by From within, we are able to make close-up surveys of our Galaxy’s stellar population and 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 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 characteristics to our own.
They discovered that the Galaxy has twins – lots 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.
To begin their search, the researchers reduced their sample size by selecting only those galaxies that matched what we know about the Milky Way in three broad categories. First, they were filtered for galaxies with a similar total mass to that of the Milky Way. Second, they excluded galaxies with a very different “bulge-to-total ratio” (the size of the galaxy compared to its bright central core). Finally, they selected 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. There are other improvements possible to the Hubble classification system, including bar-shaped centers in some spirals, for example, but the idea was to use the classifications to find rough approximations of the Galaxy from which to start more detailed work.
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 by taking into account how stellar winds drive 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 in different regions of galaxies.
So what did they find?
It turns out that there are indeed galaxies out there that look a lot like our own. 56 of the 138 galaxies in the sample ended up being a close match to home.
What characterizes these Milky Way-like galaxies is that they have long timescales on which star formation occurs in their outer regions, steadily giving birth to new stars in a relaxed fashion. The inner region, on the other hand, experiences a dramatic period of intense star formation early in the galaxy’s history, driven by a stream of gas drawn towards the center from the outer region. Later, a much slower period of star formation occurred in the core, relying on recycled gas ejected from older stars in the outer region. These new stars, made from recycled material, have a higher level of metallicity, grafted with heavier elements that were missing from the original generation of stars. We see this pattern here at home and in our own galaxy.
But this is not the case for all 138 galaxies studied. A significant portion of galaxies that at first glance looked like the Milky Way turned out to look 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 experience star formation uniformly, in a long slow extended process without the wild explosion at the core. In these galaxies, stars in both the inner and outer regions look identical.
The second category, meanwhile, consists of what are known as “centrally quenched” galaxies (27 out of 138), and these are perhaps the strangest of the bunch. These outliers appear to lack any significant period of recent star formation from recycled material in their cores, meaning that the radial inflow of gas from the outer regions that we see in the Milky Way is not occurring in these galaxies.
A consistent feature of these centrally quenched galaxies is that they typically appear to have completed most of their star formation in the past, implying that they may be older than the Milky Way.
If this is true, we may be looking at the future of the Galaxy itself. Our galaxy may one day end up with a faded center as well, and these galaxies thus represent a preview of the next stage of galactic evolution.
“Perhaps these galaxies are the evolutionary successors to our Milky Way, which are further along in their lives,” the authors write.
They also give 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 completely unique. There is a huge 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 peers can help us learn more about our home, giving us the next best thing to holding our galaxy up to a mirror and showing us our reflection.
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