Now you see it, now you don't. In 2002, observations from the Chandra X-ray observatory showed about what you'd expect for a nearby galaxy with an active supermassive black hole at its center: intense emissions in the UV and X-ray regions of the spectrum. By 2003, however, follow-up observations showed that emissions of energetic photons had dropped off precipitously. This odd disappearance led scientists to organize a massive campaign of follow-up observations, the results of which were released by Science today.

The new observations suggest that a large cloud of gas has walled off the black hole from Earth's line of sight to the galaxy. But the cloud is moving so fast and has persisted so long that it must be drawing material away from the environment around the black hole.

The galaxy in question, NGC 5548, belongs to a class of objects called Seyfert galaxies. These have active black holes that are feeding on gas, creating a highly energetic accretion disk. These galaxies don't emit as much radiation as a quasar (the brightest objects in the Universe), but they have the advantage of being found much closer to Earth than quasars, making them more amenable to study.

NGC 5548 has "been studied for decades" according to the new paper, and it was imaged as recently as 2012 by the Chandra X-ray observatory. The data from Chandra showed a relatively gradual increase in emissions as the energy of the photons increased from 0.1 to 10 kilo-electronVolts. But in 2013, observations showed that the lower end of this region dropped precipitously. To find out what was going on, a massive observational campaign was organized, involving XMM-Newton, the Hubble Space telescope, Swift, the Nuclear Spectroscopic Telescope Array (NuSTAR), the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL), and, once again, Chandra.

Previous work had shown that there was a cloud of ionized gas outside the immediate area of the black hole that absorbed some of the radiation released by the accretion disk. This "warm absorber" was still present but much less ionized, suggesting something was shielding it from the black hole. That "obscurer" (their term, not ours) also showed its presence through a number of absorption lines in the UV region, all of which are blue shifted, suggesting it's moving rapidly.

Since the object absorbs X-rays and most of the UV light, it must lie beyond 1014 meters; since it allows some UV and visible light through, it must be closer than 1017 meters from the black hole itself. This places it outside the accretion disk and near a region of turbulent gas that surrounds the black hole. That suggests it's drawing material from this region of turbulent gas, perhaps some from the accretion disk itself.





And it's drawing a lot of material. The blue shifts in its absorbance indicate it's moving at up to 5,000 kilometers a second, and the obscurer has been present for several years, which indicates that gas is being fed into it as part of an ongoing process.

We already knew that the dynamics of the gas near a supermassive black hole are complex—despite their reputations, black holes end up spewing away a lot of material before they can swallow it. But these new observations provide greater detail on where and how the outflows of material might arise, with possible implications for the behavior of quasars. And scientists will undoubtedly be watching to see when the light from NGC 5548 returns and seeing whether we might learn anything further from that.

Science, 2014. DOI: 10.1126/science.1253787 (About DOIs).