Gravitational Lenses

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284 Nature Vol. 295 28 January 1982 to the photobleaching measurements, provide a useful alternative to explain the observed surface density of some 25 per cent of the bungarotoxin conventional viscometric techniques. quasars. Since only a minority of the acetylcholine receptor complexes were Further, the association of fluorescently galaxies we see locally appear capable of immobile on the time scale of the derivitized actin with existing fibres in live producing large amplifications, the mean measurement, another finding not cells is being examined at the Weizmann density of the Universe has to be very high confirmed by Poo's new experiments. Institute using a combination of micro if the model is to work. Success in pre These discordant results raise the injection and photobleaching techniques. dicting d log NQ/dmQ is not sufficient to possibility that the macromolecular In another novel application, investigators remove these doubts. ligand, a-bungarotoxin, has retarded the at the University of North Carolina and In the second of the papers Avni 6 lateral mobility of the native receptor either Penn State University have shown that the addresses a closely related point. He by direct interaction with other cell surface diffusion of macromolecules, such as lgG discusses Turner's suggestion that ampli components or, indirectly, by enhancing and bovine serum albumin, microinjected fication by gravitational lenses may the ability of the receptor to interact with into the cytoplasm of living fibroblasts is significantly perturb the measured other cellular components. Alternatively, retarded 70-fold compared with aqueous statistics of quasars, perhaps accounting the rate of diffusion might be reduced by a buffer values; these results suggest that the for the apparent increase in the number photochemical process unique to this cytoplasmic matrix hinders the free density of quasars with increasing redshift. labelled ligand-receptor combination in diffusion of such molecules sterically Quasar analyses are customarily per the muscle cell membrane. and/or through binding interactions formed on flux-limited samples, which will While photobleaching techniques have between the diffusant and the matrix. preferentially include quasars whose flux so far been largely directed towards These emerging applications of the has been amplified by an intervening gravi problems in membrane biology, an exciting photobleaching techniques to the study of tational lens. Avni's improvement on development has been their application to molecular movement both within cell Turner's model is to require average flux the study of macromolecules in solution membranes and within the cytoplasm conservation, which implies that the mean and within the cytoplasm of living cells. At suggest a great potential for this approach amplification has to be unity. This is an Syracuse University, photobleaching tech to provide unique dynamic information on important, obvious requirement which niques are being used to study the cellular structures, particularly when these considerably reduces the effect of lenses on polymerization of actin monomers in measurements can be interpreted in the the quasar statistics below that computed solution by monitoring the growth of the context of available biochemical and by Turner. A vni correctly concludes that it immobile fraction with time and should ultrastructural data. D is unlikely that the gravitational lens effect can account for the apparent evolution of the density of quasar with redshift. The third paper takes a different direction. Paczynski and Gorski' suggest that a cluster of quasars 8 at redshift 2.05 Gravitational lenses may be another example of multiple imaging by a gravitational lens. However, from Charles Alcock there are some difficulties - the objects have a large separation (minutes, rather ONE of the most exciting recent speculative suggestions involving gravita than seconds, of arc) and their redshifts are developments in astronomy has been the tional lenses. In the first, Tyson5 revives an different (by about 103 km s· 1). The discovery of multiple imaging of quasars. old suggestion of Barnothy and Barnothy authors produce model lenses consisting of There are three confirmed examples 1•3 of that quasars may be the nuclei of Seyfert two large clusters of galaxies which can this phenomenon, and in one case, the galaxies and appear very luminous because account for the large angular separations. gravitational lens responsible for the an intervening gravitational lens has ampli The model cannot account for the redshift multiple imaging has been detected4 • It is a fied the flux from the nucleus. The differences but the time delays between the large elliptical galaxy near the centre of a apparent increase in the density of quasars three images are 102-103 years, and it is cluster of galaxies. with increasing redshift can then be suggested that the quasar's redshift might The great virtue of the phenomenon is accounted for by the larger probability that change during this time interval. that, by the standards of extragalactic amplification will occur if the Seyfert The velocity dispersions of the two astronomy, it can be fairly unambiguously galaxy is at a greater distance. What is new clusters of galaxies in the models of interpreted. In each case the optical in Tyson's approach is that he uses his own Paczynski and Gorski are large(> 1,500 km emission line spectra of the different data (obtained using automated pattern s· 1). They point out that if there is a positive images are essentially the same and have recognition techniques) on the number cosmological constant the necessary identical redshifts. density of galaxies on the surface of the velocity dispersion decreases. This suggests The general properties of gravitational celestial sphere as a function of measured that, should the clusters be discovered and lenses are easy to understand. A galaxy or flux to calculate the logarithmic slope of their velocity dispersion measured, one cluster of galaxies will bend light rays the number density of quasars as a function could then estimate the cosmological through small angles, producing (in of measured flux. The answer, d Jog constant. general) an odd number of images of a NQ/dmQ = 0.9 (where mQ = 2.5 log flux) It is clear that gravitational lenses can in background quasar. Usually there will be is within the range of observed slopes principle do many remarkable things. The only one image, but fortuitous projection (0.86-0.95). studies described here are very speculative, of a quasar behind a galaxy can result in The primary difficulty with this model is and definitive answers await further work. three or more images. The image bright that the large (>15 times) amplification nesses differ from the brightnesses that required is difficult to obtain with gravi I. Walsh, D., Carswell, R.F. & Weymann, R.R. Nature would be observed if the lens was absent tational lenses. Furthermore, high ampli 279, 381 (1979). because the beams are focused. fication should 2. Weymann, R.T. eta/. Nature 2115, 643 (1980). be accompanied by 3. Weedman. O.W., Green, R.F. & Weymann, R.J. Nature Three papers in a recent issue of the multiple imaging but multiple images have (submitted). Astrophysical Journal explore some more only been detected in three cases out of the 4. Young, P.J., Gunn, J.E., Kristian, J., Oke, J.B. & Westphall, nearly 1,000 known quasars. In addition J .A. Astrophys J. 241, 507 (1980). Charles Alcock is Associate Professor in the 5. Tyson, J.A. Astrophys. J. Lett. 248, L89 (1981). there would need to be a large number of 6. Avni, Y. Astrophys. J. Lei/. 248, L95 (1981). Department of Physics, 6-207, Massachusetts 7. Paczyilski, B. & Gorski, K. Astrophys. J. Lett. 248, LIO! Institute of Technology, Cambridge, distant galaxies, at present undetected, (1981). Massachusetts 02139. capable of producing amplification to 8. Burbidge, E.M. et al. Astrophys. J. Lett. 242, L55 (1980).