radio source or has an optically-bright quasar nucleus. These outflows could be a significant source of entropy for the gas in the Universe, and may provide a vital feedback mechanism regulating the formation and evolution of massive galaxies and their associated clusters."

"Well the short answer to this question is regrettably `no’, although we do seem to be in the midst of a period over which knowledge is increasing rapidly. Concerning active galaxy formation, the last few years have seen several groups develop `semi-analytic’ models which work by bolting simple prescriptions for physical processes like star formation and AGN fueling onto a standard gravity-driven model for structure formation. First attempts at explaining active galaxy evolution are highly encouraging, although as yet they reproduce only the gross evolutionary trends evident in the data. A slight worry with the semi-analytic models is that they include large numbers of free parameters so that even misleading observational data, most notably the first versions of the curve of global star formation rate as a function of redshift, can be quite successfully fitted. However, experts in this field assure me that the semi-analytic approach has matured to a point where robust model predictions are now significantly at odds with observational data, signalling problems with incorrect or missing physics, or with the underlying cosmological model for structure formation. My suspicion is that the incorporation of AGN feedback, accounting for both radiated and bulk kinetic outputs from an accreting central object, will prove vital in resolving these difficulties. It is also worth bearing in mind that important physics, for example the influence of magnetic field on structure formation, are not yet included in the models. Concerning AGN evolution, there has also been significant recent progress. For powerful radio galaxies we now have some understanding of the gross features controlling the time evolution of their radio luminosity, modulo significant uncertainties concerning their environments: upcoming X-ray satellites should map the gas surrounding radio lobes and resolve these problems. There is still no consensus over even basic questions concerning the time evolution of radio-quiet quasars, or how to unify the radio-quiet and radio-loud popu1lations."

"The astronomical community as a whole — and not just radio astronomers —view the development of the ALMA, the Atacama Large Millimeter Array, as a very high priority for future research into the young Universe. With planned sensitivity and resolution limits exceeding present instrumentation by more than an order of magnitude, this will be a truly revolutionary instrument. While SCUBA struggles to see even the most prodigiously star-forming galaxies at high redshifts, the ALMA will detect galaxies no more extreme than our own Milky Way. With millimetre-wave spectroscopy of high-redshift galaxies we will be able to study the rotation curves of molecular material in their galactic disks and, using gravitationally-lensed objects, within circumnuclear star-forming regions; these observations will give us dynamical measures of the masses of galaxies and their black holes as a function of redshift. Also, we will be able to make the first serious studies of gas and dust masses as functions of cosmic time and galaxy mass, determining whether any galaxies can be treated as closed boxes, or whether all are subject to large-scale inflow and/or outflow of gaseous material. Perhaps most excitingly, the ALMA may also be the instrument to open up studies of objects in the Universe at epochs corresponding to redshifts above ten. Objects rendered effectively invisible in the optical by redshifting and obscuration effects could be strong sources of millimetre-continuum emission on which will be superimposed various emission and absorption lines. Thus, the ALMA may become the primary redshift-measuring machine at very high redshifts. Only with this instrument will we get anywhere near a complete picture of the evolution of star formation with redshift."

STEVEN GREGORY RAWLINGS was born on 11 October 1961.

He studied Physics and Theoretical Physics at the University of Cambridge and was awarded his PhD in Radio Astronomy in 1988.

From 1988 to date, Rawlings has held several positions at the Universities of Cambridge and Oxford (Director of Studies in Mathematics for Natural Sciences, Fitzwilliam College, Cambridge; SERC Advanced Fellow in Astrophysics, University of Oxford, among others).

He has a large teaching experience, both for undergraduates and postgraduates covering subjects such as theoretical mechanics, mathematical physics, quantum theory, electronics and computing.

Rawlings’s research interests focus on the cosmic evolution of galaxies and active galactic nuclei (AGN); high-redshift galaxies and quasars; observational constraints on cosmological models; rich clusters: formation, properties and evolution; gravitational lensing; physics of extragalactic radiosources, jets and AGN; and the study links between star formation, radio and AGN activity.

He has been member of the Panel for the Allocation of Telescope Time, UKIRT Telescope Allocation Group, (1992-1995, 1999-); member of ROSAT X-ray Telescope Panel (1995-1998); member of Gemini Working Group (1997); Chair of the UK working group on Cosmology with the LMA (1998), and member of PPARC Research Assessment Panel (1997,1999), among other committees.