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 Milky Way’s shape and features like other galaxies, mostly because we live in it. But we have some advantages. From there we can study the stellar population of the Milky Way and its chemical composition up close. This gives researchers the tools they need to compare our own galaxy to the millions of others in the universe.

This week, an international team of researchers from the US, UK and Chile published a paper doing just that. They dug through a catalog of ten thousand galaxies compiled by the Sloan Digital Sky Survey, looking for galaxies with properties similar to our own.

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

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digging through the data

To begin their search, the researchers narrowed their sample size by selecting only those galaxies that agreed with what we know about the Milky Way in three broad categories. First they filtered for galaxies with a total mass similar to that of the Milky Way. Second, they excluded galaxies with wildly different bulge-to-total ratios (the size of the galaxy compared to its bright central core). Finally, they only selected galaxies with a similar “Hubble type”: a classification system that groups galaxies according to their shape. Some galaxies, like our own, are spiral-shaped, while others, usually older ones, are shaped more like fuzzy blobs and are known as elliptical galaxies. Other refinements are possible within the Hubble classification system, including for example bar-shaped centers of some spirals, but the idea was to use the classifications to find rough approximations of the Milky Way from which more detailed work could begin.

A simple representation of the Hubble classification, with spiral galaxies on the right (striped galaxies on the lower branch) and elliptical galaxies on the left. Photo credit: Cosmogoblin (Wikimedia Commons).

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

They fitted the data into a model that predicts star formation, taking into account how stellar winds blow away from star systems excess gas that can be pulled towards the centers of galaxies. The model also took into account the chemical composition and metallicity of materials in different regions of the galaxies.

So what did you find?

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

What sets these Milky Way-like galaxies apart is that they have a long timescale in which star formation occurs in their outer regions, steadily producing new stars at a leisurely pace. The inner region, on the other hand, is experiencing a dramatic period of intense star formation early in the galaxy’s history, fueled by a flow of gas being drawn inward from the outer region toward the center. Later, a much slower phase of core star formation occurred, relying on recycled gas blown off older stars in the outer region. Made from recycled material, these new stars have a higher level of metallicity, while grafting heavier elements that were absent in the first generation of stars. We also see this pattern here at home in our own galaxy.

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

The first category (consisting of 55 of the 138 galaxies) are galaxies that appear to have no distinction at all between their inner and outer regions. These galaxies experience steady star formation in a long, slow, extended process without the wild outburst at the core. In these galaxies, the stars appear identical in both the inner and outer regions.

The second category, meanwhile, consists of so-called “centrally erased” galaxies (27 out of 138), and these are perhaps the strangest of the group. These outliers do not appear to have a significant period of recent star formation from recycled material at their cores, meaning that the radial influx of gas from the outer regions that we see in the Milky Way does not occur in these galaxies.

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

If true, we may be looking at the Milky Way’s own future. Our galaxy too may one day have an erased center, and these galaxies therefore represent a preview of the next stage in galactic evolution.

“Perhaps these galaxies are the evolutionary successors of the Milky Way, more advanced in life,” the authors write.

They also offer 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 chews 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 play by the same rules as the Milky Way, and many are in the same life stage. Studying these doppelgangers can help us learn more about our own home and give us the next 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 Aragón-Salamanca, Michael Merrifield, Brett H Andrews, Niv Drory, Richard R Lane. “Are Milky Way-like galaxies like the Milky Way? A view of SDSS-IV/MaNGA.”

Featured image: Pablo Carlos Budassi (Wikimedia Commons).

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