The Missing Pieces of the SMBH Accretion Puzzle: Revealing Extreme Accretion Modes

Informations projet

ExSMBHs

N° de convention de subvention: 950533

DOI

10.3030/950533

Date de signature de la CE 1 Septembre 2020

Date de début 1 Octobre 2021

Date de fin 30 Septembre 2027

Financé au titre de

• EXCELLENT SCIENCE - European Research Council (ERC)

Coût total

€ 1 684 750,00

Contribution de l’UE

€ 1 684 750,00

Coordonné par

TEL AVIV UNIVERSITY
Israel

Periodic Reporting for period 2 - ExSMBHs (The Missing Pieces of the SMBH Accretion Puzzle: Revealing Extreme Accretion Modes)

Période du rapport: 2023-04-01 au 2024-09-30

Résumé du contexte et des objectifs généraux du projet

How do supermassive black holes (SMBHs) turn their accretion “on” and “off”? How fast can they grow?
How is this related to their circumnuclear stellar and gaseous environments?
What are the consequences for the emergence of the first SMBHs and their co-evolution with galaxies?
The key to answering these and other questions is found in understanding SMBH accretion, in all possible modes and cosmic epochs.

Decades of intensive work have brought huge progress to our understanding of several key aspects of SMBH accretion and evolution, but mainly focused on systems accreting at modest rates, over timescales as long as hundreds of millions of years, which are relatively well-understood and by now easy to identify observationally. In contrast, recent progress in both theory and observations strongly support other, extreme modes of SMBH accretion, namely abrupt changes seen on timescales of weeks-to-months and the possibility of extremely fast, so-called "super-critical" accretion. Both of these extreme accretion scenarios may have observational characteristics that are very different from what we know from “normal” accreting SMBHs (AGN).

This multi-faceted research program is designed to reveal, survey, and characterize extreme modes of SMBH accretion, and to link it to what we know about normally-accreting SMBHs.
This is enabled by the most recent advancements in time-domain and multi-wavelength observational surveys, and informed by state of the art models.
The various research efforts pursued in this program include: (1) Responsive, multi-wavelength, and spectroscopic follow-up observations of hyper-variable accreting SMBHs, on timescales of days to weeks, to gainnew insights as to what starts or stops SMBH accretion; (2) Understanding how common these events are, and whether they share any distinctive properties, based on the huge set of multi-epoch spectroscopic dataset obtained through the SDSS-V survey; (3) Using "flaring" supermassive black holes as indicators of super-critical accretion (beyond the Eddington limit), in order to identify distinctive emission features across the electromagnetic spectrum; (4) A detailed study of highly accreting SMBHs in the local universe, as well as a new exploratory survey of extreme, advection-dominated SMBHs at significant redshifts. These and other new insights will be contextualized and compared with newly established, highly complete distributions of the black hole masses and accretion rates at both low and high redshifts, to understand the role of extreme accretion modes in the general SMBH population and the growth of the most massive black holes in the universe.

Travail effectué depuis le début du projet jusqu’à la fin de la période considérée dans le rapport et principaux résultats atteints jusqu’à présent

As of the spring of 2024, the ExSMBHs project has accomplished several key goals, which I detail below.

1) Through intensive monitoring and multi-wavelength, multii-epoch observations, we have identified several exciting types of transient phenomena occuring in accreting supermassive black holes (SMBHs).
This includes the fastest (spectroscopically confirmed) "changing look AGN" - a system with spectral transitions occuring on timescales as short as a month (compared with expectations of many thousands of years). This system provides the best evidence in hand that such transitions, identified in visual-range spectroscopy, may be driven by varying obscuration along the line of sight, whereas previously such events were only explained by variable accretion flows.
Another interesting result is the detailed study of a flaring SMBH which presented Bowen fluorescence features, telling us that the flare had to be extremely luminous in the UV regime. This is the first time that such an event was clearly shown to occur in a previously-known active galactic nuclei (AGN). A peculiar rebrightening after about a year made us realize that this may be a common feature in such events, which now focuses our attention on relevant physical interpretations. One exciting, albeit speculative, possibility is that we're witnessing the recurring partial stripping of a star (or stars) passing close to the SMBH.

2) We used the data obtained during the first year of the new SDSS-V survey to construct the largest sample to date of "changing look AGN", and other related extremely variable sources. With over 200 sources, we're now able to start assessing the properties of these peculiar systems in a statistical manner. We were able to constrain the occurrence rate of these events to roughly 1% of all previously-known, luminous AGN, and showed that such dramatic events occur preferentially in systems with lower rates of gas accretion than the general population. As we continue our searches for more events, we are turning our attention to the host galaxies in which such extreme (accretion) events occur.

3) We have completed the census of black hole masses and accretion rates in the local (present-day) universe by using data gathered through the BASS project. We managed to show that the distribution of accretion rates differs between those systems that are, or are not, enshrouded in significant reservoirs of (dusty) gas. This can be interpreted as evidence that radiative pressure on the gas, which is exerted when the black holes accrete at high rates, is an efficient way to clear out the circumnuclear region.
At the high-redshift frontier, there are new and exciting results from the James Webb Space Telescope revealing new populations of lower-mass black holes, and the host galaxies of high-mass ones. These will help us stitch together a more complete picture of SMBH evolution through "normal" and "extreme" accretion episodes.

Progrès au-delà de l’état des connaissances et impact potentiel prévu (y compris l’impact socio-économique et les conséquences sociétales plus larges du projet jusqu’à présent)

Some of the results mentioned above are clearly beyond the state-of-the-art.
We are revealing and surveying ever faster, and more extreme accretion episodes in SMBHs, and identifying peculiar emission features.
We are revealing systems that can only be explained by processes that, up to now, were neglected in other studies.
We are constructing the largest, and most complete, samples to date of some of these extreme phenomena.
At the highest redshift frontiers, we are revealing surprisingly large populations of black holes in rich environments.

By the end of the project, we expect to obtain enough data to turn the study of the most peculiar, extreme & rare events into a more statistically sound population (or populations).
With this improved demographic dataset in hand, we'll hopefully be able to construct a physically-motivated, phenomenological model that explains cosmic SMBH evolution accounting for (short) episodes of extreme accretion.
Most importantly, we hope to be able to identify distinctive radiative traits of super-critical accretion, so that such systems can be identified and studied in detail across cosmic epochs.