New research answers an important detail about how galaxies age, showing that the more advanced a galaxy is, the more chaotic the orbits of its stars collectively become.
our sun orbits its center Milky Way galaxy every 225 million years, at an average speed of 514,495 mph (828,000 km/h). Astronomers call this a “galactic year.” path around the sun galaxy It is nearly circular, although it moves up and down a bit relative to the plane of the galaxy.
But movements in other galaxies stars They have a greater degree of randomness as their orbits adopt a wide range of speeds and angles relative to the plane of the galaxy. Inside elliptical galaxiesThis is generally easy to explain: It is the result of a large galaxy merger that created the elliptical galaxy and mixed all the stars together like a hornet’s nest. However, these random motions may also dominate in disk galaxies. It’s strange because the stars are one disk galaxy It forms in a narrow, gas-rich plane called the “thin disk”. In fact, there is a thin disk in our Milky Way galaxy, within which we find our sun and most of the stars visible to the naked eye in the night sky. It is this movement of stars and gas clouds that creates the apparent rotation of a galaxy.
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The stars in the thin disk follow a more or less circular path around the galaxy and move in an orderly manner, due to collisions between the molecular gas clouds that form these stars. This has the effect of spreading out any excess motion by narrowing what is called the “velocity dispersion”, which describes the difference between the fastest and slowest orbital speeds. A low-speed scatter should see most stars in circular orbits, whereas a high-speed scatter results in more random orbits.
Previous studies have found that the mass of a galaxy and how densely packed its surroundings are with neighboring galaxies can play a role in controlling the tendency for random stellar motions. But new research conducted by Australian astronomers reveals that mass and environment are not the direct cause of random movements, but the real cause is something more insidious: age.
The “age” of a galaxy does not necessarily describe how long that galaxy has existed; The galaxies are all thought to have formed at roughly the same time, about 13 billion years ago. Rather, age in this case is a condition that directly refers to the star-forming activity of the galaxy. A galaxy that is still giving birth to new stars is considered “younger”, while a galaxy where star formation has stopped is described as “old”.
“When we did the analysis, we found that age was always the most important parameter,” said Scott Croom from the University of Sydney, who led the study. press release. “When you take age into account, there’s essentially no environmental bias, and they’re similar in terms of mass. If you find a young galaxy, it will rotate in whatever environment it’s in, and if you find an old galaxy, it will have more galaxies in random orbits, whether in a dense environment or in a vacuum.” “
But age and environment (and environment and galaxy mass) are related. For example, galaxies in a denser environment tend to experience more collisions and mergers with other galaxies and, as a result, grow in mass faster.
“We also know that age is affected by the environment,” said team member Jesse van de Sande from the University of Sydney. “If a galaxy falls into a dense environment, it will tend to stop star formation. So galaxies in denser environments are older on average.”
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There are two ways to stop star formation in a galaxy. The first is through a phenomenon known as “ram-pressure stripping”; The thick, hot gas that lives in galaxy clusters can heat the galaxy’s gas as it dives into the cluster. This would strip the galaxy of gas, leaving it without any material to form new stars.
In dense environments, gravitational interactions between nearby galaxies can also stir up a galaxy’s gas and send it into a frenzy of star formation called a “starburst.” Feedback in the form of radiation from hot, newborn stars in a star explosion or jets from an active star supermassive black hole This star, activated by the large amount of matter directed towards its mouth through interactions, can prevent stars from forming by heating the gas in a galaxy and blowing it into intergalactic space.
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Croom and van de Sande’s team conclude that the random stellar motions seen in older galaxies can be blamed on a combination of age-related effects. The first are stars that are born “hotter” (meaning they are too energetic to adapt to boring circular orbits) early in the life of the galaxy, and then the feedback from them drives further star formation, quickly extinguishing it before the galaxy has a chance to form again. Create a thin stellar disk with lower velocity dispersion.
It turns out our Milky Way galaxy was one of the lucky ones. The estimated age of its thin disk is 8.8 billion years. This thin disk, about 350 light-years deep, is embedded within a “thick disk”, a much older torus of ancient stars. These stars were born hotter with more random motions, which contributes to the thick disk being 1,000 light-years deep because these random motions are at steeper angles to the galactic plane.
Croom and van de Sande’s team’s conclusions are based on observations of 3,000 galaxies of various ages, masses, and different environments; these are all conducted by the SAMI (Sydney-AAO Multi-Object Integral Field Spectrograph) Galaxy Survey. The Anglo-Australian Telescope at Siding Spring Observatory in Australia. The results were published on April 3. Monthly Notices of the Royal Astronomical Society.