Alma gives astronomers a unique glimpse of a black hole’s eating habits

A team led by astronomers from Chalmers and Onsala Space Observatory have used the powerful telescope Alma to catch an unexpected glimpse of an extreme place in space: the base of a powerful jet close to a distant, hungry black hole.

There are supermassive black holes — with masses up to several billion solar masses — at the hearts of almost all galaxies in the Universe, including our own galaxy, the Milky Way. In the remote past, these bizarre objects were very active, swallowing enormous quantities of matter from their surroundings, shining with dazzling brilliance, and expelling tiny fractions of this matter through extremely powerful jets. In the current Universe, most supermassive black holes are much less active than they were in their youth, but the interplay between jets and their surroundings is still shaping galaxy evolution.

A new study, published today in the journal Astronomy & Astrophysics, used Alma to probe a black hole jets in a very distant and active object called PKS 1830-211. Light from PKS 1830-211 had to travel for about 11 billion years before reaching us (a redshift of 2.5). The light we see was emitted when the Universe was just 20% of its current age.

In PKS 1830-211, Ivan Martí-Vidal (Chalmers and Onsala Space Observatory) and his team observed a supermassive black hole with a bright and very active jet in the early Universe. It is unusual because its brilliant light passes a massive intervening galaxy on its way to Earth, and is split into two images by gravitational lensing. Einstein’s theory of general relativity predicts that light rays will be deflected as they pass a massive object such as a galaxy. The lensing can create multiple images as well as distort and magnify the background light sources.

From time to time, supermassive black holes suddenly swallow a huge amount of mass, a star or a cloud of gas and dust, which increases the power of the jet and boosts the radiation up to the very highest energies. And now Alma has, by chance, caught one of these events as it happens in PKS 1830-211.

The remote active galaxy PKS 1830-211 from Hubble and Alma. Credit: I. Martí-Vidal"The Alma observation of this case of black hole indigestion has been completely serendipitous. We were observing PKS 1830-211 for another purpose, and then we spotted subtle changes of colour and intensity among the images of the gravitational lens. A very careful look at this unexpected behaviour led us to the conclusion that we were observing, just by a very lucky chance, right at the time when fresh new matter entered into the jet base of the black hole," says Sebastien Muller, a co-author of the paper.

The team also looked to see whether this violent event had been picked up with other telescopes and were surprised to find a very clear signal in gamma rays, thanks to monitoring observations with the Fermi-LAT satellite. The process that caused the increase of radiation at Alma’s long wavelengths was also responsible of boosting the light in the jet dramatically, up to the highest energies in the Universe. This energy is emitted as gamma rays, the shortest wavelength and highest energy form of electromagnetic radiation.

"This is the first time that such a clear connection between gamma rays and submillimetre radio waves has been established as coming from the real base of a black hole's jet," adds Sebastien Muller.

The observations are just the start of Alma's investigations into the workings of jets from supermassive black holes, near and far. The unique object PKS 1830-211 is expected to be the focus of much future research with Alma and other telescopes.

"There is still a lot to be learned about how black holes can create these huge energetic jets of matter and radiation," concludes Ivan Martí-Vidal. “But the new results, obtained even before Alma was completed, show that it is a uniquely powerful tool for probing these jets — and the discoveries are just beginning!"

Image (of the Alma Observatory above): Clem & Adri Bacri-Normier (wingsforscience.com)/ESO


More about Onsala Space Observatory
Onsala Space Observatory is Sweden's national facility for radio astronomy. The observatory provides researchers with equipment for the study of the earth and the rest of the universe. In Onsala, 45 km south of Gothenburg, it operates two radio telescopes and a station in the international telescope Lofar. It also participates in several international projects. The observatory is hosted by Department of Earth and Space Sciences at Chalmers University of Technology, and is operated on behalf of the Swedish Research Council.

Contacts:
Robert Cumming, astronomer and communications officer, Onsala Space Observatory at Chalmers University of Technology, +46-31-772 55 00 or +46-70-493 31 14, robert.cumming@chalmers.se
Ivan Martí-Vidal , astronomer, Onsala Space Observatory, at Chalmers University of Technology, +46 31 772 5557, ivan.marti-vidal@chalmers.se

More about Alma
The Atacama Large Millimeter/submillimeter Array (Alma), an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile. Alma is funded in Europe by the European Southern Observatory (ESO), in North America by the U.S. National Science Foundation (NSF) in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and in East Asia by the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Academia Sinica (AS) in Taiwan. Alma construction and operations are led on behalf of Europe by ESO, on behalf of North America by the National Radio Astronomy Observatory (NRAO), which is managed by Associated Universities, Inc. (AUI) and on behalf of East Asia by the National Astronomical Observatory of Japan (NAOJ). The Joint Alma Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of Alma.

More about the research
These research projects are presented a paper, “Probing the jet base of the blazar PKS 1830−211 from the chromatic variability of its lensed images: Serendipitous ALMA observations of a strong gamma-ray flare”, by I. Martí-Vidal et al. in the journal Astronomy & Astrophysics.
The team is composed of I. Martí-Vidal (Chalmers University of Technology, Onsala Space Observatory, Onsala, Sweden), S. Muller (Onsala), F. Combes (Observatoire de Paris, France), S. Aalto (Onsala), A. Beelen (Institut d’Astrophysique Spatiale, Université Paris-Sud, France), J. Darling (University of Colorado, Boulder, USA), M. Guélin (IRAM, Saint Martin d’Hère, France; Ecole Normale Supérieure/LERMA, Paris, France), C. Henkel (Max-Planck-Institut für Radioastronomie [MPIfR], Bonn, Germany; King Abdulaziz University, Jeddah, Saudi Arabia), C. Horellou (Onsala), J. M. Marcaide (Universitat de València, Spain), S. Martín (ESO, Santiago, Chile), K. M. Menten (MPIfR), Dinh-V-Trung (Vietnam Academy of Science and Technology, Hanoi, Vietnam) and M. Zwaan (ESO, Garching, Germany).

Links
ESO press release: http://www.eso.org/public/news/eso1344/
Pictures of ALMA: http://www.eso.org/public/images/archive/category/alma/​