Radio astronomy at decimetre and centimetre wavelengths has not reached any fundamental sensitivity limits. New astronomical thresholds beckon to be crossed as the capabilities of existing telescopes, such as the LT, are systematically enhanced by a planned programme of technological developments.
Large radio telescopes have a distinguished record of fundamental discoveries. To give just a few examples, the Lovell telescope and the Parkes telescope in Australia played vital roles in the discovery of quasars, while the Lovell telescope made the observations which led directly to the discovery of gravitational lenses. The Arecibo telescope discovered millisecond pulsars and, in Nobel prize-winning work, established the energy loss in a stellar binary system due to gravitational radiation; it also found the first extra-solar planetary system (around a pulsar). None of these discoveries could have been anticipated at the time that these telescopes were being planned. Two broad lessons can be drawn from history. The first is that general purpose instruments, with at least a decade of frequency coverage to ensure a wide range of the scientific impact, have dominated the discoveries made by radio astronomy. The second is that raw sensitivity is a certain path to success.
The contribution of the LT to astrophysics remains outstanding. Recent scientific highlights include:
As a stand-alone instrument:
The methanol maser transition at 6.7 GHz is particularly important. It is one of the very strongest cosmic maser emission lines and masing appears to take place over a broader range of physical conditions than for other molecular species. Methanol masers are usually associated with compact HII (ionized hydrogen) regions and infrared sources which are pointers to sites where massive stars are being formed. In broad terms these masers signpost warm (100-400K) dense gas in star-formation sites. Methanol therefore provides the link between the cold (~10 K) molecular phase of the interstellar medium and the ionised photosphere of the protostar (1000s K). These masers are also found where there are no other signposts of star-formation, suggesting that methanol masers may be the first general indicators that star-formation is occurring.
A survey for methanol masers is, therefore, the best and possibly the only feasible way to take a complete census of star-formation in the Galaxy. As the 6.7 GHz maser is a relatively recent discovery (1991) there has not yet been a systematic survey of the whole Galaxy. In the northern hemisphere, there have only been targetted searches of previously known star-forming sites made with relatively modest sensitivity (approximately 1 Jy). There is a compelling case for carrying out such an untargetted survey with the upgraded LT using a dedicated multibeam receiver (at least 7 beams) and a suitably large spectrometer (14 inputs, each of 2000 spectral channels, covering 10 MHz with a spectral resolution of approx 5 kHz). NRAL has extensive experience in constructing and operating multi-beam receivers and spectrometers and hence is ideally placed to exploit this new opportunity to the maximum.
With such a powerful combination of telescope and instrumentation, the definitive survey for sites of massive star- formation in the Galaxy could be carried out. One year of observation (integrated over several calendar years) with a sensitivity of 0.1 Jy would yield over 1000 new sources. The catalogue would provide a resource for many subsequent investigations, including
The upgraded LT would also have the sensitivity to detect methanol masers in nearby galaxies.
The multibeam system would also have unique capabilities for surveying other important molecular lines of OH and formaldehyde in star-forming regions, as well as the hydrogen recombination lines from the ionized gas (HII regions) around the massive young stars.
While the physics of pulsars themselves is a fascinating study, most science has probably come from their use as tools for measuring and testing other natural phenomena. No other class of object in the Galaxy can claim the extreme properties of these exotic stars which have been used to probe many fields of physics, often with unbelievable precision. For instance, the spiral-in of two neutron stars in the binary system 1913+16, resulting from the loss of energy by gravitational radiation, is about 1 mm per orbit and yet it has been determined with an accuracy of better than 1%. We shall briefly outline some other examples of the uses of pulsars.
Most of the studies of pulsars have been carried out by the LT and two other large radio telescopes around the world (at Parkes and Arecibo). The first stage in such work is the search for new pulsars, exploring new regions of parameter space, such as short period, low flux density and orbital acceleration. Modern searches are limited by raw sensitivity and by data processing technology. The past decade has seen the completion of surveys led by Manchester astronomers, the 400-MHz all-sky surveys for millisecond pulsars at Parkes and Jodrell Bank (e.g. Manchester et al. 1996, Lyne et al. 1998), and the start of a new 1400-MHz multibeam survey of the galactic plane primarily for young and distant pulsars. The all-sky surveys were aimed specifically at discovering millisecond pulsars and in this they have been remarkably successful, resulting in the discovery of 102 new pulsars, of which 19 were millisecond pulsars, compared with a total of 4 which were known at the start in 1992 (e.g. Lorimer et al. 1996, Bailes et al. 1997). Figure 1 illustrates the advance in recent years, which was due in large part to this work.
Figures 1 to 3|
The galactic distributions of known millisecond and binary pulsars. The period variations in PSR J1741-3056. The Crab pulsar observed at the end of 1997.
The primary task of the LT lies in the follow-up observations of the pulsars discovered in the surveys, firstly in establishing evidence for any orbital motion and then measurement of the positions and period derivatives and any orbital parameters (e.g. D'Amico et al. 1998). More detailed observations are conducted on the more interesting objects such as the binary and millisecond pulsars. Most of the special pulsars discussed below were found serendipitously in untargetted searches. One cannot anticipate the nature of future discoveries.
In the past 10-15 years, further highlights of the work involving the LT include:
We also propose to increase the precision of pulse timing measurements by using wide-bandwidth receivers between 2 and 5 GHz. The use of the high frequency will mitigate the propagation effects and the wide bandwidth will increase the sensitivity required to make the most of these wonderful cosmic clocks.
The best way to carry out the high-frequency survey is in conjuction with the 25m Mk2 telescope located ~450m away from the LT on the Jodrell Bank site. The two telescopes can be operated as a very wide bandwidth, and hence sensitive, interferometer. The factor of six higher angular resolution provided by this 450-m baseline compared with the LT alone provides two advantages. Firstly, the LT-Mk2 survey will automatically provide accurate (1 arcsec) positional information which is required for the identification of the sources and subsequent multi-wavelength follow-up. Secondly, the higher resolution makes the survey much less prone to "confusion" arising from the inability to distinguish between closely-spaced faint sources. The ability to make such a faint-source survey in an interferometer mode is unique to the LT-Mk2 combination.
The new LT-Mk2 survey would reliably reach ten times deeper (complete down to a few mJy) than existing surveys. It would use the same multi-beam receiver on the LT as the molecular masers survey described above. The new survey is bound to throw up many individually interesting new sources and in particular many new active stars, as well as being a valuable starting point for a multitude of other investigations in the years to come. There will be many synergies with surveys in other wavelength regimes. It is notable that the discovery of the first gravitational lens (0957+561A,B) was a direct result of an unbiassed radio source survey carried out with the LT (Walsh et al. 1979).
The stellar radio luminosity function is poorly known, though it is clear that only a small fraction of stars of all types have been detected in the radio. A complementary, targeted survey of ~1000 stars with known distances, going to even fainter flux levels (~100 microJy) than the large-area survey described above, can be carried out with the LT-Mk2 combination. This survey would make a major contribution to understanding which types of star show radio emission at a given luminosity.
The MERLIN National Radioastronomy Facility and the European VLBI Network (EVN).
MERLIN is uniquely placed to satisfy the consequent demand for < 0.1 arcsec imaging in the radio and indeed already supports a broad range of astronomical programmes for its users. These include: fundamental astrometric reference frames; velocities of pulsars; distances in the Galaxy; young stars and their outflows; stellar mass loss and circumstellar environments; planetary nebulae; colliding winds in binary star systems; ``micro quasars''; novae; starburst galaxies; extragalactic mega-masers; Seyfert Galaxies (both continuum and line absorption studies); radio quiet quasars; optical and radio jets in active galactic nuclei; radio galaxies; distant star-forming galaxies in the Hubble Deep Field; gravitational lenses.
VLBI: In addition to operating MERLIN, the National Facility also plays a major role in the European VLBI Network (EVN), in particular by contributing data from radio telescopes at Jodrell Bank (including the LT) and Cambridge. The EVN is run by a consortium of institutes which operate telescopes in both Europe and Asia. Since its formation in 1980 the EVN has grown to include 12 active institutes with 16 radio telescopes in 10 different countries. The EVN telescopes operate as a Network for up to 12 weeks per year, a part of which also involves simultaneous observations with MERLIN.
The capabilities of European VLBI are currently undergoing a radical upgrade in sensitivity. At the heart of the change is the new 1 Gbit/sec data acquisition system, coming into operation in 1999, and the wide-band data processor (correlator) currently being commissioned at the Joint Institute for VLBI in Europe (JIVE) and due to come into routine operation in late 1999. [For further information on the EVN see the JIVE WWW pages at http://www.nfra.nl/jive.]
The three large instruments at Jodrell Bank, Effelsberg (Germany) and Westerbork (The Netherlands) form the highly- sensitive heart of the EVN and this `closure triangle' of telescopes enables it to study fainter radio sources than are accessible to other arrays. However the LT can presently only contribute to at wavelengths longer than 18cm. Upgrading the LT will, therefore also have a major impact on the the scientific potential of the EVN at the international standard frequency of 5 GHz and at the frequencies of molecular maser emissions.
Some of the outstanding new research programmes which become possible when the upgraded LT is included in MERLIN and the EVN are now described.
There are several recent indications that magnetic fields play a key role in star formation, both in helping to remove angular momentum from collapsing molecular clouds and later in channelling the outflow from YSOs (e.g. Hutawarakorn & Cohen 1998). Recent developments in radio interferometry have opened up the possibility of using masers to probe the three-dimensional structure of the magnetic field in starforming regions. The Zeeman effect can be used to measure the field strength and the linear polarisation enables the direction of the magnetic field vector to be determined. Some of this work has already commenced but, because the polarized intensity is only a fraction of the total intensity, such obserations invariably have limited signal-to-noise ratio. The inclusion of the LT in MERLIN and the EVN will therefore have a major impact on studies of cosmic magnetic fields.
Excited OH: The excited OH lines at 6 GHz are particularly valuable for Zeeman investigations and the first EVN maps of these masers have recently been made (Desmurs et al. 1998). The source W3(OH) yielded 37 Zeeman pairs, which indicate a large-scale order to the magnetic field. It should be noted that the EVN is the only VLBI network with 6-GHz capability. The excited OH lines at 4.7 GHz, which are accessible to both MERLIN and the EVN, were thought to be unpolarized but recent observations have proven this to be wrong. There is a great opportunity for MERLIN and EVN to map out the magnetic field in star-forming regions by means of multi-transition studies of OH masers. Different evolutionary stages (bipolar outflows, cometary HII regions, and ultra-compact HII regions) can be studied.
Methanol: Recent pioneering observations of methanol masers with the EVN and US Very Long Baseline Array (see Figure 5) have revealed disks around high mass stars. If Keplerian rotation about a central mass is assumed, the data are consistent with central masses in the range 10-100 times solar. Although the formation of disks of dust and gas around low (~1 solar mass) stars is considered to be a normal feature of their formation, this is the first direct evidence of such disks around young massive stars. Disk-like morphology has also been seen with the the Australia Telescope Compact Array and recent sensitive infrared observations at 10 microns of G339.88-1.26 have revealed an elongated structure consistent with a dust disk around this hot massive star (see Figure 5). The putative dust disk is aligned along the axis defined by the methanol maser emission. It is important to note that some models of star and planet formation predict that protoplanetary disks will not survive around massive stars. These methanol maser results show that those models are wrong - disks can survive the intense stellar winds and radiation.
Newly formed stars are expected to be surrounded by rotating disks of molecular gas. Radio frequency spectral lines from methanol appear to be the best tracers for such disks.
In summary: MERLIN and EVN measurements of methanol masers, both on their own and in combination with other instruments, can provide the most detailed picture yet of part of the evolution of star and pre-planetary disk formation from early stages to the point at which the star switches on. This maser work is complementary to the extensive programme of studies of star-formation proposed for the international mm-wave array which will study cooler material at somewhat lower resolution.
Star-formation: The mechanism driving the mass loss from Young Stellar Objects (YSO), and its role in reversing the infall stage of star formation, is not known. Determining the geometry of the outflow will be a vital step towards answering this question, as a surprising variety of disc-like and jet-like structures have already been seen. The ionised gas in the wind is expected to recombine to neutral material at radii of about 100 AU (just over twice the size of the Solar System). This is equivalent to 0.1 arcsec at a typical distance of 1 kpc, and so this can be resolved by MERLIN at 5 GHz but only a few of the known luminous massive YSOs are currently bright enough to map with MERLIN.
The multiple star formation areas of the Orion nebula are the closest regions where we know that massive stars are being formed and hence they are intensively studied. The richest and densest young cluster in Orion is the Trapezium Cluster, whose hottest and most luminous member, Theta 1 Ori C, powers the Orion Nebula (NGC 1976). As the Trapezium cluster is one of the youngest (105-106 years) known ensemble of stars, and is swept clean of obscuring ambient matter, it offers a unique opportunity for finding and understanding phenomena related to star and planet formation. The HST and MERLIN are already playing a complementary role in this study but the inclusion of the upgraded LT will allow MERLIN to have a much greater impact on this fundamental work.
Early Hubble Space Telescope images revealed that many low mass stars in the Trapezium cluster possess disks comparable in size to our planetary system, surrounded by envelopes extending to sizes of 900 AU. These dense, dusty regions are silhouetted against the background optical emission of the Orion Nebula. The majority of these disks are photo-ionized on their outer surfaces by the hard radiation from Theta 1 Ori C and are therefore detectable at radio wavelengths. These have been dubbed `proplyds'. This HST identification (see Figure 6) built upon earlier observations in the optical, near infrared and radio. Proplyds are now believed to be a distinct class of young low-mass stars accompanied by material of the type thought to give rise to planets.
An HST image of the Trapezium cluster in the Orion Nebula which reveals the `proplyds' with elongated tails.
A broad-ranging observational programme at high angular resolution (0.1 arcsec) is in progress to reveal vital aspects of the `proplyds'. HST/STIS observations are to be made to determine the mass loss rates and exact nature of the flowing ionized gas. Maser sources are being sought in the protoplanetary disks and their discovery would be the key to investigating the conditions prevailing in this environment. MERLIN is capable of imaging the thermal bremmstrahlung from the ionized gas of the `proplyds' with signal-to-noise ratios and angular resolution comparable with the HST images. The shapes of their ionized structures, unaffected by the absorption by intrinsic and residual ambient dust, are just starting to be revealed in initial `pilot' observations. Recognising the promise of such radio observations a proposal to obtain a deep integration with MERLIN at 6 cm of a 2 arcmin diameter area of the core of the Orion Nebula (M42), at a resolution of 75 mas, has just been awarded Key Programme status. Within three years, with the upgraded LT, this Key Programme could proceed almost ten times faster than is currently possible, thus revealing the structure and key physical parameters of all of the ionized gas in this uniquely important cluster of YSOs.
Hot massive stars: MERLIN observations of the Luminous Blue Variable P Cygni (Skinner et al. 1997a) have fully resolved the core of the stellar wind, showing peak brightness temperatures of 14000-18000 K. Continued monitoring shows rapid (less than a month) variability of small clumps in the outflowing wind, which is not yet well explained. No other star has been similarly studied. The proposed increase in the sensitivity of MERLIN would bring at least another four similar stars within reach. In P Cygni the clumps in the stellar wind could be tracked further from the star. By comparison with stellar rotation rates, these observations will also reveal the effects of rotation in flattening radiatively- driven winds, and test the proposition that the appearance of Be stars is due to equatorial shocks around rapid rotators.
Red Giants: Apart from the Sun, radio emission from a region close to the stellar surface has only been mapped for one star, Alpha Orionis (Betelgeuse). The instruments used were MERLIN and the Very Large Array (VLA) (Skinner et al. 1997b; Lim et al. 1998). A handful of other red giants and supergiants can be detected but not mapped by the VLA since its resolution is too low. With the upgraded LT, MERLIN will be able to produce detailed maps of the photospheres of these giant stars. For those red giants with circumstellar masers, near-simultaneous mapping of the masers and the stellar continuum will provide new insights into the nature of the stellar pulsations and the physics of mass loss.
Novae and interacting winds: All novae are thought to arise from a thermonuclear runaway in mass accreted onto the surface of a white dwarf from an orbiting companion. In some cases these may be Type 1 supernova precursors.
In classical novae the donor is a main sequence dwarf star like the Sun. Since 1992, MERLIN has provided the earliest radio images of the novae ejecta, notably V1974 Cyg, V705 Cas and V723 Cas, and followed their evolution for several years. However the very earliest and latest phases are too faint to study given the present sensitivity. With the upgraded LT included in MERLIN, it will be possible to investigate all these astrophysically interesting phases of the stellar explosion.
Radio observations of symbiotic stars are fundamentally important to the understanding of the interaction of stellar winds in binary systems, the impact of ionising flux on the circumstellar material, and the effects of the orbital rotation on the whole system. Once this is understood we can directly investigate further the thermonuclear runaways on the surface of the dwarf component. At present detailed radio maps can only be made of five such stars (e.g. Eyres et al. 1996; Richards et al. 1998). In some cases radio peaks are resolved on the scale of the binary orbit and proper motions can be measured. The resolution of MERLIN at 5 GHz is similar to that of the HST in the ultraviolet and large, complementary observing programmes with the HST are currently underway. Comparison of the HST and MERLIN images will enable us accurately to locate the stars in relation to their winds. A three-fold increase in sensitivity at 5 GHz will allow us to image all of the ~25 such stars which have currently been detected in the radio. The nebulae around quiescent symbiotic stars can be observed for the first time and their properties disentangled from the effects of outbursts. Only MERLIN enhanced with the upgraded LT will have the required combination of sensitivity and resolution for this work.
Wolf-Rayet stars (WR) are very hot (40,000 K), massive, highly evolved stars containing more heavy elements such as carbon or nitrogen than they do hydrogen. They lose mass very rapidly and in some cases this material collides with the wind from a companion. Complementary MERLIN and HST images (Williams et al. 1997; Niemela et al. 1998) solved the puzzling simultaneous detection of both thermal and non-thermal emissions from WR 147 by showing that they came from different locations in the binary star system (see Figure 7). However it is still not clear how important the presence of a binary companion is to the occurrence of this phenomenon or at what stage in late stellar evolution it occurs and for stars in what mass range. Continuing investigations require studies of a larger sample than is accessible at present. The increased sensitivity at 5 GHz with the LT in MERLIN will help to achieve this since at least 10 more WR stars will come within its range. Detailed observations will show shocks and structural changes in the winds at different locations, and these may be causally related to changes in the stellar mass loss rates.
MERLIN maps of WR147 from which variations in both the thermal and non-thermal components can be derived. A model of the WR147 colliding wind system.
RS Cvn stars and Algol-like binaries emit by the gyro-synchrotron mechanism from electons whose energies vary from a few eV to a few MeV. These sources exhibit both flaring and quiescent behaviour in the radio. Radio eclipse mapping of RS Cvn and CF Tuc (Gunn et al. 1997) shows that the radio emission comes from the region between the two stars, as expected on the interaction model. However early high resolution VLBI measurements during a flare on UX Ari showed emission from the two stars as well. More recent observations show a more complicated situation with emission from both between and outside the stellar orbit. The basic problem is that the emission is weak and it is only during fortunate circumstances, such as a strong flare, that detailed measurement can currently be made.
Radio Emitting X-ray Binaries and Transients: The effects of mass exchange are even more dramatic when one of the stars in the binary system is degenerate and compact. X-ray emission results when the compact star is a neutron star or a black hole, and the release of gravitational energy by accreting matter onto the compact star results in temperatures of 107K or more. Some 200 or so X-ray binaries are known in our galaxy, of which about 15% have radio emission (the radio emitting X-ray binaries or REXRB). In addition X-ray transients occur, during which the X-ray emission can rise by a factor of 104 over several days or weeks. These X-ray novae have radio flares which are correlated with complex changes in the X-ray spectral states and which may be associated with advection dominated collapse of the inner parts of the acretion disk. Because they can be thought of as small-scale analogues of active galactic nuclei, X-ray binaries are sometimes called `micro-quasars'. Their study has opened up the possibility of gaining insights into phenomena which are common in active galactic nuclei, with the advantage that the characteristic dynamical timescales vary as the mass of the compact object. Thus events happening over days in a micro-quasar correspond to analogous phenomena occurring over thousands of years in a galactic system containing a 108 solar-mass black hole.
Like quasars many REXRB have collimated radio jets, and so the unstable infall onto the neutron star or the black hole is associated with the high velocity collimated ejection of hot plasma. The radiation is non-thermal requiring electrons with energies of >100 Mev in strong magnetic fields. The energy required to form the radio jets in the transient outburst is several Eddington luminosities for a 10 solar mass object, which poses a severe problem for the physical models of these intriguing objects. Many of the radio sources are weak and difficult to observe and only the more dramatic sources have been studied to date.
The addition of the upgraded LT to MERLIN and EVN will give a huge boost to research in this area. These instruments are ideal for studying the evolution of objects in our Galaxy, when the relativistic expansion speeds mean that inter-continental VLBI has too much resolution to be able to follow the event, while the VLA can only follow the longer lived outbursts when the components have already moved further out. This was well demonstrated by our recent observations of GRS 1915+105 (Fender et al. 1999) with MERLIN. The increased sensitivity will enable the 20 or so weaker REXRB to be studied and also allow a search for jets in these objects. It may well be that all the REXRB have radio jets - only the new instrumentation with the improved Lovell telescope will be able to find out.
The SNR in a starburst are all at essentially the same distance and hence can be observed with the same linear resolution and sensitivity. Their luminosity and size distributions can thus be measured and their variabilities compared directly. One starburst galaxy, M82, has been studied in great detail in the radio (see Figure 8). MERLIN and EVN observations of the remnants in M82 (Pedlar et al. 1999) have revealed the expected shell-like structures and by measuring the changing sizes over several years the expansion velocities of some of the more compact SNRs have been measured directly (see Figure 8). They are typically ~10,000 kms/sec which implies that these remnants are only a few decades old.
MERLIN and VLA radio images of the central region of the nearby starburst galaxy M82 and individual supernova remnants.
The addition of the upgraded LT into MERLIN and the EVN at 5 GHz will enable more starbursts to be studied at high resolution in an obvious future Key Programme. The strongest SNR in more distant starbursts can be detected to >100Mpc and hence a comparison of the IR, radio continuum and SNR methods can be made on several hundred galaxies with high star-formation rates. The calibrated total-radio-continuum technique can then be used with confidence to cosmological distances - indeed the majority of radio sources in the Hubble Deep Field (see below) are starburst galaxies. Radio observations with the upgraded LT in MERLIN and the EVN can therefore provide an improved estimate of the cosmological star-formation rate.
The Hubble Deep Field overlaid by contours of radio intensity and a radio source in the field-of-view of MERLIN but just outside the HDF
Almost all the radio sources in the HDF are resolved in the MERLIN+VLA images showing that the sources have angular sizes in the range 0.2 to 2 arcseconds, about the same as the parent galaxies imaged by the Hubble (see Figure 9). The properties of ~70% of these faint sources are consistent with starburst events in large disk galaxies in the redshift range 0.4 to 1, while ~20% seem to be low-luminosity Active Galactic Nuclei in giant elliptical galaxies at redshifts close to 1. However ~10% of the radio sources are not visible in the optical; recent sensitive infrared observations suggest that many of these objects will turn out to be dust-shrouded starburst galaxies at redshifts >3. Without the LT it would have taken over 100 days to acquire enough data to make a radio image of the same signal-to- noise ratio. It is not possible to dedicate such a huge amount of the MERLIN National Facility time to a single project and hence, without the LT, MERLIN would have been unable to contribute to the world-wide study of the HDF. As a result of the success of this programme proposals to observe two other deep fields at 20 cm by independent UK teams have recently been given `Key Programme' status by the PPARC PATT Time Allocation Committee. The proposed upgrade will enable the LT to join MERLIN in making such deep field observations at 6 cm is currently available at 20 cm and with three times higher resolution. These new radio images will enable the properties of ultra-faint distant star- forming galaxies to be better understood.
Figures 10 & 11|
A MERLIN radio picture at 20-cm wavelength of the first gravitational lens to be discovered, 1956+561. An infrared picture of the gravitational lens system 1933+504 made with the NICMOS camera system on the Hubble Space Telescope.
The foundations of the best observational work in the radio have been surveys using the VLA by a Manchester-led consortium; almost 15000 radio sources have been examined. All candidate lenses have been diligently followed-up with MERLIN, working at 6cm wavelength, providing the vital high-resolution imaging. One of the scientific highlights of MERLIN in the 1990s has been this pivotal role in the discovery of 17 new arcsec-scale gravitational lens systems (Browne et al. 1998).
Using the Very Large Array good progress has been made on a few objects (e.g. Biggs et al. 1999) in monitoring variations in the lensed images - but the VLA is far from ideal for this work since it does not stay in the high resolution A configuration for long enough. The constant availability and enhanced sensitivity will make MERLIN the instrument of choice for measuring time delays in gravitational lenses.
Obtaining accurate mass distributions of the lensing galaxy is a vital part of the Hubble constant determination. In order to solve the `optics' of each lens, detailed pictures of the lensed images are required (see Figure 11). It is by mathematically modelling the varying degrees of distortion from image to image that the lensing galaxy's mass distribution is obtained. Compared with optically selected lens systems, those selected in the radio have one great advantage; the individual images can be mapped with milliarcsecond resolution with VLBI. With sufficient detail the matrix elements which transform one image into another can be deduced and compared with model predictions.
For this detailed imaging, sensitivity is at a great premium. The extra sensitivity provided by the upgraded Lovell in VLBI arrays will therefore open up many more lenses to study.