Galaxies are broadly divided in star forming and passive. Understanding the causes and mechanisms responsible for quenching star formation in galaxies, and transforming them
into passive systems, is one of the main topics of modern astrophysics.
Two main categories of quenching mechanisms have been invoked:
1) removal of gas, primarily through outflows, hence removing fuel for star formation,
2) "starvation", which consists in preventing the galaxy from accreting fresh supplies of gas,
3) suppression of star formation efficiency.
Moreover, any of these mechanisms can be associated either with the galaxy properties (e.g. mass, star formation rate,
accretion on supermassive black holes) or with their environment (e.g. the surrounding density of galaxies, whether
it is a central or a satellite galaxy).
The aim of this project was to use multi-band observations to identify what are the primary mechanisms responsible for quenching
star formation in galaxies, as a function of cosmic epoch and for different categories of galaxies.
Some of the key facilities to be used in this project are the Atacama Large Millimetre Array (ALMA), the James Webb Space Telescope (JWST,
and in particular its main spectrograph NIRSpec), MOONS (which is the next generation optical/infrared multi-object spectrograph at the
Very Large Telescope, VLT), as well as other VLT instruments.
The main results and conclusions of the project are the following:
- The main mechanisms responsible for quenching star formation in galaxies are starvation (lack of fresh gas accretion) and reduction of the star formation efficiency.
- Outflows do not have a major role in star formation quenching in terms of instantaneous 'ejective mode' (except possibly for the central region), but they can have a role in heating the circumgalactic medium (CGM).
- For central and massive satellite galaxies, black hole mass is the primary galaxy parameter directly associated with galaxy quenching, implying that the integrated black hole accretion (hence integrated injection of energy, mostly in the CGM) is reponsible for star formation quenching.
- For low-mass satellites, environment (galaxy overdensity) is primarily associated with galaxy quenching.
- Star formation quenching occurs (at high redshift) independently of galaxy morphological and dynamical transformation (which happens at later times).
- Outflows contribute significantly to the enrichment of the CGM on large scales.
- The data revealed star formation occurring inside galactic outflows. This new mode of star formation, with stars forming with large radial velocities, can have important implications on the evolution of some of the galactic components, as well as for the in-situ enrichment and heating of the CGM.
