Dust and gas really make up far more of the universe than most of us like, but its really hard-to-detect nature can lead to some fascinating realities. For instance, consider Andromeda. This nearby galaxy appears distant and alone, but it turns out, its outer halo reaches well beyond its starry limits and actually stretches halfway to our galaxy. 

The core of this nearby system is just visible in dark sites as a smudge in the night sky. Its main disk of stars would appear several moon-widths across if we could see it without a telescope. Now, thanks to a paper also appearing in The Astrophysical Journal, we know its outer halo would appear as wide as the Big Dipper if we could see it at all, which we can’t except as interference in the light of more distant quasars. Project AMIGA (Absorption Map of Ionized Gas in Andromeda) found interference from Andromeda in the light of 43 quasars. This interference took the form of spectral lines signaling the presence of rarified and ionized gas. 

It’s amazing to think that Andromeda is interfering with that much of our view of the evening sky. This shows, once again, dust and gas are everywhere, and one person’s noise is always someone else’s data. 

More Information

NASA Hubblesite press release 

“Project AMIGA: The Circumgalactic Medium of Andromeda,” Nicolas Lehner et al., 2020 Aug. 27, Astrophysical Journal (Preprint on arxiv.org)