Mathematics – Logic
Scientific paper
Jul 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997eso..pres....9.&link_type=abstract
ESO Press Release, 07/1997
Mathematics
Logic
Scientific paper
New infrared observations of a gravitational lens
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. http://vela.astro.ulg.ac.be/imaproc. 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.
Caption to ESO PR Photo 21/97 [JPEG, 55k]
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 ESO Press Photo 21/97 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 (`A'; the upper one in ESO PR Photo 21/97) 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, cf. ESO Press Photo 17/97. 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
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