Is the Milky Way… normal?

Is the Milky Way… normal?

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 features of the Milky Way as we do of other galaxies, largely because we live in it. But we do have some advantages. From the inside, we are able to conduct close-up studies of the Milky Way’s stellar population and its chemical composition. This gives researchers the tools they need to compare our own galaxy with the many millions of others in the universe.

This week, an international team of researchers from the US, UK and Chile completed a paper that does just that. They dug through a catalog of ten thousand galaxies traversed by the Sloan Digital Sky Surveylooking for galaxies with similar characteristics to ours.

They found that the Milky Way has twins — many of them — but just as 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.

Digging through the data

To begin their search, the researchers narrowed their sample size by selecting only those galaxies that matched 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 ruled out galaxies with vastly different bulge-to-total ratios (the size of the galaxy compared to its bright central core). In the end, they chose only 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 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, rod-shaped centers for some spirals, but the idea was to use the classifications to find rough approximations of the Milky Way to begin the more detailed work.

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

At the end of this process, the team was left with 138 galaxies superficially similar to ours. From there, they could dig into the details to see just how close our galactic cousins ​​actually 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 galaxies, which can be pulled into the centers of galaxies. The model also took into account the chemical composition and metallicity of materials in different parts of the galaxies.

What did they find?

It turns out that there are indeed galaxies much like ours. 56 of the 138 galaxies in the sample eventually matched well with home.

What characterizes these Milky Way galaxies is that they have a long timescale in which star formation takes place in their outer regions, with new stars being born steadily in a relaxed fashion. The inner region, on the other hand, is experiencing a dramatic period of intense star formation early in the galaxy’s history, spurred by a gas stream drawn in from the outer region toward the center. Later, a much slower period of star formation occurred in the core, relying on recycled gas vented from older stars in the outer region. These new stars, made from recycled material, have a higher level of metallicity, with heavier elements grafted into them that were missing from the first generation of stars. We also see this pattern here at home in our own galaxy.

But this does not apply to all 138 studied galaxies. A significant portion of the galaxies that initially resembled the Milky Way looked very different upon closer inspection. These fall into two categories.

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

The second category, meanwhile, consists of what are known as “centrally extinct” galaxies (27 out of 138), and these are perhaps the strangest of the bunch. These outliers do not appear to have a significant period of recent star formation from recycled material in 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 consistent feature of these centrally-extinct galaxies is that they generally 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 may also end up with an extinct center one day, and these galaxies therefore represent a foretaste of the next phase of galactic evolution.

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

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

There’s still a lot to learn, but this study opens up a lot of new possibilities to chew on when it comes to galactic evolution. Essentially, it shows that we are not completely unique. There is a huge variety of galaxies 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 look-alikes can help us learn more about our own home, giving us the best alternative to holding our galaxy in front of a mirror and showing 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? A representation of SDSS-IV/MaNGA.”

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

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