Probing the High Redshift Intergalactic and Interstellar Medium with Gravitational Lenses





Here is how it works: according to General Relativity the gravitational field of a massive object (e.g., a galaxy or galaxy cluster) can bend light. Under certain conditions this effect will distort the image of a point source into a ring or produce multiple images. The sketch below shows how a galaxy lens (red dot) produces two displaced images (open star symbols) of a background QSO, which, without the lensing effect would appear as a single point source (filled star symbol). Imagine now that there is a gas cloud (green ellipse) somewhere between the lens and the QSO. It may cause absorption in both lines of sight if it is large enough. The angular magnification produced by the lens makes this cloud appear as if it is much bigger in projection on the sky (dotted green ellipse). The higher the redshift of the cloud the smaller is the separation between the lines of sight intersecting it. The angular separation at which the images appear on the sky is of course constant for all redshifts.Thus the combination of ground-based telescope and gravitational lens works like a gigantic microscope for the intervening matter between the lens and the QSO redshift (it is really a microscope and not a telescope as it is often said, because the `instrument' is much bigger than the object observed).

The angular magnification in principle becomes arbitrarily high (there are some practical limits related to the finite size of the QSO emission region) as one approaches the QSO redshift. In practice it has been possible to study QSO absorption systems at transverse line of sight separations of a few tens of parsecs at redshifts 3-4. The angular magnification here would amount to several hundred (for example, we may see common absorption in two lensed images that appear 2 arseconds apart, while the proper separation of the lines of sight to the naked eye (i.e., unaided by the lensing effect) may be only a few milliarcseconds and would be impossible to resolve from the ground.















































The following figure shows two spectra (red and turqoise) in adjacent lines of sight to a lensed QSO, UM673. The absorption is caused by the CIV (1548, 1551) doublet at redshift 1.94. The pattern to the left of the origin of the x-axis comes from the 1458 A transition. It repeats itself to the right of the origin in the 1551 A line (though with lower optical depth because of the lower oscillator strength). The transverse separation between the lines of sight at that redshift is about 1.7 kpc for a Hubble constant of 50 km/s/Mpc. Note the strong shift between the absorption groups near -400 km/s, in contrast to the nearly identicial appearance of the sharp absorption line near -50 km/s in both images. The spectra were taken with the Keck I telescope and the HIRES spectrograph on Mauna Kea.