Astronomers have recently discovered that the galaxy Andromeda is surrounded by an enormous halo of hydrogen gas, the same substance that makes Goodyear blimps float. The gas is not shining of its own accord and can only be search for in ultraviolet light, making it a challenge to find until now.
It is hard to detect this halo gas for the same reason that it is difficult to know if the fog has rolled in just by looking down an unlit street on a pitch black night. In this scenario, if a passerby should be walking towards you holding a flashlight, then by the unusual pattern of the light you would probably surmise that fog was affecting the flashlight beam. Otherwise, you might not know until you unwittingly bumped into a broken street lamp (but hopefully not)!
Similarly, even though we cannot see the foggy halo surrounding Andromeda just by looking at it, we can detect it by shining a flashlight through the gas. As such, one can propose a straightforward test: if light from the background flashlight was perturbed from how a flashlight normally looks like then we would know that the hydrogen ‘fog’ surrounding Andromeda was the culprit.
Now then, how does one shine light through an object 2.5 million light years away? And, even if we could build powerful enough flashlight, which we cannot, how can we possible expect to wait that long? To finish this absurd scenario, even our descendants did patiently wait for light from our flashlight to reach Andromeda they would be disappointed as light would be going the other direction (away from Earth)!
Analogous to the street fog story, what we need is to find a powerful flashlight situated behind Andromeda and one that is shining such that the light has already made it to Earth (thus eliminating the long wait time). Still better is to find more than one such flashlight, each shining through a different region of Andromeda so one can trace out the size and shape of this mysterious halo.
Astronomers have succeeded in finding just such a flashlight in the form of objects called quasi-stellar objects (QSOs). QSOs are the piercingly-bright centers of certain types of galaxies. They are located at large distances behind Andromeda and have no physical connection to Andromeda whatsoever. They act merely as distant flashlights illuminating the foreground material, which in this case is the hydrogen gas.
We are fortunate to know of a handful of distant QSOs probing the outer regions of Andromeda at just the right distances to detect the hydrogen gas halo if it were there. We know that hydrogen gas has its strongest signals at ultraviolet wavelengths, a part of the electromagnetic spectrum that (thankfully) does not make it to Earth’s surface in large amounts. Thus this test must be carried out in space from the Hubble Space Telescope (Hubble), which is above the protective ozone layer which stops most but not all of this ultraviolet light.
This proposed experiment was carried out recently on Hubble, and as a result we now know that Andromeda has a gigantic halo which is massive and reaches half-way between it and the Milky Way. Interestingly, as the Milky Way and Andromeda are thought to be twins in some ways, it is possible that the Milky Way also has a halo extending a significant fraction of the way to Andromeda. If so, then we are witnessing the early stages of an imminent collision of the two galaxies (but please do not pack your bags, by ‘imminent’ astronomers mean in this case about four billion years until full impact).