The origins of spiral galaxies like our own Milky Way are still unknown, but there are two prevailing theories: They either form from collapsing giant gas clouds or are the result of smaller galaxies combining into one. Solar systems appear to form from gas clouds and galaxies share many of those features at a larger scale. The European Space Agency's Gaia space telescope observed and measured over two billion stars within our galaxy, discovering evidence of previous absorption of smaller galaxies in the form of tidal streams. Scientists continue to study galaxy collisions such as in a study by Dr. R. Pierre Martin of the University of Hawaii at Hilo and international researchers at Université Laval in Québec, Canada simulating past and future collisions between two observed galaxies, NGC 2207 and IC 2163. By observing solar system formations, tidal streams in our galaxy, and galaxy mergers, astronomers hope to find the answer to how spiral galaxies form.
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Astro Brief is a 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
Welcome to Astro Brief, a weekly science review produced in cooperation with the Missouri Space Grant and the Missouri State University Department of Physics, Astronomy and Material Science.
Welcome to Astro Brief. I'm Mike Reed. One of the outstanding questions concerning ours and other galaxies is whether they form from a giant, giant cloud of gas individually, or if they grew by absorbing and merging with other galaxies. Our Milky Way galaxy is about 12 billion years old, and, like most spiral galaxies, has three parts a bulge, a disk, and a halo. The bulge and disk are the components we usually see when looking at other galaxies, as halos are, as suggested by their naming very diffuse and faint, the bulge, or central Roundy, part of a spiral galaxy, is round because the orbits of the stars are randomly directed. That is, they can orbit around in any direction, thus creating a sort of ball-like structure. However, the orbits of the disk have to be quite organized. The stars are all going around in the same direction and roughly on the same plane. If they weren't, the disc would quickly disappear because of the coherent disc motion of stars and the flat shape of our own solar system. The giant gas cloud model was originally preferred. That is certainly the easier method to produce a flat disk. As a gas cloud collapses, there will be some excess motion in one direction, and that will naturally form a flattened disk. Mission accomplished. That is most likely how our solar system formed, and we see other solar systems doing the same at various stages of development. So that makes it only natural to think galaxies could do the same just on a much, much larger scales. The European Space Agency's Gaia telescope operated from 2014 until 2025 and measured precise motions of over two billion stars in our galaxy.
Those data are still being calibrated and analyzed, but already over 100 tidal streams have been detected. Tidal streams are disrupted groups of stars from globular clusters and small dwarf galaxies that our Milky Way has consumed over its long history. So we already know that external objects have been absorbed into our galaxy, according to a map of streams in a review article in New Astronomy by doctors Anna Bonaca and Adrian Price-Whelan. The vast majority of streams are associated with our halo and not our disk. Also, the fraction of streams is quite small compared with the mass of our halo. But those streams are still observable today and are not representative of the original components of our galaxy. And so the question remains as to whether our galaxy formed from many smaller galaxies representing a merger, or if it formed from a giant, humongous cloud of gas. One possible way to answer this question is to study galaxy mergers that we can observe today. Doctor Pierre Martin of the University of Hawaii at Hilo, and PhD student Camille Poitras of Quebec, published a study in the Monthly Notices of the Royal Astronomical Society of two interacting spiral galaxies. The interactions are not head on collisions, but rather grazing passes. And one of the galaxies designated as NGC 2207 is about five times more massive than the other designated as IC 2163. Spiral structure is still obvious for both galaxies, and they are heading into just their second encounter, with the first happening about 160 million years ago. The disks of galaxies are nearly perpendicular to each other, so they will certainly warp and be disturbed.
The next closest approach will occur in a few tens of millions of years. And then the third approach in another 100 million years will roughly be when they merge with each interaction, a gravity induced density wave passes through each galaxy, creating a burst of star formation. That star formation uses up the gas in the galaxies. And so by the time the merger is complete, most of the gas will have been converted into stars, stifling the birth of future stars with the mass ratio of roughly 5 to 1. It would be interesting to know if the disk of the larger galaxy survives, simulations performed by Martin and Poitras indicate it will not, transforming the galaxy into an elliptical galaxy. However, there are a lot of variables at play, so it is not certain. It would be very interesting to watch and see how it plays out. Unfortunately, I don't have the 200 million years to do that. One other interesting note is that Camil Poitras, the lead author of the study, did much of the work as an undergraduate student. She had a summer internship with Doctor Laurent Drissen of the University of Laval in Quebec, and that initiated the study, culminating in their paper as a predominantly undergraduate institution here at Missouri State University. We also include a lot of undergraduate students in our research groups. It is not uncommon for undergraduate students to be coauthors or even first authors on our research papers, like Camille is.
This has been Astro Brief. I'm Mike Reed, and until next week, goodbye.
Astro Brief is available online at ksmu.org.