The reason this galaxy has remained hidden is that it’s incredibly faint – in fact, it’s the faintest satellite galaxy we’ve found to date. And the discovery suggests that there might be plenty more of these dark galaxies lurking out in deep space. Finding out more about our galaxy’s satellites could be the key to finally understanding how galaxies form, and the role mysterious dark matter plays in holding everything together.
There are already around 50 galaxies that we know of orbiting our own, and roughly 40 of them are faint, which puts them in the category of so-called dwarf spheroidal galaxies.
Fifty satellite galaxies might sound like a lot, but the problem is that our current understanding of dark matter and how it helps galaxies form suggests that we should have hundreds of satellites orbiting our own.
This is what’s known as the missing satellite problem, and so far, astronomers haven’t been able to explain what’s going on – either our understanding of dark matter is wrong, or a whole lot of these satellites are hiding in plain sight.
Now new research suggests the latter might be possible.
Until recently, we haven’t had the ability to detect any galaxies much fainter than an absolute magnitude of -8. Absolute magnitude is the brightness of a celestial object as it would be seen at a distance of 10 parsecs away (that’s 32.6 light-years)…
So while -8 is pretty faint, this newly discovered satellite is a whole order of magnitude darker than that, with an absolute magnitude of -0.8, making it the faintest satellite galaxy yet found.
The new satellite has been named Virgo I, because it lies in the direction of the Virgo constellation in our night sky.
Virgo I was discovered using the 8.2-metre Subaru Telescope in Hawai’i, and because of its large aperture, it can take in a whole lot more light than other telescopes.
The Subaru telescope was able to scan a large section of the night sky using a tool called the Hyper Suprime-Cam (HSC), and looked for any areas with an over-density of stars. It was then able to examine those regions more closely to look for evidence of very faint galaxies.
Dark matter, which makes up 80% of the universe according to observations of how galaxies move, might be composed of some new as yet undiscovered kind of matter. Another option is that gravity does not behave as we think at certain distances. Another option is that there is no observable universe, that we are in a simulation.