In trying to understand the formation and evolution of galaxies, a major challenge faced by astronomers and astrophysicists is pinpointing a viable energy source that can explain the galaxy luminosity function and how it is shaped. In the context of ΛCDM cosmology, simulations consistently yield a power-law mass function for galaxies. But comparison with observational data reveals divergences at both low and high luminosity ends.
At the lower end of luminosity, the influence of supernova (SN) feedback on the Interstellar Medium (ISM) emerges as a key explanatory factor. Supernovae generate pressure, propelling the ISM beyond escape velocity and impacting star formation. Yet, at higher luminosities, the observed deficit of galaxies requires more energy than supernovae alone can provide. So, where does this huge amount of additional energy come from?
A widely accepted theory involves harnessing energy from Active Galactic Nuclei (AGNs), which release an immense amount of energy into the galaxy. The energy released by supermassive black holes (SMBHs) while they feed and grow is more than 80 times the amount of energy keeping the galaxy’s bulge together! This means that even a small fraction of the energy released by the SMBH will significantly impact the host galaxy's ISM. This process is known as AGN feedback.
There are two main mechanisms for transferring energy from the central active SMBH into the galaxy: jets and winds. While jets show relativistic and highly collimated power, they are only observed in a small fraction of AGNs in the local Universe, ~5-10%. Winds, although less powerful and collimated, are present in the majority of active galaxies and can potentially halt star formation in the galaxy, if carrying sufficient kinetic power.
As we look into the eye of the Universe, every gleaming galaxy tells a unique story of how the cosmic web intricately connects everything - everywhere - at all times.