A team of astronomers led by Dr Alex Markowitz from the University of California, San Diego and the Karl Remeis Observatory in Bamberg, Germany, has discovered giant clouds of gas circling supermassive black holes at the centers of galaxies.

Once thought to be a relatively uniform, fog-like ring, the accreting matter instead forms clumps dense enough to intermittently dim the intense radiation blazing forth as these enormous objects condense and consume matter.

Evidence for the clouds comes from records collected over 16 years by NASA’s Rossi X-ray Timing Explorer, a satellite in low-earth orbit equipped with instruments that measured variations in X-ray sources.

Those sources include active galactic nuclei, brilliantly luminous objects powered by supermassive black holes as they gather and condense huge quantities of dust and gas.

By sifting through records for 55 active galactic nuclei, Dr Markowitz and his colleagues found a dozen instances when the X-ray signal dimmed for periods of time ranging from hours to years, presumably when a cloud of dense gas passed between the source and satellite.

The findings published in a paper in the Monthly Notices of the Royal Astronomical Society confirm what recent models of these systems have predicted.

The clouds the team observed orbit a few light-weeks to a few light-years from the center of the active galactic nuclei.

One, in a spiral galaxy in the direction of the constellation Centaurus designated NGC 3783, appeared to be in the midst of being torn apart by tidal forces.

______

A. G. Markowitz et al. First X-ray-based statistical tests for clumpy-torus models: eclipse events from 230 years of monitoring of Seyfert AGN. MNRAS, published online February 12, 2014; doi: 10.1093/mnras/stt2492