Special Spiral Galaxy

Yao-Yuan Mao (Utah), with images from the DESI Legacy Surveys Sky Viewer

The Milky Way is special because it is our home. No matter where we are on Earth we can see its arc of light overhead if the night is dark enough. But how similar is our galaxy to others? Is it an unusual spiral galaxy, or is it rather typical in the cosmos?

Before we had discovered exoplanets, astronomers generally thought our solar system was rather typical. Sure, there would be differences, but the general arrangement of rocky worlds close to the Sun and cold gas giants in the outer system made sense. However when we studied planetary systems we found ours was rather unusual. Most planets orbit red dwarfs, not sun-like stars, and large gas giants often orbit close to their star. Now that we have sky surveys of galaxies throughout the Universe, we can answer the same question of the Milky way, as a recent study shows.¹

The study is based on the Satellites Around Galactic Analogs (SAGA) Survey, which began collecting data in 2013. The goal of SAGA is to look at the small galaxies which orbit large galaxies. The team looked at 101 galaxies with masses similar to the Milky Way and found 378 satellite galaxies for them. Because of observational limits, this only covers satellites with a mass of about a million Suns or more. In this range our galaxy has four satellites. We know of many more, but most of them are below the mass cutoff.

This would seem to indicate that the Milky Way is rather typical. But then the team looked at those galaxies with a large companion, like the Large Magellanic Cloud we see in the southern hemisphere. For those galaxies the number of satellites is typically much larger than four. The Milky Way has an unusually low number of satellites. One reason for this may be that the Large Magellanic Cloud entered our sphere of influence rather recently on the cosmic timeline.

A second study based on the SAGA data looked at star formation in the satellite galaxies.² It found that the closer a satellite is to the main galaxy the more likely it is to still be producing stars. This is similar to what we see among the Milky Way satellites. So it seems that while the Milky Way is a little unusual, it isn’t unique among galaxies of similar mass.

But it will always be our special spiral galaxy.