A multitude of objects inhabit space, from tiny dust grains to enormous black holes. However, the focus of astronomers is primarily on these objects’ formations, held together by gravity. At the smaller scale are planets and their moons; planetary system. Then there are stars and their respective planets, forming a planetary system. Beyond that, we encounter stars, black holes, along with gas and dust in between, referred to as a galaxy. On a grander scale, the assembly of very large objects that creates larger patterns throughout the universe is termed structure. An example of such a structure is a galaxy cluster, composed of hundreds to thousands of galaxies.
Astronomers are keen to understand the influence that being part of a larger structure, such as a galaxy cluster, has on its individual objects, especially as these structures evolve over time. One research team investigated what transpires when a galaxy encounters the Abel 496 cluster, which harbors a mass approximately 400 trillion times that of the Sun and is relatively nearby, at about 140 megaparsecs or approximately 455 million light-years away from Earth.
Their goal was to study how the galaxy evolved after joining the cluster. They observed 22 galaxies within Abel 496 to identify any differences in star formation rates post-infall. Specifically, they aimed to pinpoint the last billion years, focusing on when the cluster’s regular star-forming galaxies ceased creating new stars.
The research team merged two distinct types of data regarding light emissions from the observed galaxies. The first is the long-wavelength emissions from neutral hydrogen atoms present in the interstellar dust; H I, pronounced “H One”. Analyzing these emissions helps determine how much the galaxy is being influenced by its neighboring galaxies and how much gas remains for star formation. These H I emissions were observed using the National Radio Astronomy Observatory’s Very Large Array.
The second dataset comprised short-wavelength emissions from recently formed stars, which have a mass between two to five times that of the Sun. These stars are short-lived, averaging a lifespan of less than 1 billion years. Researchers utilized luminosity patterns from these ultraviolet measurements to calculate the star formation frequency within the galaxies. These observations were conducted using the Ultra Violet Imaging Telescope aboard the AstroSat Satellite.
By combining this data, the team could delineate the history of each galaxy, assessing how long star-forming gas reserves persist and when star formation starts being influenced by the presence of other galaxies. The spatial positioning of each galaxy within the cluster was also examined to understand how the process of falling into the cluster altered their evolutionary trajectories.
The researchers found that galaxies located at the cluster’s edge experience star formation rates perceived as undisturbed, consistent with the Main Sequence. Additionally, it was noted that over half of the 22 galaxies under study reside at the center of the cluster, closely bound by gravitational forces and subject to secondary effects. Nevertheless, none of these central galaxies have fallen into the cluster for the past hundreds of millions of years, implying that they have not yet reached the region closest to the actual center of the cluster.
The team developed a five-stage evolutionary model for galaxies falling into clusters. Initially, galaxies begin their descent into clusters and continue their standard main sequence star formation, termed pre-trigger. In the second stage, other galaxies within the cluster disrupt the neutral hydrogen of the falling galaxies, triggering increased star formation.
The third stage sees a significant disturbance of the galaxy’s neutral hydrogen, escalating star formation to peak levels, designated as star formation peak. Next, during the fourth stage, the emissions of newly formed stars decline, though the galaxies are still quite disturbed, referred to as star-forming fading. The researchers estimate that these first four stages could span hundreds of millions of years. In the fifth stage, the depletion of neutral hydrogen leads star formation rates to fall below the pre-trigger main sequence, termed extinction.
In conclusion, the researchers asserted that their methodology successfully reconstructed the evolutionary history of galaxy clusters. However, they encouraged future teams to develop accurate measurement methods for both star formation and neutral gas within distant galaxies. They recommended utilizing larger samples of galaxies within clusters for more robust statistical analyses and investigating multiple clusters across various local environments to gain deeper insights into how galaxies evolve within vast structures.
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Source: sciworthy.com
