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The Dust Devils in Galaxy Centres

Periodic Reporting for period 1 - DUSTDEVILS (The Dust Devils in Galaxy Centres)

Période du rapport: 2018-10-01 au 2020-09-30

The vast majority of the universe is empty thanks to the cosmological expansion of space itself. Gravitation led to the formation of islands within this tremendous emptiness. These are called galaxies and they consist billions of suns dancing in grand design patterns around the galaxy centres bound in a cosmic cycle of birth and destruction to the material they are born from and resolve in, gas and dust. But apart from the stars, gas and dust, there is something else lurking in the very centre of the galactic gravitational well, something that swallows all matter alike, a supermassive black hole.
While it is established by now that at least all big galaxies host supermassive black holes in their centres, it is far less clear how, where and when they were born, and how they grew to their spectacular masses of up to 10^10 solar masses! Furthermore, it seems that these objects are not just an isolated curiosity of nature but they seem to even play an important role in the evolution of the galaxies by blowing material from the inner regions outwards, sometimes even out of the galaxy. This in turn can suppress (or amplify) the star formation within the galaxy and thus influence the further evolution of the whole system. Therefore, the study of the galactic black holes is well motivated as one of the pieces required for understanding the universe.
This project focuses on the growth and feedback phase of the galactic black holes during which large amounts of radiation are released throughout all the electromagnetic spectrum. The nuclei of these galaxies will shine many times brighter than all the rest of the galaxy together. They are visible even over cosmic distances and are called active galactic nuclei (AGN).
The powerful emission is primarily generated by an accretion disk forming as a results of the gravitational pull of the black hole and angular momentum conservation from gas and dust that somehow crossed into the gravitational sphere of influence of the black, either as a result of a galaxy merger, or a secular process like galactic disk or bar instabilities. A large part of the in-falling matter is actually not consumed by the black hole. Instead, it is lifted up from the swirling accretion disk and taken up in vertical direction, like a dust devil in the desert. Of course, the underlying physics of such vertical, or polar, outflows are fundamentally different from dust devils, but recent results indicate that the former are, in fact, surprisingly dusty, and they extend out to kiloparsec scale, way beyond the sphere of influence of the black hole. Thus, here we see the black hole feedback onto the galaxy in action!
The polar dusty outflows have only recently been discovered, and they are have the potential to significantly advance our understanding of the AGN structure. Their base together with the accretion disk could give rise to the angle dependent obscuration that is required by the AGN unification models, and thus replace the so far canonical but purely phenomenological torus scenario. Therefore, polar dusty outflows are the subject of this research action.
In order to establish the general importance of the polar dust emission in AGN, we observed a test sample of nearby galaxies with the VLT/VISIR. We successfully detected polar dust emission in 90% of the cases, indicating the polar dust might well be a ubiquitous phenomenon in AGN. Furthermore, the estimated lower limits on the on the extent of the polar dust provide evidence that this component is actually dominating the total mid-infrared emission of the
AGNm and thus an integral part of its structure by reprocessing a significant part of the primary radiation. We also found tentative evidence of a physical connection between the size of the polar structure and the accretion rate of the black hole relative to its mass. This indicates that the polar dust is indeed part of a radiation driven wind that extends from the outer (dusty) accretion disk to galactic scales.
These results were published in a refereed journal article (Asmus 2019) and presented on several international conferences (TORUS 2018, EWASS 2019, NAM 2019).

Motivated by the above findings, we undertook a data-mining search for AGN in the local Universe to complete our census of black hole growth at present times. Here particular emphasis was on those black holes that grow in a highly obscuring environment, which makes it difficult to find them. The polar dust radiating in the infrared can be a large help here owing to its large extent. For this purpose, we constructed a new survey, the Local AGN Survey (LASr), with the goal of identifying AGN unbiased against
obscuration. First, a highly complete all-sky galaxy sample within 100 Mpc was assembled from the public astronomical databases, resulting in ~49,000 galaxies. Among those, ~4,300 are already known to harbour an AGN. Using the infrared all-sky survey from WISE, we identified further AGN candidates based on their infrared colour with particular emphasis on the more powerful, but much rarer, AGN. The new candidates found expand the known such population by nearly 40%. Most of these sources are estimated to be highly obscured and will be detectable with the upcoming X-ray surveys. In preparation of further follow-up work we also estimated the total number of luminous AGN in the local Universe which, a third of which still remains to be identified. These results were published in a refereed journal article (Asmus et al. 2020) but could not yet be presented on conferences owing to COVID-pandemic related cancellations.

Finally, we worked on comparing the observational data of the polar dust structures with state-of-the-art models for dust structures around AGN, developed by our team and collaborators. First results look promising and were partly published in Stalevski et al. (2019) and shown at the TORUS 2018 conference.
The results of this research action significantly advance our understanding of the black hole growth and feedback face by establishing dusty polar winds as a new paradigm and retiring older dust scenario. They set the stage for the upcoming James Webb Space telescope, which will be able to follow up on this work in much greater depth.

Furthermore, the new local AGN survey, LASr will reveal the missing population of highly obscured AGN and already brought us many new candidates to be follow-up in the nearby future. This work is paving the way for the upcoming X-ray all-sky surveys to complete our census of local black hole growth.
Thermal-infrared image of the nuclear region of NGC 1365 after subtracting the point source.