Periodic Reporting for period 4 - QUENCH (Star formation quenching and feedback in galaxies throughout the cosmic epochs)
Reporting period: 2021-04-01 to 2023-03-31
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.
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.
By combining data from ALMA and the VLT, the properties and multiple phases of galactic outflows in the local universe were investigated in detail. The cold phase dominates the mass, energy and momentum in outflows. While outflows (especially AGN-driven) are likely effective in ejecting gas from the central region, even the most powerful of them are incapable of quenching star formation across the entire galaxy.
ALMA data of distant quasars and starburst galaxies, revealed the presence of fast and extended cold outflows. However, even in these extreme systems the inferred outflow energy is not sufficient to shut down star formation in the host galaxy. The warm ionized component of outflows was also explored by using data from the VLT and JWST. Also these datasets confirm that galactic outflows are generally not capable of quenching star formation.
The reason for the outflows low effectiveness in quenching star formation has been ascribed to their poor coupling with the interstellar medium, resulting in most of the energy being deposited in the circumgalactic medium (CGM), hence heating the latter, and suppressing gas accretion, eventually leading to the quenching of star formation by starvation. Within this context, important has been the first direct detections of halo heating by AGN via the Sunyaev-Zeldovich effect with ALMA.
The scenario in which star formation is primarily shut down as a consequence of the integrated action of black hole accretion (injecting energy into the halo), has been verified by the finding that passive galaxies are directly connected with the central black hole mass.
The finding that quenching of massive galaxies happens through the black hole action, without catastrophic events such as galaxy merging, has been confirmed with JWST data revealing massive, passive rotating discs high redshift, hosting AGN driving a energetic outflows.
The same methodology has revealed that the quenching of low-mass satellite galaxies is primarily driven by environmental effects. In this case quenching is explained in terms of stripping the galaxy of its circumgalactic gas by the interaction with the overdense environment. The result, also in this case, is that the galaxy quenches by lack of gas accretion (‘starvation’).
‘Starvation’ as primary mechanisms for quenching star formation has been also inferred from the detailed analysis of the stellar metallicity difference between passive and star forming galaxies, through large spectroscopic samples. The analysis of the gas metallicity scaling relations of large spectroscopic samples (through VLT and JWST data) has revealed that gas accretion and star formation feedback have different roles in the low and high mass regimes, with a complex dependence on resolved and global properties. Importantly, the well known ‘mass-metallicity relation’ is not driven by feedback being more prominent in low mass galaxies.
SDSS data has revealed that also the reduction of star formation efficiency contributes to star formation quenching.
ALMA and VLT have also revealed that outflows have a critical role in enriching the CGM. In some cases, star formation can occur inside the outflows. The implication of this new mode of star formation is that SN explosion on large orbits can also contribute to the in-situ enrichment and heating of the CGM.
Finally, ALMA, VLT and JWST observations have revealed the importance of photoionization feedback in the early universe, both on the ISM and on the CGM.
These results have also been used to guide the observing strategies for future facilities and in particular for the next generation optical and near-IR spectrograph MOONS for the VLT.
ALMA data of distant quasars and starburst galaxies, revealed the presence of fast and extended cold outflows. However, even in these extreme systems the inferred outflow energy is not sufficient to shut down star formation in the host galaxy. The warm ionized component of outflows was also explored by using data from the VLT and JWST. Also these datasets confirm that galactic outflows are generally not capable of quenching star formation.
The reason for the outflows low effectiveness in quenching star formation has been ascribed to their poor coupling with the interstellar medium, resulting in most of the energy being deposited in the circumgalactic medium (CGM), hence heating the latter, and suppressing gas accretion, eventually leading to the quenching of star formation by starvation. Within this context, important has been the first direct detections of halo heating by AGN via the Sunyaev-Zeldovich effect with ALMA.
The scenario in which star formation is primarily shut down as a consequence of the integrated action of black hole accretion (injecting energy into the halo), has been verified by the finding that passive galaxies are directly connected with the central black hole mass.
The finding that quenching of massive galaxies happens through the black hole action, without catastrophic events such as galaxy merging, has been confirmed with JWST data revealing massive, passive rotating discs high redshift, hosting AGN driving a energetic outflows.
The same methodology has revealed that the quenching of low-mass satellite galaxies is primarily driven by environmental effects. In this case quenching is explained in terms of stripping the galaxy of its circumgalactic gas by the interaction with the overdense environment. The result, also in this case, is that the galaxy quenches by lack of gas accretion (‘starvation’).
‘Starvation’ as primary mechanisms for quenching star formation has been also inferred from the detailed analysis of the stellar metallicity difference between passive and star forming galaxies, through large spectroscopic samples. The analysis of the gas metallicity scaling relations of large spectroscopic samples (through VLT and JWST data) has revealed that gas accretion and star formation feedback have different roles in the low and high mass regimes, with a complex dependence on resolved and global properties. Importantly, the well known ‘mass-metallicity relation’ is not driven by feedback being more prominent in low mass galaxies.
SDSS data has revealed that also the reduction of star formation efficiency contributes to star formation quenching.
ALMA and VLT have also revealed that outflows have a critical role in enriching the CGM. In some cases, star formation can occur inside the outflows. The implication of this new mode of star formation is that SN explosion on large orbits can also contribute to the in-situ enrichment and heating of the CGM.
Finally, ALMA, VLT and JWST observations have revealed the importance of photoionization feedback in the early universe, both on the ISM and on the CGM.
These results have also been used to guide the observing strategies for future facilities and in particular for the next generation optical and near-IR spectrograph MOONS for the VLT.
The following activities, methodologies and results have all gone well beyond the state of the art in this field:
- the thorough characterisation of multlphase outflows in local and distant galaxies and the finding that the ejective mode plays a minor role in galaxy quenchig
- the identification of starvation as the main quenching mechanism through the metallicity difference between passive galaxies and star forming progenitors
- the use of machine learning to discover that black hole mass (hence integrated accretion) is the prime cause of star formation quenching in massive satellites and centrals
- the use of machine learning to demonstrate that environment is the prime cause of star formation quenching in low-mass satellites
- the use of JWST observations to reveal that star formation quenching happens before and independently of morphological and dynamical transformation
- the discovery of the S-Z signal tracing the hot halo around distant quasars
- the detection of star formation inside galactic outflows.
- the thorough characterisation of multlphase outflows in local and distant galaxies and the finding that the ejective mode plays a minor role in galaxy quenchig
- the identification of starvation as the main quenching mechanism through the metallicity difference between passive galaxies and star forming progenitors
- the use of machine learning to discover that black hole mass (hence integrated accretion) is the prime cause of star formation quenching in massive satellites and centrals
- the use of machine learning to demonstrate that environment is the prime cause of star formation quenching in low-mass satellites
- the use of JWST observations to reveal that star formation quenching happens before and independently of morphological and dynamical transformation
- the discovery of the S-Z signal tracing the hot halo around distant quasars
- the detection of star formation inside galactic outflows.