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Q:
The one thing I didn't get is how you could tell what the quasar was
made of so that you could extrapolate the information regarding the gas
cloud. I would assume you were not able to observe it unobscured by the
cloud?
A:
The optical spectrum of PKS 2155 tells us it is a quasar because optical
wavelengths are not affected by this intergalactic cloud. The overall
X-ray spectrum is also consistent with its being a quasar. However, the
X-ray spectrum of the quasar does show dips, or absorption lines, due to
oxygen ions at a lower red shift than the quasar, so the absorption must
be due to gas between Earth and the quasar. By looking at either side
of the dips in the spectrum, the astronomers can deduce the depth of the
dips and hence the properties of the cloud, without needing to know much
about the properties of the quasar.
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