Astronomers Identify New Galaxy Class – Green Bean Galaxies

A team of astronomers, led by Dr Mischa Schirmer of the Gemini Observatory, using observations from ESO’s Very Large Telescope (VLT), the Gemini South telescope and the Canada-France-Hawaii Telescope, has identified a new galaxy type called ‘green bean galaxies.’

Green bean galaxies, from left to right, top to bottom: J0020, J0024, J0111, J0113, J0159, J1155, J1347, J1351, J1441, J1455, J1504, J1505, J2050, J2135, J2202, J2240 and J2308 (M. Schirmer et al)

The green bean galaxies have unusual appearance – they glow in the intense light emitted from the surroundings of monster black holes and are extremely rare objects in the Universe.

Many galaxies have a giant black hole at their center that causes the gas around it to glow. However, in the case of green bean galaxies, the entire galaxy is glowing, not just the center.

The new galaxy class has been proposed after the discovery of a galaxy labeled J224024.1-092748 (or J2240 for short). This galaxy lies in the constellation of Aquarius and its light has taken about 3.7 billion years to reach Earth. The discovery will be reported in a paper in the Astrophysical Journal (ESO version / arXiv.org version).

“ESO (the European Southern Observatory) 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,” Dr Schirmer said. “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.”

After the discovery, Dr 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 color and because they are superficially similar to, but larger than, green pea galaxies.

This image shows a close-up of the sky around the green bean galaxy J2240 (CFHT / ESO / M. Schirmer)

In many galaxies the material around the supermassive black hole at the center gives off intense radiation and ionizes the surrounding gas so that it glows strongly. These glowing regions in typical active galaxies are usually small, up to 10 per cent 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. Ionized oxygen glows bright green, which explains the strange color that originally caught Dr 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,” Dr Schirmer said. “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 further analysis soon revealed another puzzle – J2240 appeared to have a much less active black hole at its center than expected from the size and brightness of the glowing region. The astronomers think 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.

“Discovering something genuinely new is an astronomer’s dream come true, a once-in-a-lifetime event,” Dr Schirmer said. “It’s very inspiring!”

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Bibliographic information: Schirmer M. 2012. A sample of Seyfert-2 galaxies with ultra-luminous galaxy-wide NLRs – Quasar light echos? Accepted for publication in ApJ; arXiv: 1211.7098