Galaxies are primarily made up of stars, gas, and dark matter in a variety of types and mixes in composition. Over time, galaxies can merge with or absorb other galaxies, growing and evolving. In a study by Carlos Marrero de la Rosa and collaborators at the Institute of Astrophysics of the Canary Islands (Instituto de Astrofísica de Canarias or IAC), the galaxy IC 1101 was observed as the largest galaxy to date.
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Astro Brief is a podcast collaboration between KSMU, the Missouri Space Grant, and MSU's Department of Physics, Astronomy and Materials Science. Hosted by Dr. Mike Reed, Astro Brief focuses on astronomical events, the field of astronomy, and astronomy-related guests. It airs Thursdays at 9:45 am on KSMU.
Transcript
In this episode, we're going to talk about the largest known galaxy. But let's begin by summarizing what galaxies are.
A galaxy is mostly a collection of stars, millions to trillions of them. There is also going to be some gas, mostly hydrogen, and how much can vary quite a lot. For a galaxy like ours that's still forming stars, it's about 15%. And in the small Magellanic Cloud galaxy, it's actually more than 50% of the visible matter. Gravity-wise, galaxies are usually dominated by dark matter, which accounts for about 90% of the mass in our galaxy.
Galaxies also come in a variety of types.
It's pretty common that when we think of galaxies, we picture the flat disk spiral structure of our galaxy with the central ball-like bulge. But there are other types of galaxies — mostly what we call elliptical galaxies. That type does not have a commonly orbiting disk of stars, but rather the stars mostly orbit the central bulge on their own paths. Elliptical galaxies also tend to not have much gas and are therefore not forming new stars. They are the largest range of sizes from a few million stars to over a trillion.
Galaxy sizes and locations are important for understanding how galaxies form and grow and how the universe itself evolves. As an example, our Milky Way galaxy has grown over the course of its 12 billion year history by accumulating other smaller galaxies. And it's not known whether most of our galaxy formed from a single supermassive cloud of gas or via the merger of several moderate sized galaxies.
For galaxies to merge or subsume smaller galaxies, they must be somewhat close together.
Our Milky Way is not alone in our region of space, but is part of our local group roughly 10 million light years across. Our local group consists of two larger spiral galaxies — us and Andromeda, a smaller spiral galaxy, Triangulum, and about 50 much smaller dwarf galaxies — most of which are ellipticals. Our group is gravitationally bound together, and so the motions bring galaxies around and close to each other from time to time. In the distant future, we will merge with Andromeda, messing up our spiral disks into elliptical galaxy and probably many of the smaller galaxies will get absorbed by one of the larger ones.
There are also much larger clusters of galaxies containing hundreds to thousands of major galaxies. I say major galaxies which would be like us, Andromeda, and Triangulum. At the large distances of most galaxy clusters, the smaller dwarf galaxies are too faint to be seen. While galaxy clusters have many times more galaxies than our local group, they tend to span about the same size in space. For example, the galaxy cluster of interest today is Abel 2029, which contains over 1000 galaxies. It is only 80% the size of our local group at 8 million light years across.
As you might expect with so many galaxies in such a small space, interactions and collisions occur more frequently. That or near the centers of those clusters exist the largest galaxies from the accumulated collisions of tens to hundreds of galaxies. Those galaxies are called Brightest Cluster Galaxies or BCGs. And that begs the question — which is also the title of an article recently posted to the Archive Preprint Server by Carlos Marrero de la Rosa of the Institute of Astrophysics of the Canary Islands and collaborators — and that is,
"How large can galaxies be?".
Because our universe ages, more galaxies can merge, making ever larger galaxies. But at our current age of 14 billion years, what's the largest galaxy that has been made so far?
So far, that seems to be a BCG galaxy called IC 1101 at the center of the Abel 2029 galaxy cluster. Marrero de la Rosa and collaborators used the Wide Field Camera on the Isaac Newton Telescope to obtain deep images of the galaxy cluster. But just looking at IC 1101 isn't enough to determine its size. Galaxies tend to be very faint around their periphery, so they had to very carefully characterize the camera by taking images of dozens of stars of varying brightness and then generating a model of the camera's response. With that information, they could search for the very outside dimensions of IC 1101.
They found that it is about 1.7 million light years across or 17 times larger than our Milky Way. It contains over 3.4 trillion solar masses of stars, nearly 100 times more than our galaxy. That makes IC 1101 the largest galaxy observed so far, and it's still eating as they found some other galaxies around its periphery.
In the distant future, our own galaxy will merge with Andromeda, our local group will also merge with nearby galaxy clusters, and perhaps someday in the distant future, we will be part of a supermassive BCG galaxy.