Is the Milky Way… normal?

Studying the large-scale structure of our galaxy is not easy. We don’t have a clear view of the shape and characteristics of the Milky Way like we do other galaxies, largely because we live there. But we have some advantages. From within, we can conduct close-up studies of the Milky Way’s stellar population and their chemical compositions. This gives researchers the tools they need to compare our own galaxy to the many millions of others in the Universe.

This week, an international team of researchers from the US, UK and Chile published a paper this does exactly that. They dug up a catalog of ten thousand galaxies produced by the Sloan Digital Sky Surveylooking for galaxies with attributes similar to our own.

They discovered that the Milky Way has twins – lots of them – but also many that are only superficially similar, with fundamental differences buried in the data. What they discovered has implications for the future evolution of our own galaxy.

Exploring the data

To begin their search, the researchers narrowed down the sample size by selecting only those galaxies that match what we know about the Milky Way in three broad categories. First, they filtered out galaxies with a total mass similar to that of the Milky Way. Second, they ruled out galaxies with a very different “bulge-to-total ratio” (the size of the galaxy compared to its bright central core). Finally, they only chose galaxies with a similar “Hubble Type” — a classification system that groups galaxies based on their shape. Some galaxies, like ours, are spiral-shaped, while others, usually older, are shaped more like fuzzy blobs and are known as elliptical galaxies. There are other refinements possible within the Hubble classification system, including bar-like centers in some spirals, for example, but the idea was to use the classifications to find rough approximations of the Milky Way from which to start work more detailed

At the end of this process, the team was left with 138 galaxies superficially similar to our own. From there, they could dig into the details to see how close our galactic cousins ​​really are to ourselves.

They plugged the data into a model that predicts star formation, taking into account how stellar winds expel excess gas from star systems, which can be dragged into the center of galaxies. The model also took into account the chemical composition and metallicity of materials within different regions of galaxies.

So what did they find?

It turns out that there are indeed galaxies that look a lot like ours. 56 of the 138 galaxies in the sample ended up being a close match at home.

What characterizes these Milky Way-like galaxies is that they have a long timescale over which star formation occurs in their outer regions, slowly giving rise to new stars. The inner region, on the other hand, experiences a dramatic period of intense star formation early in the galaxy’s history, spurred by an inflow of gas stretching inward toward the center from the region outdoor. Later, a much slower period of star formation occurred in the core, based on recycled gas ejected from older stars in the outer region. These new stars, made of recycled material, have a higher level of metallicity, with heavier elements grafted onto them that were missing from the initial generation of stars. We also see this pattern here at home in our own galaxy.

But this is not true for the 138 galaxies studied. A significant number of galaxies that at first glance looked similar to the Milky Way ended up looking very different upon closer inspection. These fall into two categories.

The first category (consisting of 55 of the 138 galaxies) are galaxies that appear to have no differentiation between their inner and outer regions. These galaxies are undergoing star formation uniformly, in a long, slow process and without the wild explosion at the core. In these galaxies, the stars in the inner and outer regions appear identical.

The second category, on the other hand, consists of what are known as “centrally extinguished” galaxies (27 of 138), and these are perhaps the strangest of the group. These outliers appear to have no significant period of recent star formation from recycled material in their cores, meaning that the radial inflow of gas from the outer regions we see in the Milky Way does not occur in these galaxies

A consistent feature of these centrally extinguished galaxies is that they generally appear to have completed most of their star formation in the past, suggesting that they may be older than the Milky Way.

If this is true, we may be looking at the very future of the Milky Way. Our galaxy may also one day end up with a dull center, and so these galaxies represent a preview of the next stage of galactic evolution.

“Perhaps these galaxies are the evolutionary successors of the Milky Way, which are later in their lives,” the authors write.

They also raise some other possible explanations, such as an overly active galactic core that could suppress star formation in the inner regions of galaxies.

There’s still a lot to learn, but this study offers plenty of new possibilities to chew on when it comes to galactic evolution. Basically, it shows that we are not entirely unique. There is an enormous variety of galaxy types in the Universe, but at least some of them follow the same rules as the Milky Way, and many are in the same stage of life. Studying these fellow beings can help us learn more about our own home, giving us the best thing to hold our galaxy up to a mirror and show us our reflection.

The paper is available in preprint format on ArXiv:

Shuang Zhou, Alfonso Aragon-Salamanca, Michael Merrifield, Brett H. Andrews, Niv Drory, Richard R. Lane. “Are Milky Way-like galaxies like the Milky Way? Viewed from SDSS-IV/MaNGA.

Featured image credit: Paul Carlos Budassi (Wikimedia Commons).