Understanding the causes and mechanisms responsible for quenching star formation in galaxies and transforming star forming galaxies into passive, quiescent systems, is one of the main goals of modern astrophysics.
I have been awarded an Advanced Grant from the European Resarch Council to investigate this issue by exploting multi-band observations.
The following provides some of the highlights obtained so far.
(Additional results can be found in the Research sections)
We have investigated the properties of quasar-driven outflows both in local and in the early Universe. Although these outflows are very massive and energetic, they appear insufficient to totally clean galaxies of their gas content. Therefore, they do not seem to be the primary mechanism responsible for quenching star formation in massive galaxies.
Read more about our research on galactic outflows in the dedicated section.
By investigating the stellar metallicities and ages of local passive galaxies we have revealed that their quenching must have involved both an extensive phase of starvation (during which the accretion of fresh gas supply was prevented) and gas removal via galactic outflows or ram pressure stripping.
Starvation was more prevalent in massive galaxies, while outflow and ram pressure stripping dominated quenching in low mass galaxies.
Black Holes: delayed feedback
By using spatially resolved spectra from thousands of local galaxies we have found that the property of any sub-galactic region to be quenched depends primariy on the mass of the central supermassive black hole. This is interpreted as a consequence of the total integrated energy from active galactic neucleus (AGN) feedback, likely resulting in the heating of the halo and therefore a delayed feedback effect in terms of suppression of cold accretion, eventually resulting in galaxy starvation.
Halo heading by quasar-driven winds in the early Universe should be discernible through the associated Sunyaev–Zeldovich (S-Z) effect on the CMB.
We have detected the S-Z signature in the direction of one of the most powerful quasars at high redshift, tracing heating of its halo localized on scales of ~70 kpc out to very large scales of ~300 kpc. This result confirm that quasars can have a prominenet, delayed feedback effect onto their host galaxy via heating of its Circumgalactic Medium.
Positive... and then negative
We have obtained the first observational evidences that stars can form inside galactic outflows. Since stars formed inside galactic outflows have high radial velocities, this new mode of star formation has several far reaching implications, such contributing to the formation of the spheroidal component of galaxies (bulge, halo, elliptical galaxies). However, it can also have a negative delayed effect. Indeed, supernovae exploding in the halo would efficently release their energy to the halo gas (in contrast to supernovae exploding in the disc, whose energy is mostly radiated away), which would contribute to the halo heating and to a delayed negative feedback onto the galaxy.
The role of environment
By investigating the stellar metallicity difference and age difference between passive and star forming galaxies as a function of environment, we found that enviornment has no effect on the star formation quenchig of central galaxies, but it does play a role in quenching satellite galaxies (albeit smaller than previously thought), likely as a consequence of satellite galaxies plunging in dense environment being stripped of the reservoir of gas in their halo and therefore being queched primarily as a consenquence of starvation (lack of fresh gas supply, in this case often referred to as 'strangulation') rather than gas removal from ram pressure stripping.
Efficiency of star formation
By using two different, independent techniques we explored the gas content and star formation efficiency (star formation per unit gas mass) for several thousands galaxies and for a smaller sample, but in a spatially resolved way. We found that the quenching of star formation is both due to the decrease in gas content (fuel for star formation) and drop in star formation efficiency.
The multiple routes of quenching
Snakes and Ladders game played during the Lorentz Centre Workshop "Reality and Myth of AGN Feedback" (2017) illustrating the multiple routes of galaxy quenching and transformation. Most participants voted for the delayed feedback mode as primary quenching mechanism.