Galaxy-wide echoes from the pastDecember 05, 2012
Many galaxies have a giant black hole at their centre that causes the gas around it to glow. However, in the case of green bean galaxies, the entire galaxy is glowing, not just the centre. These new observations reveal the largest and brightest glowing regions ever found, thought to be powered by central black holes that were formerly very active but are now switching off.
Astronomer Mischa Schirmer of the Gemini Observatory had looked at many images of the distant Universe, searching for clusters of galaxies, but when he came across one object in an image from the Canada-France-Hawaii Telescope he was stunned -- it looked like a galaxy, but it was bright green. It was unlike any galaxy he had ever seen before, something totally unexpected. He quickly applied to use ESO's Very Large Telescope to find out what was creating the unusual green glow .
"ESO granted me special observing time at very short notice and just a few days after I submitted my proposal, this bizarre object was observed using the VLT," says Schirmer. "Ten minutes after the data were taken in Chile, I had them on my computer in Germany. I soon refocused my research activities entirely as it became apparent that I had come across something really new."
The new object has been labelled J224024.1−092748 or J2240. It liesin the constellation of Aquarius (The Water Bearer) and its light has taken about 3.7 billion years to reach Earth.
After the discovery, Schirmer's team searched through a list of nearly a billion other galaxies  and found 16 more with similar properties, which were confirmed by observations made at the Gemini South telescope. These galaxies are so rare that there is on average only one in a cube about 1.3 billion light-years across. This new class of galaxies has been nicknamed green bean galaxies because of their colour and because they are superficially similar to, but larger than, green pea galaxies .
In many galaxies the material around the supermassive black hole at the centre gives off intense radiation and ionises the surrounding gas so that it glows strongly. These glowing regions in typical active galaxies are usually small, up to 10% of the diameter of the galaxy. However, the team's observations showed that in the case of J2240, and other green beans spotted since, it is truly huge, spanning the entire object. J2240 displays one of the biggest and brightest such regions ever found. Ionised oxygen glows bright green, which explains the strange colour that originally caught Schirmer's attention.
"These glowing regions are fantastic probes to try to understand the physics of galaxies -- it's like sticking a medical thermometer into a galaxy far, far away," says Schirmer. "Usually, these regions are neither very large nor very bright, and can only be seen well in nearby galaxies. However, in these newly discovered galaxies they are so huge and bright that they can be observed in great detail, despite their large distances."
The team's further analysis of the data soon revealed another puzzle. J2240 appeared to have a much less active black hole at its centre than expected from the size and brightness of the glowing region. The team thinks that the glowing regions must be an echo from when the central black hole was much more active in the past, and that they will gradually dim as the remnants of radiation pass through them and out into space .
These galaxies signal the presence of a fading galactic centre, marking a very fleeting phase in a galaxy's life. In the early Universe galaxies were much more active, growing massive black holes at their centres that swallowed up surrounding stars and gas and shining brilliantly, easily producing up to 100 times more light than all the stars in the galaxy together. Light echoes like that seen in J2240 allow astronomers to study the shutdown processes of these active objects to understand more about how, when, and why they halt -- and why we now see so few of them in younger galaxies. This is what the team aims to do next, by following up on this research with further X-ray and spectroscopic observations.
"Discovering something genuinely new is an astronomer's dream come true, a once-in-a-lifetime event," concludes Schirmer. "It's very inspiring!"
 The astronomers studied the object using the powerful X-shooter spectrograph on the VLT. By splitting the light up into its component colours they could find out the composition of the glowing material and why it was shining so brightly.
 The search was made using the huge online database of the Sloan Digital Sky Survey (SDSS).
Green Pea galaxies are small, luminous galaxies undergoing vigorous star formation. They were first spotted in 2007 by participants in the astronomical crowd-sourcing project Galaxy Zoo. Unlike green beans, these galaxies are very small -- our Milky Way galaxy contains a mass equivalent to that of around 200 average green pea galaxies. The similarity between green pea and green bean galaxies is limited to their appearance, as most of them are not closely related.
 In many active galaxies the view of the central black hole is blocked by large amounts of dust, making it difficult to measure the activity of the black hole. To check whether green bean galaxies are indeed different from other galaxies with hidden centres, the astronomers looked at data from these galaxies at much longer infrared wavelengths that easily penetrate even very thick dust clouds. The central regions of J2240, and the other green bean galaxies, turned out to be much fainter than expected. This means that the active nucleus is now really much weaker than suggested by the brightness of the glowing regions.
This research was presented in a paper, "A sample of Seyfert-2 galaxies with ultra-luminous galaxy-wide NLRs - Quasar light echos?", to appear in The Astrophysical Journal.
The team is composed of M. Schirmer (Gemini Observatory, Chile; Argelander-Institut fur Astronomie, Universitat Bonn, Germany), R. Diaz (Gemini Observatory, Chile), K. Holhjem (SOAR Telescope, Chile), N. A. Levenson (Gemini Observatory, Chile) and C. Winge (Gemini Observatory, Chile).
Photos of the VLT:http://www.eso.org/public/images/archive/category/paranal/
The year 2012 marks the 50th anniversary of the founding of the European Southern Observatory (ESO). ESO is the foremost intergovernmental astronomy organisation in Europe and the world's most productive ground-based astronomical observatory by far. It is supported by 15 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world's most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world's largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning the 39-metre European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become "the world's biggest eye on the sky".
The Gemini Observatory is an international collaboration with two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located on Mauna Kea, Hawai'i (Gemini North) and the other telescope on Cerro Pachon in central Chile (Gemini South); together the twin telescopes provide full coverage over both hemispheres of the sky. The telescopes incorporate technologies that allow large, relatively thin mirrors, under active control, to collect and focus both visible and infrared radiation from space.
The Gemini Observatory provides the astronomical communities in seven partner countries with state-of-the-art astronomical facilities that allocate observing time in proportion to each country's contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF); the UK Science and Technology Facilities Council (STFC); the Canadian National Research Council (NRC); the Chilean Comision Nacional de Investigacion Cientifica y Tecnologica (CONICYT); the Australian Research Council (ARC); the Argentinean Ministerio de Ciencia, Tecnologia e Innovacion Productiva; and the Brazilian Ministerio da Ciencia, Tecnologia e Inovaçao. The observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.
The Canada-France-Hawaii Telescope (CFHT) is operated by the National Research Council of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii.
Science Fellow, Gemini Observatory
La Serena, Chile
Kamuela, Hawaii, USA
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