by The more developed a galaxy is, the more chaotic the orbits of its many stars are together, new research shows, answering important details about how galaxies age.
Our sun orbits the middle of the The Milky Way Galaxy once every 225 million years, at an average velocity of 514,495 mph (828,000 kph). Astronomers call this a “galactic year”. The path of the sun around the galaxy almost circular, although it moves up and down slightly relative to the plane of the galaxy.
In other galaxies, however, the movements of the stars have more randomness, with their orbits taking a wide range of velocities and angles relative to the plane of their galaxy. I elliptical galaxies, this is often easy to explain: It is the result of a large galaxy merger that formed the elliptical galaxy and fed all the stars like a hornet’s nest. But these random motions can also dominate in disk galaxies. This is strange, since stars i disk galaxy form in a narrow plane, gas-rich called the “thin disk.” In fact, our Milky Way galaxy has a thin disk, inside which we find our sun and most of the stars visible to the unaided eye in the night sky. The movement of these stars and gas clouds creates the apparent rotation of a galaxy.
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The stars in the thin disk take a more or less circular path around a galaxy, traveling in an ordered way due to collisions between the molecular gas clouds that formed these stars. This has the effect of smoothing out any extreme motions, narrowing what is known as the “velocity spread”, which describes the difference between the fastest and slowest orbital velocities. Low-velocity scattering should see most stars on circular orbits, but high-velocity scattering results in more random orbits.
Previous studies have found that a galaxy’s mass and the closeness of its environment to nearby galaxies may play a role in controlling the propensity for random stellar motions. However, new research led by Australian astronomers finds that mass and environment are not the direct cause of the random movements, and that the real reason is something more insidious: age.
The “age” of a galaxy does not necessarily describe how long that galaxy has existed; All the galaxies are thought to have formed in the same era about 13 billion years ago. Rather, age is a condition in this case, which refers directly to galaxy formation activity. A galaxy that is still giving birth to new stars is considered “younger,” while one that has stopped forming stars is described as “old.”
“When we did the analysis, we found that age … is always the most important parameter,” Scott Croom of the University of Sydney, who led the research, said in a statement press release. “When you account for age, there’s basically no environmental trend, and it’s like mass. If you find a young galaxy, it’s going to be rotating, no matter what environment it’s in, and if you find an old one -galaxy, it will have more. random orbits, whether in a compact environment or void.”
However, there is a link between age and environment – and the environment and galaxy mass. For example, galaxies in a denser environment experience more collisions and mergers with other galaxies, and subsequently grow in mass more quickly.
In addition, “we know that the environment has an effect on age,” said team member Jesse van de Sande, also from the University of Sydney. “If a galaxy falls into a dense environment, it will tend to shut down star formation. Therefore, galaxies in denser environments are, on average, older.”
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There are two ways that star formation can be stopped within a galaxy. One of them is through a phenomenon called “pressure-ram stripping” – the thick, hot gas that resides in galaxy clusters is able to heat galactic gas as it collapses into the cluster. This causes the gas to be stripped from the galaxy, leaving no material to form new stars.
In dense environments, gravitational interactions between nearby galaxies can also stir up galactic gas and spur it into a frenzy of star formation called a “starburst.” Feedback in the form of radiation from hot stars, newborns in a starburst, or jets from an active supermassive black hole which has progressed thanks to a large amount of material being escorted towards its star by the interactions, it can heat the gas in a galaxy and blow it into interstellar space, preventing the formation of stars .
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Croom and van de Sande’s team conclude that the random bursts seen in older galaxies can be blamed firmly on a combination of age-related effects. One is stars born “hotter” (meaning they are too energetic to take on boring circular orbits) early in a galaxy’s life, and their subsequent feedback acts to quickly stop any further star formation. before the galaxy has a chance. build a thin disk of stars with a lower velocity dispersion.
Our Milky Way galaxy, it seems, was one of the lucky ones. Its thin disk has an estimated age of 8.8 billion years. That thin disk, which is about 350 light-years deep, is embedded in a much older “thick disk” of ancient stars. Those stars were born hotter with more random motions, which contributes to the thick disk being 1,000 light-years deep, because those 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 across a range of ages, masses and environments, all by the SAMI Galaxy Survey (Sydney-AAO Multi-Object Integral field spectrograph) being carried out on the Anglo-Australian Telescope at Siding Spring Observatory in Australia. The results were published on 3 April in the Monthly Notices of the Royal Astronomical Society.
