Feedback from Young Stars Influences Galaxy Evolution
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When you look at the Milky Way stretching across the sky, it shows the result of billions of years of evolution.
Collisions and mergers play a huge role in that process. However, so does star formation. It turns out to be a major driver in galaxy evolution.
Researchers led by Ohio State University graduate student Debosmita Pathak, wanted to know more about how starbirth regions change galaxies. So, they studied 18,000 star-forming regions in nearby spiral galaxies to understand the many different appearances and effects of star formation. Their goal was also to get a greater understanding of the starbirth process and its galactic effects.
They used data from Hubble Space Telescope, the James Webb Space Telescope, and the Atacama Large Millimeter/Submillimeter Array as part of the Physics at High Angular resolution in Nearby GalaxieS (PHANGS) survey to probe galaxies at different wavelengths of light. The goal is to understand the physics of gas and star formation, and measure the interplay with galactic structure and evolution. The process of star formation is a complex one that begins in clouds of hydrogen gas called HII regions.
Areas of those clouds coalesce under the influence of gravity, and when conditions are ripe, protostars form and eventually light up as a full-blown star. Hot young stars heat up their surroundings as they grow and evolve. They emit light that energizes (ionizes) atoms of gas in the birth clouds, and they can emit jets of material that plays havoc with surrounding regions of space.
As part of the survey, Pathak's team found that in normal galaxies, pressure from star-ionized gas plays a role. Essentially, it drives the expansion of young star-forming regions. However, whether these zones continue to grow or remain stagnant is strongly dependent on their surrounding environment, according to Pathak.
“When young massive stars are born, she said, "they’re very energetic and pump out a ton of photons into their surroundings. In that process, they disrupt their local environments and start to drive interstellar material out of the area.” *Newborn stars carved out a cavern in this cloud of gas in the Large Magellanic Cloud. Their UV radiation eats away at the gas, sculpting the cavern where they remain.
Courtesy: ESO* That driving mechanism is called "stellar feedback". It includes the action of stellar winds on nearby clouds of gas, as well as shock waves from supernova explosions, and jets from newborn stars carving out caverns in the birth clouds. Such feedback can either stimulate new starbirth in nearby regions or it can choke off the star forming process by eating up gas clouds.
Additionally, a galaxy's chemical evolution can get affected by star formation. If, for example, a given set of newborn stars burn off great swaths of gas clouds, that destroys any materials that could have formed planets. The center of the Orion Nebula, where hot young stars are steadily affecting their local environment.
The Milky Way has many regions of starbirth scattered across its spiral arms. It forms (on average) one star a year, according to Pathak. Other galaxies can produce huge numbers of stars, likely spurred by another process: collisions.
"Galaxies that have an abnormally high amount of stars typically underwent a more violent process to form, such as a major merger, where two galaxies collide," she said. The results of violet galaxy collisions can be seen across the Universe. Our own Milky Way, for example, formed through repeated mergers of smaller galaxies.
In fact, it's still cannibalizing smaller dwarf galaxies, although the results is not spectactular waves of star formation.
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