Astronomers using the Subaru Telescope in Japan have discovered a new satellite of the Milky Way that they think is the faintest companion ever discovered.
The galaxy, called Virgo I, joins roughly 50 known companions to our galaxy. It is located 280,000 light-years away and is 124 light-years in diameter – tiny even for a dwarf galaxy. For comparison, the Milky Way is about 100,000 light-years across.
The galaxy was only recently discovered because it was below the detection limit of previous surveys. Looking at it in the sky, it appears 1.5 billion times fainter than the Large Magellanic Cloud, our galaxy’s biggest companion, and it’s even dimmer than most stars. The galaxy has an absolute magnitude of -0.8, making it 1.6 times fainter than our Sun, which is quite average.
Reported in the Astrophysical Journal, this discovery implies that Virgo I might be the first of many ultra-faint dwarf companions.
“This discovery implies hundreds of faint dwarf satellites waiting to be discovered in the halo of the Milky Way,” senior author Masashi Chiba, from Tohoku University, said in a statement. “How many satellites are indeed there and what properties they have, will give us an important clue of understanding how the Milky Way formed and how dark matter contributed to it."
Location (left) and a density graph (right) of newly discovered galaxy Virgo I.
Tohoku University/National Astronomical Observation of Japan
The structures in the universe are believed to form in a bottom-up fashion, meaning that the bigger stuff got this big by accumulating lots of little things. Large galaxies, like the Milky Way, are known to cannibalize their smaller companions, stealing their gas and stars. The presence of a large population of small galaxies can solve some of the issues with our current understanding of the cosmos.
The “dwarf galaxy problem” is one of these issues. Simulations suggest that there should be more small galaxies than what we have observed, and maybe the solution is that our instruments are only getting good enough and we have simply overlooked many of the objects out there. Virgo I was found in the early data release of the Subaru Strategic Survey. The data represent about only 0.3 percent of the whole sky, so there could be many more tiny galaxies out there.