The James Webb Space Telescope (JWST) has once again revolutionized our understanding of the cosmos, this time shedding light on the enigmatic phenomenon of massive quenched galaxies in the early universe. These galaxies, which ceased their star formation much earlier than expected, present a conundrum that the JWST's advanced capabilities have helped unravel. The story of ZF-UDS-7329, a galaxy that quenched just two billion years after the Big Bang, is a microcosm of this intriguing puzzle.
What makes this discovery particularly fascinating is the timing. As the JWST peeks further back in time, it should detect fewer massive galaxies. Yet, it has consistently revealed these behemoths, only to find that some have already shut down their star formation. This discrepancy has led to the realization that there were post-Starburst (PSB) galaxies in existence hundreds of millions of years sooner than previously thought. The JWST's ability to capture such early and massive galaxies challenges our existing models of galaxy evolution.
The key to understanding these galaxies lies in the mechanisms that quench star formation. There are only two fundamental ways this can happen: by removing the cold gas from which stars form, or by disrupting the gas without removing it. Gas stripping, often caused by tidal interactions with other galaxies or galaxy clusters, is one such mechanism. Supermassive black hole/Active Galactic Nuclei (AGN) feedback can heat the gas and introduce turbulence, further stifling star formation. These processes, among others, can trigger the quenching of galaxies.
To unravel the mysteries of these early quenched galaxies, researchers analyzed the light from 120 PSB galaxies, focusing on the range 0.5 < z < 3. This period captures the rise, peak, and aftermath of the Cosmic Noon, when star formation in galaxies was at its zenith. The study, led by Professor Omar Almaini, aimed to understand why these massive galaxies stopped forming stars so early.
The JWST's Public Release IMaging for Extragalactic Research (PRIMER) survey played a pivotal role in this research. By studying the structural parameters of the PSB sample, the researchers discovered a trend. At z > 1, the most massive galaxies had already formed their stars rapidly and quenched first. However, as cosmic time progressed, the most massive galaxies became quenched, and now it is the lower mass galaxies that are quenching.
One of the most intriguing findings was the presence of disturbance indicators in the PSB sample. These indicators, including asymmetry and residual asymmetry, and RFF (residual flux fraction), suggest that the massive PSBs at high redshifts experienced enhanced structural disturbances. These galaxies are significantly more compact than their normal passive counterparts, indicating a powerful and violent event that quenched them.
The researchers propose that these PSBs likely suffered violent mergers, which drove gas into the galaxies' centers, triggering rapid star formation. Some of the gas was turned into stars, while the rest was removed by powerful AGN feedback, leaving behind a dense, spherical, quenched galaxy. Simulations support this theory, showing that collisions between gas-rich galaxies produce very compact remnants.
However, the story doesn't end there. Later, in the Cosmic Afternoon, a different scenario unfolded. These PSBs retained their disk-dominated structure rather than becoming compact spheroids, indicating a gentler quenching process. A minor merger or gas stripping by interactions with a galaxy cluster could be responsible for this change. Once starved of their star-forming gas, these galaxies burned through their final gas in a brief, more modest star-formation phase, becoming passive disks without losing their disk-dominated shape.
In conclusion, the JWST has once again demonstrated its power in unraveling the complexities of the universe. The discovery of massive quenched galaxies in the early universe and the understanding of their quenching mechanisms highlight the telescope's ability to provide unprecedented insights. However, there is still much to learn, and future research incorporating stellar kinematics will help constrain the mechanisms and timings revealed in this study. The JWST continues to push the boundaries of our knowledge, offering a glimpse into the diverse physical routes through which galaxies transition from star-forming to quiescent states across cosmic time.
