Ordinary Meeting, 2008 May 28
The New Generation of Gravitational Lens Surveys
Prof. Hewitt explained that the theme of his talk would be dark matter and dark energy. He added, however, that despite their surprisingly frequent mention in popular science, very little was actually known about the physical nature of either of them – hence the rather vague and non-descriptive names which they had been given. The only concrete things that could be said about either were descriptions of the observations which pointed to their existence, and these would be the subject of the remainder of the talk.
The speaker began his talk by showing a recent all-sky image of the radio emission called the cosmic microwave background (CMB) as observed by NASA's WMAP satellite, explaining that this radio radiation dated from when the Universe had been a mere 500,000 years old, and that it had been travelling through space ever since. Up until that time, termed the epoch of recombination, all of the material in the Universe had been at such high temperatures that all of the hydrogen – the bulk of the material in the early Universe – had been in a highly opaque ionised form – the same form which was present in the Sun today, and which made its disk opaque. At an age of 500,000 years, this hydrogen had cooled to below 6,000°C for the first time in its history and had been able to form into transparent neutral atoms, like those found in interstellar space today. Even today, 13 billion years later, if a radio telescope was pointed in any direction, and it looked beyond any foreground galaxies which might lie in its field of view, it saw a surface covering the whole sky at a distance of 13 billion light years, where the final emission of this ancient glowing ionised hydrogen could still be seen. This emission was the CMB – originally it had glowed red-hot at a similar temperature to the surface of the Sun, but now it had been heavily redshifted to radio wavelengths by the subsequent expansion of the Universe.
Prof. Hewitt explained that the CMB was of central importance to modern cosmology because it was rather like a photograph of the Universe as it had been in the distant past. Those parts of the sky where the CMB was brighter than average were understood to correspond to regions of the Universe which had had a slightly above-average density of hydrogen at the epoch of recombination; conversely, parts of the sky where the CMB was fainter were understood to correspond with regions of the Universe where the density of hydrogen had been lower. The picture presented in this fashion was of a remarkably smooth Universe: gas appeared to have been spread evenly throughout space at the epoch of recombination, and the formation of structures such as stars or galaxies appeared not yet to have happened. Prof. Hewitt added that this result was a little troublesome: several indicators of the ages of galaxies pointed to their being very old structures – nearly as old as the Universe itself – and so it seemed that they must have formed incredibly rapidly after the epoch of recombination.
The speaker went on to explain the relevance of this to dark matter. He explained that the small variations in density seen in the CMB were understood to have been subsequently amplified to form the structures seen in the Universe today. The enhanced inward gravitational pull of denser regions was understood to have pulled them together and caused them to collapse to form clusters of galaxies. He explained that as the two forces involved in this collapse – gravity pulling the material together and gas pressure pushing it apart – were both well understood, theoretical models could be run on supercomputers to see how the process depended upon the amount of mass in the Universe. The answer was that it seemed to do so very strongly, and so by comparing the structures seen in the Universe today with those seen in the CMB, and trying to build theoretical models to mimic both, it was possible to obtain a remarkably precise calculation of the mass of the Universe.
However, a problem remained. The estimated ages of galaxies could not be reproduced by such models: the collapse of clumps after the epoch of recombination was too slow in the models for galaxies to be formed as rapidly as they seemed to have been in the Universe. The problem could be solved by the introduction of a new kind of material – dark matter – which did not absorb or emit any light. Models showed that the collapse of hydrogen gas into galaxy-like structures had been inhibited prior to the epoch of recombination. Any such collapse, in the process of increasing the density of this luminous ionised hydrogen, would have rapidly increased the corresponding brightness of the light it emitted. The intense radiation produced by the collapsing region would produce an outward force called radiation pressure, which would rapidly overcome the gravitational force causing it to collapse and cause the gas to disperse. After the epoch of reionisation, when the hydrogen gas had become transparent, it had been freed from radiation pressure, and become able for the first time to make significant progress towards collapsing into galaxies.
By contrast, dark matter, if it produced no light at all, would always have been free from radiation pressure, and could have begun collapsing immediately after the Big Bang. After the epoch of recombination, the hydrogen gas would then have fallen very rapidly into the regions where the dark matter had already begun collapsing at earlier times, explaining the very rapid way in which galaxies seemed to have formed.
Such models were now strongly favoured on account of their excellent reproduction of the kinds of structures seen in the Universe today, as well as the observed ages of galaxies. Comparing these models to the latest observations from the WMAP satellite, it had been concluded quite precisely that the Universe contained a mixture of 4% baryonic matter – all of the matter which made up everything we could see in the Universe – 26% dark matter, and 70% dark energy. The speaker explained that very little was known about this final term.
Prof. Hewitt granted the audience that they might feel some justifiable scepticism about these percentages, given that they seemed to be based upon little that observers could easily relate to. He went on, therefore, to describe some experiments which provided independent evidence for the existence of dark matter. The oldest of these involved studying the rotation of spiral galaxies such as the Andromeda Galaxy (M31), which could be thought of as spinning disks of gas and stars. When such galaxies lay in edge-on orientations, as M31 did, their rotation speeds could be measured quite easily by taking spectra of their stars and looking for a Doppler shift. Those stars which were being carried away from the Earth by the galaxy's rotational motion appeared redshifted, meanwhile stars on the other side of the galaxy, that were moving towards the Earth, appeared blueshifted by a corresponding amount. The magnitudes of these red- and blueshifts indicated the rotational speed of the galaxy.
For a galaxy to sustain any given rotation speed, theory said that the centrifugal forces on the stars needed to be exactly counterbalanced by the inward gravitational forces that they felt, otherwise they would either fly outwards, if the centrifugal force was greater, or sink inwards, if the gravitational force was greater. However, this was difficult to reconcile with observations. When the visible stars in galaxies such as M31 were counted, and a mass estimated for each star, it became clear that there was far too little mass present to keep the outer parts of the galaxy rotating at the speeds observed; it seemed a mystery why they didn't fly off into intergalactic space. Even if there were assumed to be some stars hidden behind dust clouds, it was hard to find enough mass. To avoid this fate, it was inferred that galaxies such as M31 must contain huge amounts of material – more than the total mass contained in all of their stars – in some invisible form, termed dark matter.
Further evidence for the existence of dark matter came from studies of gravitational lenses. The speaker explained that sometimes, when a very distant galaxy lay precisely behind another much nearer galaxy – or more normally, cluster of galaxies – the light rays from the distant galaxy were bent by the gravitational attraction of the nearer object as they skimmed past it. The effect was rather as if they had passed through a refracting telescope lens. The amount by which the rays were bent depended upon the mass of the nearer lensing object.
One key difference between telescope lenses and gravitational lenses was that whilst the former were carefully figured to produce focused images, the latter were less painstakingly constructed, and tended to produce multiple distorted images, often stretched out into arc-like shapes. Sometimes, if many images of the distant galaxy could be seen, it was possible to determine a significant amount of information about how the matter in the lensing object was distributed, but in most cases, it was only possible to determine the amount of mass in the lens.
Clearly gravitational lenses could provide valuable information about dark matter, since they provided another independent method for estimating the masses of galaxies. However, the speaker explained that until recently their usefulness had been restricted by the difficulties faced in finding significant numbers of lensed objects on the sky. Traditionally, search programmes for gravitational lenses had operated by compiling databases of galaxies which showed signs of being at very great distances. Optical images were then obtained for each object and checked for any unusual morphological features which might be attributed to lensing. Where such features were seen, their spectra needed to be obtained before they could be conclusively confirmed to be the result of gravitational lensing.
This was labour-intensive work. One of the most successful projects to have followed this strategy had been the CLASS survey, undertaken by the Jodrell Bank Observatory in collaboration with colleagues in Holland and the US. However, even this project had had a remarkably modest yield: after looking at over 11,000 objects over the past decade, it had discovered only 22 confirmed lenses. Part of the problem with these traditional surveys was that they surveyed all distant galaxies, each of which had a very small chance of being lensed. The speaker explained that modern surveys were trying to devise ways to predict which galaxies were most likely to be lensed, and to spend observing time on only the most promising targets.
The speaker closed by describing the SLACS survey, which was searching for lensed objects in the areas of sky around galaxies which were known to be very massive, and which were likely to be large enough to produce lensed images of any galaxies which lay behind them. Instead of looking for objects with unusual morphologies, which were often difficult to spot, the survey looked for evidence of lensing in the spectra of these galaxies. Normally, the spectra of massive galaxies were remarkably uniform, but galaxies whose light was entangled with that of lensed images of more distant objects had an assortment of unusual features added to their spectra.
Once a database of massive galaxies with strange spectra had been compiled, these objects were imaged by the Hubble Space Telescope (HST)'s Advanced Camera for Surveys (ACS) to search for unusual morphologies to clinch the case. This search strategy was still labour-intensive – taking spectra of galaxies was a slow process – but it was aided by the work of the pre-existing Sloan Digital Sky Survey (SDSS), which had been operating a dedicated 2.5-m telescope in New Mexico for the past eight years. One of the SDSS's principal activities was taking spectra and images of distant galaxies, and it was releasing all of its data into the public domain. To date, it had obtained spectra for 1.3 million galaxies. Using this data, the SLACS survey had been able to find several hundred lenses, at a rate of several tens for every night of dedicated observing time required. In due course, this huge new sample of gravitational lenses would provide substantial new insights into the amount of dark matter in galaxies and its distribution within them.
Following the applause for Prof. Hewitt's talk, the President introduced the evening's second speaker, Dr Carolin Crawford, from the X-ray Astronomy Group of the Institute of Astronomy, Cambridge.