Fast-Flowing Gas Curtails Galaxy’s Glow

NGC 5548. Bootes Penryn, California May 2008. M 250 @ f 9.3 (ag, ST-4)
NGC 5548. Bootes Penryn, California May 2008. M 250 @ f 9.3 (ag, ST-4)

Main Points:

The bright core of a spiral galaxy has unexpectedly dimmed, according to a new study by an international team of astronomers. The nucleus of galaxy NGC 5548, which contains a region of powerful X-ray light surrounding the galaxy’s central black hole, has been obscured by a fast-flowing stream of gas. Such behavior, which is rarely seen in the heart of this type of galaxy, casts new light on the poorly understood processes governing the interaction between galaxies and their central black holes.

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Published in Science today, the study provides the first direct evidence for the long-predicted shielding process that is needed to accelerate powerful gas streams, or “winds,” to high speeds.

“This is a milestone in understanding the interaction of supermassive black holes and their host galaxies.” says Professor Graziella Branduardi-Raymont, one of the researchers from UCL Mullard Space Science Laboratory (MSSL). She added, “I was delighted when our consortium agreed to focus the campaign on NGC 5548, as I first became interested in this active galaxy almost three decades ago, observing it with the X-ray instruments operating in space at the time.”

In 2013 and 2014 the team including researchers from UCL, University of Cambridge and University of Oxford, led by Dr. Jelle Kaastra, SRON Netherlands Institute for Space Research, conducted the most extensive monitoring campaign of an active galaxy ever with major ESA and NASA space observatories: XMM-Newton, the Hubble Space Telescope, Swift, NuSTAR, Chandra, and INTEGRAL.

Megan Whewell, the UCL MSSL PhD student who worked on the study, said: “The journey from receiving unexpected data to building a coherent theory of the obscuration has been an incredibly exciting one. Being part of this international research effort at an early stage in my career has been invaluable in understanding the process of scientific discovery.”

All big galaxies have supermassive black holes at their centers, but most are dormant as they have very little material near them. In some galaxies, however, gas, dust and even stars are close enough to the black hole to fall into it. Astronomers call these ‘active galaxies’, as the matter spiraling into the black hole forms a disk that heats up and shines extremely brightly, emitting radiation, including X-rays and ultraviolet.

The ultraviolet radiation can launch winds outward, which may be so strong that they can blow off gas that would have otherwise fallen onto the black hole, slowing its growth down again. This feedback mechanism means black hole winds can regulate both the growth of a black hole and its host galaxy; however, the winds only come into existence if their starting point is shielded from X-rays. The newly discovered stream of gas in the galaxy NGC 5548 — the first of its kind in one of the best-studied active galaxies — provides this protection, and it appears that the shielding has been going on for at least three years.

Supermassive black holes in the nuclei of active galaxies, such as NGC 5548, are known to expel large amounts of matter through powerful winds of ionized gas. For instance, the persistent wind of NGC 5548, known for two decades, reaches speeds exceeding 1,000 kilometers per second. But now a new obscuring wind has arisen, much stronger and faster than the persistent wind.

“The new wind reaches speeds of up to 5,000 kilometers per second but is much closer to the nucleus than the persistent wind,” Kaastra said. “The new gas outflow blocks 90 percent of the low-energy X-rays that come from very close to the black hole, and it obscures up to a third of the region that emits the ultraviolet radiation at a few light-days distance from the black hole.”

Because of this shielding, the persistent wind far away from the nucleus receives less radiation and cools down, causing new features to arise in the spectrum of the wind detected, allowing researchers to pinpoint the location of the strongest persistent wind component.

Strong X-ray absorption by ionized gas has been seen in several other sources where it has been attributed to passing clouds, but in this case, combined data from XMM-Newton and Hubble informed the team that it is a fast stream of outflowing gas very close to the nucleus which may originate from the bright disk surrounding the galaxy’s central black hole.


Kaastra et al. A fast and long-lived outflow from the supermassive black hole in NGC 5548. Science (2014).


Dr. Rebecca Caygill, UCL Press Office, +44 (0)20 3108 3846,

Further Information:

About UCL MSSL (Mullard Space Science Laboratory)

UCL’s MSSL (Department of Space and Climate Physics, is a world-leading research organization and is the UK’s largest university-based space research group. MSSL delivers a broad, cutting-edge science program, underpinned by a strong capability in space science instrumentation, space-domain engineering, space medicine, systems engineering and project management. MSSL led the build of one of the instruments flying on XMM-Newton (Optical Monitor) and contributed to a second, the Reflection Grating Spectrometer.

About UCL (University College London)

Founded in 1826, UCL ( was the first English university established after Oxford and Cambridge, the first to admit students regardless of race, class, religion or gender and the first to provide systematic teaching of law, architecture and medicine. We are among the world’s top universities, as reflected by our performance in a range of international rankings and tables. According to the Thomson Scientific Citation Index, UCL is the second most highly cited European university and the 15th most highly cited in the world. UCL has nearly 25,000 students from 150 countries and more than 9,000 employees, of whom one third are from outside the UK. The university is based in Bloomsbury in the heart of London, but also has two international campuses — UCL Australia and UCL Qatar. Our annual income is more than £900 million.

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