When does a gravitational lens appear

Radio studies of gravitational lenses

A.R. Patnaik, R.W. Porcas, C. Henkel and K.M. Ments

Gravitational lenses and their properties. Particularly compact accumulations of matter in the universe can deflect electromagnetic radiation, e.g. light and radio waves, in such a way that a distant background object is imaged several times, like with a lens. The observer sees two or more images of the same object with identical spectral properties, with the generated image of the background object being enlarged or reduced and additionally distorted, depending on the geometry of the system. Images that appear on opposite sides of the lens are reversed.

This so-called gravitational lensing effect occurs in different size scales, depending on whether it is caused by stars, galaxies or even galaxy clusters. The background objects imaged several times by the lens are distant other galaxies in the early phases of their development or quasars, which often show strong radio emissions.

The study of gravitational lenses is of great interest to astronomers in many ways. On the one hand, there is often an enlargement of the background object. This fact makes it possible to examine in detail objects that are located at very great distances and thus in considerably earlier epochs of the universe. On the other hand, estimates of the mass distribution of the distracting object are possible. It is also of great interest that observations of gravitational lens systems in principle allow a determination of the Hubble constant H0 and thus the age of the universe. (Due to the expansion of the universe since the Big Bang, radiation from objects at cosmological distances is shifted to longer wavelengths due to the Doppler effect. The distance of an object is given by (c / H0) z, where c is the speed of light and z is the observed redshift. ) If, for example, the background object is variable over time, as there are many quasars, then because of the different optical path lengths (and thus transit times) a change in its intensity will be observed in the different images at different times. The measurement of this time delay in connection with a model of the lens enables a direct determination of the distance of the lens and thus enables the Hubble constant to be determined. Compared to other methods, this new technique has fewer systematic sources of error.

Radio observations with high and highest spatial resolution provide detailed images of gravitational lenses. With the Very Long Baseline Interferometry (VLBI) method, a worldwide network of radio telescopes, angular resolutions in the milli-arcsecond range are achieved. The radio images generated by means of VLBI observations allow tests of the theory of gravitational lenses and provide the basis for models of the lenses.

The following two examples are intended to illustrate the great potential of radio studies of gravitational lenses. In particular, a particularly interesting, new branch of research is presented: The discovery of spectral lines of different molecules of the imaging object in absorption against the radio emission of the background system.

B0218 + 357 - A picture book example. Many of the properties mentioned at the beginning can be illustrated using the example of the radio source B0218 + 357, the smallest gravitational lens system known to date. This source consists of two compact radio sources at a distance of 335 milli-arcseconds from each other (approx. 1/5000 of the moon's diameter), and a so-called Einstein ring, which has the same diameter. Such a ring is created when the source is directly behind the lens as seen by the observer. Einstein predicted this phenomenon as early as the 1930s. However, he estimated the possibility of his observation to be very small, since he only considered individual stars as lenses and not, as in the case of B0218 + 357, a galaxy with a mass of 100 billion (1011) solar masses.
The picture above right in Fig. 1 shows a radio map of B0218 + 357, which was generated with MERLIN, a radio interferometer consisting of several individual telescopes in England, with an angular resolution of 50 milli-arcseconds. The interpretation of these observations shows that the background quasar consists of a compact core and an elongated outflow (jet). The core is shown in the two compact images A and B and the jet in the Einsteinring.

The Very Long Baseline Array, which consists of ten different telescopes in the USA, was used to examine the fine structure of the A and B sources, which, as required by the gravitational lens theory, both turn out to be double sources, with a resolution that is a hundred times higher. The provisional result of the time delay of the variability of the two sources, which is important for the determination of the Hubble constant, gave a still rather uncertain value of 12 days.

Molecular gas in gravitational lenses. In the case of B0218 + 357 the lens is a galaxy with a relatively high redshift (z = 0.69), which is very difficult to observe directly. However, it is relatively easy to observe interstellar gas in this galaxy in absorption against the very intense radio radiation of image A of the background quasar. The surprising discovery of molecular gas in B0218 + 357 and in another radio-gravitational lens system 1830-211 (z = 0.89, see Fig. 2), enables investigations of the interstellar medium of galaxies in early epochs. At these redshifts, molecules are observed when the universe was only a third of its present age. Fig. 1 bottom left shows a spectrum of the J = 1-0 rotation line of the CS molecule, which was observed with the 100 m radio telescope in Effelsberg. Fig. 2 shows a radio map from 1830-211 (note the mirror symmetry of the images!) Together with the spectra of various molecules that were observed with the Very Large Array of the National Radio Astronomy Observatory in New Mexico, which consists of 27 radio antennas. From these observations the relative abundances of the molecules, and in some cases even of isotopes (see H.12CO+ and H13CO+ in Fig. 2). Surprisingly, it turns out that both molecular and isotope frequencies are very similar to the values ​​found in our Milky Way.

Max Planck Society Yearbook 1997. Copyright © 1997 Max Planck Institute for Radio Astronomy.

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