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Dust Clouds Causing the Puzzling Features in Active Galactic Nuclei

Findings of a new study suggest that mystery features observed in light released from active galactic nuclei could be due to minimal obscuration by dust clouds. Large galaxies often feature a bright core region known as an Active Galactic Nucleus (AGN) that is powered by matter that spirals into a huge black hole. Gas clouds in the broad-line region then release light at characteristic wavelengths but what had puzzled astrophysicists is the complexity and variability that characterizes these emissions.

A new study by researchers at UC Santa Cruz explains the puzzle of the AGN as the impact of small dust clouds that can partly obscure the AGN’s innermost areas. Martin Gaskell, a research associate in astronomy says that they have shown that numerous mysterious properties of the AGN can be explained by the small dusty clouds.

The recent findings have significant implications since researchers make use of optical emissions coming from the broad-line region to make conclusions concerning the behavior of gases in the inner areas of the black hole. Gaskell says that there is poor understanding of the nature of these gases.

Peter Harrington, a UCSC graduate explains that the spiraling of gases towards a galaxy’s core black hole triggers the formation of a flat accretion disk containing superheated gas which then produces intense thermal radiation. There is reprocessing of that light by hydrogen and other gases that swirl on top and under the disk in the broad-line region. The region of dust is above and beyond the accretion disk.

The impacts of the dust clouds on the light released is to make the light emanating from behind them look both faint and red in the same way that the Earth’s atmosphere makes the sun at sunset look fainter and redder. In their publication, Harrington and Gaskell provide observational evidence that supports the existence of the said dust clouds in the inner regions of the AGN. The two researchers developed a computer code that can model the results of dust clouds on the broad-line region.

Harrington says that they wrote the computer code with the intention of adjusting parameters such as gas distribution in the broad-line region, its speed of movement and the system orientation then introduce dust clouds and observe their effect on the emission-line profiles.

The findings suggest that the inclusion of dust clouds in their model can replicate numerous emission features from the broad-line region that have puzzled astrophysicists for long. Thus, the gas doesn’t have a changing and asymmetrical distribution that is complex but features a uniform, symmetric and turbulent disk around the black hole.