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Hints About Dark, Light-Bending Matter in the Distant Universe

New infrared observations of a gravitational lens

25 Luglio 1997

About 20 cases of gravitationally lensed (GL) quasars are known. This special physical effect, also known as a cosmic mirage, occurs when the rays of light of a distant quasar on their way to us pass near a massive object, for instance a galaxy. As a result, two or more images of the same quasar will be seen near each other. This phenomenon is described in more detail in the Appendix. A new study by a group of three European astronomers, headed by Frederic Courbin ( Institut d'Astrophysique, Universite de Liege, Belgium, and Observatoire de Paris-Meudon, France) [1], has led to the discovery of the object responsible for the double images of a remote quasar in the gravitational lens HE 1104-1805 . The investigation is based on infrared observations at the ESO La Silla Observatory in Chile and the `lensing system' turns out to be a distant, massive galaxy. Nevertheless, the geometry of the object is unusual and an additional gravitational lens of `dark' (invisible) matter may possibly be involved.

This gravitational lens is also particularly well suited for future cosmological studies that aim at the determination of the Hubble constant and the expansion rate of the Universe.

A new and detailed study of gravitational lenses

It is rare among the relatively few, confirmed cases of gravitational lensing in the distant Universe, that the distribution of matter in the lensing system is well known. However, it is exactly this information that is needed to derive cosmological parameters by means of photometric monitoring of the brightness of the individual images in a gravitational lens [2].

The three astronomers have therefore undertaken a detailed study of some previously known gravitational lenses (or good candidate objects) with the primary aim to detect and map the associated lensing matter (refered to as the gravitational deflector or lensing object).

This is observationally quite difficult and time-consuming since the huge masses responsible for the gravitational bending of light are almost always located at very large distances from us. Thus they are quite faint and can only be observed with large telescopes and state-of-the-art equipment. Moreover, the faint images of lensing objects are located between the much brighter quasar images they lens. This makes the discovery of a lensing object and the recording of its image a most challenging task.

The advantage of infrared observations

The image of a remote galaxy is usually very faint at visible wavelengths, but it is brighter in the infra-red part of the spectrum. This is because the wavelength of maximum intensity in the spectrum of a rapidly receding, distant galaxy (a composite of the spectra of the stars of which it consists) is redshifted from the visual into the infrared region of the spectrum.

For instance, galaxies with redshifts around z = 1 [3] are best observed in the J -band near the near-infrared wavelength of 1.25 microns (about twice that of red light), while the images of galaxies with even higher redshifts and velocities are better recorded in the 2.2 micron K -band.

The present search for gravitational deflectors is therefore conducted in the infrared spectral region, using the ESO/MPI 2.2-m telescope and the IR detector IRAC 2b. Such a survey has the further advantage of revealing, if present, additional lensed images of the quasars, that may be heavily obscured by intervening dust, for example by the dust contained in the lensing galaxy.

A new and powerful image combination/deconvolution algorithm

These investigations have always been difficult because of the small angular separations in such lensed objects, of the order of one arcsecond, or even less in many cases. This corresponds to the image-smearing (seeing) effects introduced by atmospheric turbulence under common ground-based observing conditions. Detailed observations of such objects are therefore normally best made from space-based observatories, like the Hubble Space Telescope (HST).

However, an alternative method of obtaining high-resolution images is to combine numerous exposures of the same object in an optimised way; this allows to `eliminate' most of the image degradation caused by atmospheric effects. New and powerful software for this procedure has recently been developed at the Astrophysical Institute in Liege, cf. www.orca.ulg.ac.be. The new algorithm allows to treat (`deconvolve') simultaneously a large number of exposures - especially in the infrared - and yields high-resolution, combined images of the celestial objects on which precise brightness and positional measurements can be performed.

Detection of the lensing galaxy in HE 1104-1805

During the present programme, the astronomers recently observed HE 1104-1805, a gravitational lens with a doubly imaged quasar with a redshift of z = 2.316 that was discovered in 1993 at the La Silla Observatory. Observations in 1995, made in the I -waveband (0.9 micron) under poor seeing conditions, showed a very faint feature between the quasar images but the observations did not allow to ascertain the nature of this object.

New infrared images were obtained during the night of April 14-15, 1997. They were then processed with the new software and the resulting, detailed images with high-angular resolution, 0.27 arcsec, now show very clearly the lensing object, a remote, elliptical galaxy, between the quasar images.

The image displayed in eso9719a was obtained in the near-infrared J-band, where the lensing galaxy in HE 1104-1805 is quite faint, but still well visible and measurable after `deconvolution'.

The observed, infrared colour, i.e. the difference in brightness of its image in the J- and K-bands (the (J-K) index ), is compatible with that of a high-redshift elliptical galaxy, at a distance corresponding to a redshift somewhere between z = 1 and z = 1.8.

The brightest of the two quasar images shows absorption lines in its spectrum which have been redshifted at z = 1.66. Since the lensing galaxy is situated at a small angular distance from this component, it is quite likely that these spectral lines are produced by this galaxy.

Thus, the gravitational deflector in HE 1104-1805 is most probably an elliptical galaxy at redshift z = 1.66. This corresponds to a recession velocity of about 200,000 km/sec and a distance that, depending on the adopted Hubble relation, is of the order of 6,000 - 9,000 million light-years.

Since this galaxy is comparatively bright in the infrared, this may be checked in the near future by taking an infrared spectrum, for example with the future IR instrument of the ESO New Technology Telescope, SOFI.

Continued studies of HE 1104-1805

This gravitational lens is known to show brightness variations with time. It is therefore a good candidate for continued photometric monitoring which may possibly yield a new and independent determination of the Hubble constant [2], as this was recently done for another gravitational lens, PG 1115+080 [4].

If the lensing galaxy is actually located at redshift z = 1.66, then the time delay expected for brightness variations of the two lensed quasar images is of the order of 3 to 4 years, depending on the model. This should be easily measurable.

A 'Dark Lens' in HE 1104-1805?

The observed geometry of HE 1104-1805 is somewhat surprising, since current lens models predict that the position of the deflector, as seen in the sky, is closer to the fainter quasar image than to the brighter one; here the contrary is the case. This would suggest that the distribution of the lensing matter is more complex than that of a single elliptical galaxy.

In addition, the brightness of the lensing galaxy in the K-band is somewhat too high for a normal one. This may indicate the presence of a more massive object, for example a cluster of galaxies. This may not be the case, though, since the present, very deep observations would have allowed the detection of any normal cluster of galaxies between us and the quasar whose light is being split by the lens.

An interesting question is therefore: do we 'see' the effects of a lens of dark matter in HE 1104-1805? Only future observations, for instance with the ESO Very Large Telescope (VLT), will tell.

Note

[1] The group consists of Frederic Courbin (Institut d'Astrophysique, Universite de Liege, Belgium, and Observatoire de Paris-Meudon, France) and Pierre Magain (Institut d'Astrophysique, Universite de Liege, Belgium) and Chris Lidman (ESO).

[2] The careful observation of similar, but time-shifted brightness variations of the individual images of a quasar in a gravitational lens may sometimes lead to a determination of the distance to the lensing object (normally a distant galaxy).

This is because the measured time delay of such variations (from some months to several years), from the known speed of light, will provide a direct indication of the difference in the length of the two light paths, expressed in kilometres. If moreover the overall distribution of the matter that causes the lensing effect is known (from observations of the shape of the lens) and thus the relative geometry of the lensing system and the light paths, it is then possible to estimate the absolute size of the system. When this is compared with its angular size (as seen in the sky on direct images), the true distance to the lensing object can be found. Dividing this distance with the measured recession velocity (along the line-of-sight), finally gives an independent value of the expansion rate of the Universe, the famous Hubble constant .

[3] In astronomy, the redshift denotes the fraction by which the lines in the spectrum of an object are shifted towards longer wavelengths. The observed redshift of a distant galaxy or quasar gives a direct estimate of the apparent recession velocity as caused by the universal expansion. Since the expansion rate increases with the distance, the velocity is itself a function (the Hubble relation) of the distance to the object. A redshift of z = 1 corresponds to a recession velocity of 180,000 km/sec; z = 2 to 214,300 km/sec, z = 3 to 233,300 km/sec, and z = 4 to 245,500 km/sec; the non-proportionality is a relativistic effect.

[4] Detailed information about PG 1115+080 may be found in the scientific papers by Schechter et al. (1997, ApJ, 475, L85) and Courbin et al. (1997, SISSA preprint astro-ph/9705093, A&A Letters, in press).

Ulteriori Informazioni

Where to find additional information

More details about the image deconvolution techniques used for this investigation is available at the WWW pages of the Liege group.

Information about another gravitational-lens related discovery by astronomers at the Institut d'Astrophysique in Liege have been reported in ESO Press Release eso9606 (9 February 1996).

Appendix: What is a gravitational lens?

The physical principle behind a gravitational lens (also known as a cosmic mirage) has been known since 1916 as a consequence of Einstein's General Relativity Theory. The gravitational field of a massive object curves the local geometry of the Universe, so light rays passing close to the object are also curved (in the same way as a `straight line' on the surface of the Earth is necessarily curved because of the curvature of the Earth's surface).

This effect was first observed by astronomers in 1919 during a total solar eclipse. Accurate positional measurements of stars seen in the dark sky near the eclipsed Sun indicated an apparent displacement in the direction opposite to the Sun, about as much as predicted by the theory. The effect was obviously due to the gravitational attraction of the stellar photons when they passed near the Sun on their way to us. This was a direct confirmation of a new phenomenon and represented a milestone in physics.

In the 1930's, astronomer Fritz Zwicky (1898 - 1974), of Swiss nationality and working at the Mount Wilson Observatory in California, realised that the same effect may also happen far out in space where galaxies and large galaxy clusters may be sufficiently compact and massive to bend the light from even more distant objects. However, it was only five decades later, in 1979, that his ideas were observationally confirmed when the first example of a cosmic mirage was discovered.

In this connection, it is of particular interest, that this gravitational lensing effect may not only result in double or multiple images of the same object, but also that the intensities of these images increase significantly, just as it is the case with an ordinary optical lens. Distant galaxies, galaxy clusters, etc. may thereby act as natural telescopes which allow us to observe objects that would otherwise have been too faint to be detected with currently available astronomical telescopes.

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