Quasar Light Journey
The distances to the quasars
are so great that, even at the speed of light, photons have travelled for
a significant proportion of the age of the universe before reaching the
Earth. In its journey from the quasar to our telescopes, the emitted light
passes through intervening material randomly distributed along the line
of sight. This matter is too distant to be seen directly, but leaves its
signature in the qusar spectrum in the form of narrow, discrete absorption
lines. Thus quasar are veritable 'cosmic beacons', offering us a unique
view of the universe as it was at much earlier times. At least two different
types of absorbing regions produce absorption lines: haloes of galaxies
where the interstellar medium is broadly similar to our own, and intergalactic
clouds of primordial gas, which has undergone little or no metal-enrichment
through stellar nucleosynthesis. [ TIFF ]
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