My latest research - the dust around supermassive black holes

All massive galaxies, including our own Milky Way, harbor a supermassive black hole in their very centers. With gravity so strong that even light cannot escape them, black holes remain hidden from our view. However, astronomers can study them indirectly, by looking at the gas and dust in their vicinity. Pulled in by the strong gravity, gas swirling on its way to a supermassive black hole is heated to such high temperatures that it becomes very luminous and may outshine the light of all the stars in the galaxy combined. Further out from the black hole are clouds of dust, which obscure our view of the hot gas when seen from certain viewing points. These dust clouds shine in infrared light, invisible to our eyes but not to dedicated infrared space telescopes. Using information from these telescopes, scientists are trying to put together the whole picture of these fascinating objects, which are commonly called Active Galactic Nuclei (AGN).

However, interaction between the light from the central region of AGN and the dust around it is far from simple, and very often there is more than one way to interpret the data. This presents a great challenge for the researchers, who must carefully compare the observations to theoretical predictions, in their attempt to unveil the mystery surrounding the black holes. Very often this process involves performing complex calculations on large super-computers, to simulate how matter around black holes behaves. Recently, our international team of astronomers led by myself, performed such simulations based on our latest understanding of the conditions in AGN. In particular, we have investigated how the radiation travels through the dusty clouds until it escapes the AGN and finally reaches our telescopes. We found that some previously neglected effects may lead to wrong conclusions about how much dust there is around black holes.

A number of observational clues suggest that radiation from gas around giant black holes may have a profound effect on their host galaxies and even completely expel gas from their centres. How this happens exactly is still unknown. One possible explanation suggests that the radiation from the very hot gas is able to put enough pressure on the surrounding dust to push it away from the black hole. This scenario would imply that more luminous AGN have less dust around them, a prediction that was indeed reported by some previous studies. However, new findings reported by our team in the latest issue of Monthly Notices of the Royal Astronomical Society, published by Oxford University Press, challenge this scenario. Using new insights from our numerical simulations of how the light travels through the dusty clouds, we reexamined archival data of more than 500 AGN. We found that the amount of dust obscuring supermassive black holes changes very little with the luminosity of the AGN. This puts a new challenge to our understanding of AGN, as now we may need to rethink our ideas of how black holes influence their surroundings.

Animation above represents a changing map of the sky as it would be seen from the center of AGN, resulting from our simulations. The map depicts levels of optical depth, which is telling us how transparent is the dust. Regions in yellow and red indicate positions of very dens dust clouds through which light cannot penetrate. Dark blue and black regions are almost transparent.

Animation below shows changes in temperature of the dust in the equatorial plane of AGN. The inner part (white-yellow) is the hottest, reaching the temperature of around 1000 Celsius degrees, while outer, blue regions are at freezing -100. Red light streaks appear due to the very dens dust clumps which do not allow the light to pass through some directions.

This paper will appear in the latest issue of Monthly Notices of the Royal Astronomical Society.

Official journal version will be available from the publisher.

Preprint can be found on arXiv.

My collaborators in this work were Dr. Claudio Ricci (Pontificia Universidad Catolica, Chile), Prof. Yoshihiro Ueda (Kyoto University, Japan), Prof. Paulina Lira (University of Chile), Prof. Jacopo Fritz (Instituto de Radioastronomia y Astrofiısica, Mexico), Prof. Maarten Baes (Ghent University, Belgium)