An absorption-line galaxy in front of the quasar 3C 196

Not only disembodied hydrogen clouds can be found producing absorption in the spectra of QSOs - bona fide galaxies do as well, and produce quite different signatures. They produce broader and stronger hydrogen absorption, plus in many cases from heavier elements such as carbon and magnesium (the so-called metal-line systems). This example was first discovered as an absorption in redshifted 21-cm hydrogen radiation, implying a redshift z=0.437 for the foreground object. Recent HST images show that the foreground object is a clear barred spiral galaxy. Detailed radio studies show that the hydrogen is located not in front of the quasar core, but in front of the extended radio structure which indeed lies partly behind the galaxy's disk and spiral arms. The galaxy is at about half the quasar's distance in this case - the redshifts are z=0.437 and 0.871.

The H I profile was measured with the NRAO 43-m radio telescope at Green Bank, West Virginia, as observed by R. Brown et al. (ApJ 329, 138, 1988). The HST image was taken with WFPC2 in a filter centered near 7000 Angstroms; this frame shows a 25-arcsecond region centered on 3C 196, with a total exposure time of 20 minutes. The images were retrieved from the HST archive, originally obtained as part of the GHRS instrument team's observations and reported by Cohen et al (ApJ 456, 132, 1996).

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