Scientists have known for a while that there can be gas clouds circling supermassive black holes. These clouds were believed to be nothing more than a thin fog, shaped into a ring over time. A team led by Alex Markowitz of the University of California, San Diego studied data collected by NASA’s Rossi X-ray Timing Explorer (RXTE) and confirmed that this gas isn’t just a mist; it can form clouds so dense that they actually interfere with the amount of the black hole's radiation that can be observed by our telescopes. The results were published in the Monthly Notices of the Royal Astronomical Society.
Markowitz’s team analyzed data collected over RXTE’s 16 year mission. RXTE, which ended operations in 2012, detected variations in x-ray emissions from a variety of sources, including galactic nuclei. Active galactic nuclei (AGN) are incredibly bright spots at the center of a galaxy. As the supermassive black hole accumulates gas and dust, it begins to emit radiation which can be detected across the electromagnetic spectrum, from radio up to gamma rays. RXTE, naturally, studied the x-ray wavelengths.
Of the 55 AGN studied, they discovered twelve periods of time when the x-ray emissions were temporarily muted. The duration of these instances varied; while some lasted only a few hours, others were obscured for years at a time. The scientists concluded that these variations were caused by incredibly dense gas clouds. When these clouds passed between the AGN and the telescope, it obscured the signal. Though computer models have predicted this could be the case, this is the first confirmation on the topic.
Gas clouds that stray too close to the black hole (such as G2 in our own galaxy) are doomed to be torn apart. For the most part, these dense clouds are at a safe distance, ranging from a few light weeks to a few light years away from the AGN. However, the researchers did observe clouds in the galaxy NGC 3783 that were in the process of being pulled apart from tidal forces.