Galaxies are some of the largest well-defined structures in space. There are trillions of them, and many are clustered around each other. But how does this clustering affect them? This question has been around for some time, and previous work has produced conflicting results. A new paper by researchers at the University of Washington, Yale, and several other institutions analyzing millions of galaxies now shows a clear pattern that has been debated before – galaxies surrounded by other galaxies tend to be larger.
The road to this conclusion was long. Several other studies showed that galaxies in “dense environments” were both larger and smaller. But those studies were only done with a relatively limited dataset of hundreds or thousands of galaxies. So the researchers of the new paper, led by Aritra Ghosh, a postdoctoral fellow at the UW, thought, “Why not collect more data?”
And they did so by using the Subaru telescope's Hyper Suprime-Cam Subaru Strategic Program. This survey collected high-quality data on millions of galaxies for the first time, allowing researchers to select 3 million galaxies with the best data sets from the elite of the best.
Huge data sets on galaxies are becoming more common – Fraser discusses further surveying millions of galaxies.
They then drew “circles” of about 30 million light years around each of the three million galaxies and determined how dense their local neighborhood was. Statistically, the connection was obvious – galaxies in denser spatial neighborhoods were larger than their more isolated cousins.
The claim that the researchers drew circles around 30 million galaxies is incorrect, however – they used one of the myriad new machine learning tools emerging in the astronomy community. This tool, the Galaxy Morphology Posterior Estimation Network, or GaMPEN, was the focus of Dr. Ghosh's doctoral research at Yale. It specializes in estimating the size of galaxies and accounting for uncertainty in the measurement.
Given the results of the tool, the question arose: what does this mean? The idea that galaxies are larger in dense areas does not fit well with astronomers' current understanding of galaxy formation, so it's time to develop a new theory that fits the data Subaru collected and analyzed in the study.
Fraser discusses globular clusters, one of the dense states in which galaxies can exist.
A press release put forward several theories that could explain the observations. One is that densely packed galaxies are simply larger to begin with. Another is that they may merge more effectively with nearby galaxies, forming larger supergalaxies than the two original galaxies.
A third, more intriguing possibility is that dark matter could be involved. But since scientists still don't understand what dark matter actually is, this is like waving a magic wand to explain data that otherwise doesn't fit the cosmological model.
Regardless of the reason, the study is a great example of how large data sets and AI-powered tools are about to change astronomy. In some cases, it will confirm existing theories, and in others, such as the relationship between galaxy density and size, it will require a new theoretical framework. Either way, it's exciting to be a part of all these new discoveries, whether they're AI-powered or not.
Learn more:
UW – Galaxies in dense environments tend to be larger, which clarifies one cosmic question and raises others
Ghosh et al. – Denser environments lead to larger galaxies: A comprehensive study beyond the local universe with 3 million hyper-supreme-cam galaxies
UT – This distant galaxy cluster is completely relaxed and undisturbed for a billion years.
UT – A collision between gigantic galaxy clusters. Too big, too soon
Cover image:
Image of Abell 2218, a dense galaxy cluster about 2 billion light-years from Earth.
Photo credit: NASA/ESA/Johan Richard
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