The early universe presents a fascinating puzzle: why do some of the most massive galaxies stop forming stars so soon after their formation? These galaxies, known as massive quiescents (MQs), are a mystery that astronomers are eager to unravel. Understanding their premature quenching is crucial for building a more accurate picture of the universe and its complex processes.
According to observations, some early massive galaxies that formed around 3 to 4 billion years after the Big Bang ceased star production just about 1 billion years after their formation. This is a stark contrast to the Milky Way, which is over 13 billion years old and continues to produce stars, albeit at a slower pace. What mechanism could be responsible for this sudden halt in star formation?
Researchers at the Institute of Astronomy, Geophysics, and Atmospheric Sciences at the University of São Paulo, along with international collaborators, believe they have found the answer. Their study, published in Astronomy and Astrophysics, suggests a connection between dusty star-forming galaxies (DSFGs) and MQs, providing new insights into the quenching process.
DSFGs, as the name implies, are prolific star-formers, producing up to 500 solar masses of stars per year, compared to the Milky Way's modest rate of one solar mass per year. However, they are cloaked in thick dust that blocks optical light, making them invisible to our telescopes. Instead, they shine brightly in the infrared and sub-millimeter wavelengths, which can penetrate the dust.
The researchers developed models to explain both MQs and DSFGs, but they encountered a problem. Models that successfully reproduce MQs at high redshift tend to underpredict the number of DSFGs, and vice versa. This discrepancy highlights a persistent tension in galaxy formation models, suggesting that the physical mechanisms driving star formation and quenching may be at odds with each other.
To resolve this tension, the researchers ran a new model of galaxy formation on the Millennium simulation. This model produced a better match between the observed numbers of both MQs and DSFGs, suggesting that most MQs first went through a phase as DSFGs. In fact, the researchers found that the progenitors of the vast majority of MQs are DSFGs, and the most massive MQs were the brightest during their DSFG phase.
The key to quenching star formation in MQs lies in major galaxy mergers. These mergers concentrate large amounts of gas in the core, triggering an extreme burst of star formation and intense feeding of the supermassive black hole. The cold gas is rapidly consumed, while the energy released by the active nucleus heats the surrounding halo gas, preventing it from cooling and being reincorporated into the galaxy. This process blocks the supply of raw material for new stars, halting star formation in less than one billion years.
The researchers found that the rapid quenching of high-redshift MQs is driven by early mergers that result in overmassive supermassive black holes relative to the stellar mass of MQ progenitors. Consequently, less AGN feedback energy is required to quench star formation in these systems. Most galaxies, however, grow more slowly and do not follow this path, as their gas is consumed more gradually and their extinction comes later.
Despite the model's success in explaining the quenching process, there are still discrepancies between the model and observations. For instance, it cannot reproduce the number of MQs observed by the James Webb Space Telescope (JWST) in its most recent observations. However, these results provide valuable insights that will feed into future observations and modeling, allowing us to refine our understanding of galaxy evolution over time.
