Dissecting the interstellar medium of a distant galaxy
In this study we have studied the cold
in a distant galaxy — cold
in this case refers to temperatures around 100 degrees Kelvin where the gas is mostly neutral and molecular. This cold gas is essential for the evolution of galaxies as it serves as the direct reservoir for star formation. The cold gas phase is very difficult to observe directly at large cosmological distances and in this particular study we have used a lucky alignment of galaxies and a background quasar. (A quasar is a super massive black hole which accretes matter onto it and in turn releases a a lot of gravitational energy. This energy release causes the surrounding gas an extremely luminous emission of light.)
We observe a phenomenon called gravitational lensing caused by the warping of space-time around massive galaxies.
In this case, there is a very massive galaxy (the lensing galaxy) in front of the distant quasar. The gravitational field from this galaxy causes
the light from the background quasar to bend. Because of this bending of the light, we see parts of the quasar's light
that would otherwise not reach the Earth. I've indicated how this looks in actual data from the Hubble Space Telescope
(in the lower right corner). Here we see two images of the background quasar (the two bright spots) and between the
two images we see the lensing galaxy.
What makes this system even more interesting is the fact that there is a second galaxy located between the lensing galaxy and the
background quasar. We do not directly see this intervening galaxy in the image from the Hubble Space Telescope
because it's extremely far away and very faint. However, we can infer it's existence due to the fact that some light
is missing in the light from the quasar. This missing light has been absorbed by atoms and molecules in the intervening galaxy.
Thanks to our understanding of atomic physics we can count the number of different atoms and molecules in this distant
galaxy based on the fraction of light that is absorbed away. And all of this is possible even though we don't actually
see the light from the intervening galaxy.
The article is published in the scientific journal Astronomy & Astrophysics
Also available for free at arXiv.org.