The more developed a galaxy is, the more chaotic the orbits of its many stars are, according to new research that answers an important detail about how galaxies age.
Our sun revolves around the center of the galaxy once every 225 million years, at an average speed of 514,495 mph (828,000 km per hour). Astronomers call this a ‘galactic year’. The path of the sun around the universe is almost circular, although it moves slightly up and down with respect to the plane of the galaxy.
In other galaxies the movements of stars have a greater degree of randomness, with their orbits assuming a wide variety of speeds and angles relative to the plane of their galaxy. In elliptical galaxies, this is often easy to explain: it is the result of a great merger of galaxies that formed the elliptical galaxy and stirred up all the stars like a hornet’s nest. But these random motions can also dominate in disk galaxies. This is strange, since stars in a disk galaxy form in a narrow, gas-rich plane called the ‘thin disk’. Our Milky Way Galaxy does indeed have a thin disk, in which we can see our Sun and most of the stars in the night sky with the naked eye. It is the movement of these stars and gas clouds that create the apparent rotation of a galaxy.
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Stars in the thin disk follow a roughly circular path around a galaxy, traveling in an orderly manner through collisions between the molecular gas clouds from which these stars are formed. This has the effect of smearing out all extreme movements, reducing the so-called ‘velocity spread’, which describes the difference between the fastest and slowest orbital speeds. With a low velocity dispersion, most stars should appear in circular orbits, while a high velocity dispersion results in more random orbits.
Previous studies have shown that the mass of a galaxy and the density of its surroundings with neighboring galaxies can both play a role in controlling the tendency for random stellar motions. However, new research led by Australian astronomers shows that mass and environment are not the direct reason for the random movements, and that the real cause is something more insidious: age.
The ‘age’ of a galaxy does not necessarily describe how long that galaxy has been around; It is thought that galaxies all formed at about the same time, about 13 billion years ago. In this case, age is rather a condition directly related to a galaxy’s star formation activity. A galaxy that is still producing new stars is considered ‘younger’, while a galaxy in which star formation has stopped 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 study, said in a press statement. ‘If you take age into account, there is actually no ecological trend, and that also applies to the masses. If you find a young galaxy it will rotate no matter what environment it is in, and if you find an old galaxy it will have more. random orbits, whether in a dense environment or in a void.”
However, age and environment – and environment and mass of galaxies – are all connected. For example, galaxies in denser environments tend to experience more collisions and mergers with other galaxies, and then grow in mass more quickly.
Moreover, “we do know that age is influenced by the environment,” says team member Jesse van de Sande, also from the University of Sydney. ‘When a galaxy enters a dense environment, it tends to stop star formation. Galaxies in a denser environment are therefore older on average.’
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There are two ways in which star formation within a galaxy can be stopped. One of these is due to a phenomenon known as ‘ram pressure stripping’: the thick, hot gas that lives in galaxy clusters is able to heat a galaxy’s gas when it enters the cluster. This strips the gas from the galaxy, leaving no material to form new stars.
In dense environments, gravitational interactions between nearby galaxies can also agitate a galaxy’s gas and spur it into a frenzy of star formation, called a “starburst.” Feedback in the form of radiation from hot, newborn stars in a starburst, or from jets emerging from an active beam supermassive black hole which is enabled thanks to a large amount of matter funneled into its maw by the interactions, can heat the gas in a galaxy and blow it into intergalactic space, preventing it from forming stars.
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Croom and van de Sande’s team conclude that the random motions of stars in older galaxies can be firmly blamed on a combination of age-related effects. One of these is stars that are born “hotter” (meaning they are too energetic to take on boring circular orbits) early in a galaxy’s life, followed by feedback from them that quickly extinguishes further star formation before it ends. galaxy gets the chance to develop. build a thin disk of stars with a lower velocity spread.
Our Milky Way Galaxy was apparently one of the lucky ones. The 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 ‘thick disk’, a much older torus of old stars. Those stars are born hotter and have 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.
The conclusions of Croom and van de Sande’s team are based on observations of 3,000 galaxies of different ages, masses and in different environments, all by the SAMI (Sydney-AAO Multi-object Integral field spectrograph) Galaxy Survey conducted at the Anglo-Australian telescope at Siding Spring Observatory in Australia. The results were published on April 3 in the Monthly notices of the Royal Astronomical Society.