Periodic Reporting for period 1 - Red Cardinal (Unveiling the Formation of Massive Galaxies with the James Webb Space Telescope)
Período documentado: 2023-05-01 hasta 2025-10-31
In the standard cosmological scenario, galaxies form naturally out of gas and dark matter in the young universe. Then, over the course of their life, galaxies accrete gas from their surroundings and convert it into new stars, thus growing in mass and size. This model is able to explain many of the observed properties of galaxies, from our own Milky Way to the most distant galaxies that are observed when the universe was still in its infancy. However, there is one observation that has puzzled theorists for decades: when galaxies become sufficiently large, they suddenly stop forming stars and cease their growth. In order to reproduce this observation, the standard model needs a key additional ingredient called galaxy quenching, i.e. a physical mechanism that shuts down star formation and transform massive galaxies into quiescent systems. The most popular explanation for quenching is that it is due to the large amount of energy released by the infall of material onto the supermassive black hole that is found at the center of every massive galaxy. However, the details for how this so-called feedback is able to stop the formation of new stars are still unclear, and observational studies have been unable to present direct evidence causally connecting supermassive black hole feedback with galaxy quenching. There are also proposed quenching mechanisms that do not involve black holes, but are based on other processes such as galaxy mergers or gravitational effects. Clearly, identifying the physical mechanism responsible for quenching is crucial for our understanding of how galaxies form and evolve in the context of the standard cosmological scenario.
The recent launch of the James Webb Space Telescope (JWST) has revolutionized many aspects of galaxy studies, because of its unprecedented sensitivity and its ability to observe at near-infrared wavelengths that were previously inaccessible. Using some of the earliest JWST observations, we have characterized the population of galaxies at Cosmic Noon, the epoch in which the majority of massive galaxies assembled their mass and experienced quenching. We have found that most galaxies experience quenching on timescales that are very rapid, of just a few hundred million years, which rules out some of the models in which quenching happens in a gradual way. We have then explored the properties of hot, ionized gas in quiescent galaxies, finding that many of these systems host active supermassive black holes. Finally, for the first time we have been able to detect the cold gas that is in the form of neutral (not ionized) atoms. This gas is more difficult to see compared to the hot gas, but since it is much denser it can hide the majority of the mass. Surprisingly, we discovered that cold gas is outflowing from many quiescent galaxies: this can only be the result of supermassive black hole feedback. The amount of cold gas that is expelled by supermassive black holes is substantially larger than the amount of gas that is transformed into stars: it is thus likely that we are directly observing galaxy quenching. These results represent the most direct evidence linking galaxy quenching with the feedback from supermassive black holes.
The discovery of large outflows of cold, neutral gas in massive quiescent galaxies at Cosmic Noon is an observational breakthrough that was uniquely made possible by JWST. In order to study the neutral gas phase we measured the faint absorption due to neutral sodium atoms in the outflow against the background light emitted by the stars in the galaxy. Before JWST was available, the neutral gas phase was essentially invisible, and we were limited to the study of the bright emission from hot, ionized gas, which however represents only a few percent of the total gas mass in the outflow. In order to confirm this important result, additional detailed studies of the neutral gas in distant galaxies are warranted: we are currently exploring tracers of neutral gas alternative to sodium (such as calcium and magnesium) and directly testing some of the calibrations routinely used in deriving the properties of neutral gas from observations. The study of neutral outflows in distant galaxies has only begun, and has the potential to transform our understanding of galaxy evolution.