Is the Milky Way… normal?

Studying the large-scale structure of our galaxy is not easy. We don’t have as clear a view of the Milky Way’s shape and features as we do of other galaxies, largely because we live inside it. But we have some advantages. Inside, we are able to make close-up surveys of the Milky Way’s star population and their chemical compositions. This gives researchers the tools they need to compare our galaxy to the many millions in the universe.

This week, an international team of researchers from the USA, UK and Chile released a paper that does just that. They dug through a catalog of ten thousand galaxies produced by the Sloan Digital Sky Survey, looking for galaxies with similar features to our own.

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

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Data mining

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

Simple representation of the Hubble classifications, with spiral galaxies on the right (galaxies barred in the lower branch) and elliptical galaxies on the left. Image credit: Cosmogoblin (Wikimedia Commons).

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

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

So, what did they find?

It turns out that there really are galaxies that are very similar to ours. 56 of the 138 galaxies in the sample ended up being close to home.

What distinguishes these Milky Way-like galaxies is that they have a long time scale in which star formation occurs in their outer regions, steadily giving birth to new stars. On the other hand, the inner region is experiencing a dramatic period of intense star formation early in galactic history, driven by a gas flow being drawn inward toward the center from the outer region. Later, a much slower period of star formation occurred in the core, relying on recycled gas emitted by ancient stars in the outer region. These new stars, made of recycled material, have a higher level of metallicity, with heavier elements grafted into them that were lacking in the first generation of stars. We see this pattern here at home in our galaxy, too.

But this is not true for all of the 138 galaxies studied. A large portion of the galaxies that at first glance looked similar to the Milky Way, end up looking very different upon closer examination. These fall into two categories.

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

Meanwhile, the second category consists of what are known as “centrally quenched” galaxies (27 of 138), and this is perhaps the strangest group. These outliers appear to lack any significant period of recent star formation from material recycled in their cores, meaning that the radial outflow of gas from the outer regions that we see in the Milky Way does not occur in these galaxies.

One consistent feature of these centrally extinguished galaxies is that, as a rule, they appear to have completed most of their star formation in the past, indicating that they may be older than the Milky Way.

If this is true, then perhaps we are looking forward to the future of the Milky Way. Our galaxy may one day end with a quenched center, and thus these galaxies are a preview of the next stage in galactic evolution.

“Perhaps these galaxies are the evolutionary successors of the Milky Way, which are further afield in its life,” the authors wrote.

They also put forward some other possible explanations, such as an overly active galactic core that may 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 munch on when it comes to galactic evolution. Essentially, 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 stage of life. Studying these similar shapes can help us learn more about our home, giving us the next best thing to holding our galaxy in front of a mirror and showing us our own reflection.

The paper is available in preprint format on ArXiv:

Shuang Zhou, Alfonso Aragon-Salamanca, Michael Merrifield, Brett H Andrews, Nev Drury, Richard R Lane. “Are Milky Way-like galaxies like the Milky Way? A view from SDSS-IV/MaNGA.”

Featured image credit: Pablo Carlos Budasi (Wikimedia Commons).

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