Daniele Sorini
While considerable progress has been made in the understanding of structure formation in the Universe, there are still many unresolved questions on how galaxies formed. Galaxy formation depends critically on the properties of the gaseous environment that surrounds them (circumgalactic medium, CGM). Indeed, the properties of the CGM are determined by the complex interplay between inflow from intergalactic gas and outflows from supernovae and/or jets powered by the black hole at the centre of the galaxy (AGN). The highly non-linear and multi-scale nature of the physics governing these processes requires us to run numerical simulations to study galaxy formation in a cosmological context. However, such simulations are often time-expensive and memory-intensive, therefore it is necessary to implement the aforementioned feedback processes with physically motivated prescriptions that vary from code to code. It is imperative to constrain the parameters underlying such feedback models with a plethora of different observations in order to make progress in our understanding of galaxy formation. I compare observations of absorption spectra of a sample of background quasars passing at different distances from foreground quasars (in the range 20 kpc-20Mpc) with the results of the state-of-the-art Simba cosmological hydrodynamic simulation. The main result is that the average absorption properties of the CGM of quasars in the redshift range 2-3 are primarily affected by supernovae-driven (and not AGN-driven) feedback. The implication is that the amount of neutral hydrogen (HI) in the CGM is mainly set by supernovae-driven winds rather than AGN-powered jets. If this result is confirmed also by other simulations, it would suggest that one should focus on investigating the details of stellar feedback rather than AGN feedback, in order to understand the role of the HI content of the CGM in galaxy formation.
Slide 2: The top-right figure shows how quasar spectra can be exploited to probe the intervening gas between the quasar and the observer. Quasar spectra exhibit a peak in the Lyman-alpha (Ly-a) emission, and several absorption features blueward of the peak. The reason is that as Ly-a photons emitted by the quasar travel towards the observer, atoms of neutral hydrogen (HI) in the intervening intergalactic medium (IGM) can scatter the Ly-a photons away from the line of sight. The physical mechanism is shown in the lower-left cartoons: the Lyman-alpha photon is absorbed by the HI atom, which promotes the electron to the first excited state; then, the electron falls back to the ground state, re-emitting a Ly-a photon, which has in general a different direction with respect to the incoming photon. Clearly, higher HI density yields more absorption. If the line of sight passes by a foreground galaxy or quasar, we can study the properties of its CGM via the associated Lya absorption features in the spectrum of the background quasar.
Slide 3: This setting is exploited by observers. From a sample of quasar (QSO) pairs, observers can stack the spectra of the background quasars in bins of impact parameter, aligning them around the redshift-space position of the foreground quasar, and then measure the mean Lya flux within a certain velocity window around the foreground quasar. This was measured e.g. by Prochaska et al. (2013) (figure on the left).
Slide 4: We compared these observations with 5 runs of the "Simba" (Dave' et al. 2019) cosmological hydrodynamic simulation, each with different feedback prescriptions. The figure shows the mean flux profile taken from the simulations over an impact parameter range spanning 20 kpc - 10 Mpc. The data points from Prochaska et al. (2013) and Font-Ribera et al. (2013) are also included. We find that for impact parameters larger than 100 kpc all simulations match the data reasonably well. Within 100 kpc, the run the run without any feedback prescription yields much less absorption than the other runs, which give similar predictions
Our main conclusion is that The average properties of the CGM of z~2-3 quasars are primarily determined by STELLAR FEEDBACK rather than AGN FEEDBACK. If you are interested in knowing the details of our work, please feel free to check our paper on arXiv: the ID is 2005:08971.