Spectroscopy and intervening clouds

[snip]

A devious answer to this is that we can analyse clouds of hydrogen gas
that happen to be in the line of sight between us and a distant quasar
no matter how faint the cloud itself may be. The light from the quasar
allows us to see the composition and redshift of the intervening gas
cloud.

[snip]

Regards,
--
Martin Brown

~~~~~~~~~~~~~~~~~

I have often wondered how good measurements can be made particularly at
longer distances (high Z) when it is known that there may/may not be
clouds between earth and the object being studied. As above, if we see
the Quasar, how do we know what spectral effects are due to the comosition
of the Quasar and what effects are due to the one or more intervening
clouds?

"Anthony Garcia" wrote in message om...
[snip]

A devious answer to this is that we can analyse clouds of hydrogen gas
that happen to be in the line of sight between us and a distant quasar
no matter how faint the cloud itself may be. The light from the quasar
allows us to see the composition and redshift of the intervening gas
cloud.

by measuring the absorption lines in the gas. The Quasar has many emission
lines that allow its distance signature to be determined. Any gas cloud
in the path of the quasar light will impart absorption lines to the same
spectrum. All it takes is lots of photons and high resolution spectrographs
to figure out which is which. Each gas cloud (and there may be several)
imparts its unique signature to the spectrum arriving at the telescope
and spectrograph.


[snip]

Regards,
--
Martin Brown

~~~~~~~~~~~~~~~~~

I have often wondered how good measurements can be made particularly at
longer distances (high Z) when it is known that there may/may not be
clouds between earth and the object being studied. As above, if we see
the Quasar, how do we know what spectral effects are due to the comosition
of the Quasar and what effects are due to the one or more intervening
clouds?