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Shedding light on the dark supermassive black hole growth in the early Universe

Periodic Reporting for period 1 - darkBHgrowth (Shedding light on the dark supermassive black hole growth in the early Universe)

Reporting period: 2018-10-01 to 2020-09-30

"The MC project ""Shedding light on the dark supermassive black hole growth in the early Universe"" aims to investigate the interplay between active supermassive black holes (SMBHs) and galaxies since the very early stages of their growth. The feeding, feedback and cosmic growth of active galactic nuclei (AGN) must have been much more effective in the early Universe than today. However, it is yet unclear if SMBH accretion is primarily triggered by violent galaxy-galaxy mergers or via secular processes. Another open issue is whether AGN and star formation activity proceed in lockstep in all types of galaxies and at all cosmic epochs or not. In addition, no clues on the role of radio (i.e. mechanical) AGN activity in enhancing or suppressing star formation exists at redshift z>1. It is crucial to address these open questions for advancing our understanding of the role played by AGN feedback in shaping the subsequent evolution of galaxies.

Previous studies focusing on these issues have selected AGN and galaxies at particular wavelengths, and therefore biased towards a specific population. Tracing all different phases of SMBH-galaxy growth requires to make a step forward and study an unbiased sample of distant galaxies in a truly multi-wavelength approach, which is the main topic of the proposed research project. Another major limitation in previous studies of AGN-galaxy evolution was the poor reliability of star formation rate (SFR) estimates. Until now, the most robust estimates have been obtained from mid- and far-infrared imaging, which can be strongly affected by blending issues due to poor angular resolution, especially at high redshft.

For the first time, it is now possible to overcome this roadblock and infer de-blended SFR measurements with unprecedented accuracy, as part of the present project. A novel combination of deep infrared & radio data has been analysed to infer AGN and galaxy properties with unprecedented accuracy.

The main objectives of this comprehensive work have been: (i) to explore the incidence and strength of AGN activity among galaxies of different stellar mass, starburstiness (i.e. offset from the star-forming main sequence) and redshift; (ii) to build an empirically-motivated model that tracks SMBH and galaxy growth over time; (iii) to re-calibrate the radio-SFR relation from deepest radio data as a tool to take a census of radio AGN activity."
"The project has been conducted in two work packages (WPs).

WP1 consists of developing a phenomenological model that links the buildup of galaxies and supermassive black holes over cosmic time. This WP has led to two 1st-author peer-reviewed papers, five contributed talks at international conferences, four guest seminars at various research Institutes during collaboration visits. On the one hand, I made use of the latest available X-ray data as a direct indicator of ""radiative"" AGN activity. On the other hand, I used deep de-blended infrared and sub-mm photometry as a proxy for dust-unbiased SFR, for a stellar mass selected sample of over 190,000 galaxies in the COSMOS field. Such a plethora of data enabled me to connect the occurrence and strength of X-ray AGN activity to galaxy star formation, as a function of galaxy stellar mass, SFR (i.e. main sequence vs starbursts) and redshift, since z=3.

The main results that I obtained from the first sub-project of WP1 are the following: (i) SMBH accretion and star formation do not seem to evolve in lockstep, whereas AGN activity is enhanced relative to star formation in more massive galaxies; (ii) the vast majority of SMBH accretion (>80%) occurs within normal ""main sequence"" galaxies, while ""starburst"" galaxies standing well above the main sequence show a sub-dominant contribution (<20%) to the global AGN accretion history since z~5. In the second sub-project of WP1, I developed a phenomenological model that describes the buildup of SMBHs and galaxies over time. This was fully anchored to the empirically-motivated findings constrained in the first part of this work, which favours AGN activity over star formation in more massive galaxies. This has led to the following results: (i) galaxy and SMBH growth occur in two steps. At first the central SMBH starves behind the galaxy, while later the BH catches up. (ii) This twofold growth might be driven by the dark matter halo mass, that does play an important role in funnelling the gas inside the host galaxy, which, in turn, regulates the amount of gas that trickles toward the central SMBH.

WP2 is composed of two sub-projects tied to each other: re-calibrating the radio-SFR relation for an unbiased stellar mass selected sample of galaxies; and studying the distribution of radio-AGN power as a function of galaxy properties. Unfortunately, due to the Covid-19 pandemic, only the first part has been completed (1st-author paper is currently under review), while the second sub-project is on good track. In addition, all conferences at which I was supposed to disseminate the results of this WP have been cancelled or postponed to 2021. This has obviously reduced the visibility of the project. The main results achieved so far in WP2 is that the radio-SFR relation is not constant across all galaxies (contrary to what believed until now) but depends primarily on galaxy stellar mass. In particular, more massive galaxies are brighter in the radio than in the infrared (i.e. SFR) as compared to less massive analogues. The predictive power of this finding is immense, since it allows us to make first accurate predictions for the radio faint star-forming population down to micro-Jy levels, which will be routinely reached with the advent of the SKA."
"This project has also reached some additional accomplishment. That is, I have been awarded 120 hours of observations as PI with the Very Long Baseline Array (VLBA) to follow-up over 500 typical star-forming galaxies in the COSMOS field at 10 milli-arcsec resolution out to z~4 (i.e. tens of parsecs scale). This is the first time that the sharpest instrument on Earth is used to characterize the incidence of radio AGN activity in a representative sample of normal star-forming galaxies, without any prior knowledge on the presence of an AGN. Observations are currently on-going, and will be a huge leap forward in constraining the incidence of radio-faint (i.e. micro-Jy levels) AGN activity across the overall galaxy population. The final goal is to link the incidence and luminosity of radio AGN with a number of galaxy properties (SFR, M*, redshift) and model this into a novel ""radio AGN main sequence"" framework. This will serve as a benchmark to better understand the role played by the host galaxy in triggering a radio AGN.

The dataset used and the results obtained exploited in this project are also tightly aligned with the future perspective of Europe, which is preparing for large missions such as the SKA. This is expected to enhance the motivation of the international community to use those facilities and thereby bring Europe more visibility and leadership in the field.

Lastly, this MSCA allowed me to develop strong independent skills and to gain extensive experience with novel techniques and metodologies that I would not have acquired otherwise. These competences allowed me to boost my scientific profile and guided me towards a permanent staff position, which I started on October 1st, 2020 at INAF-Brera National Observatory (Milan, Italy)."