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 an object in an image from MegaCam on the the Canada-France-Hawaii Telescope, he was stunned — it looked like a galaxy, but it was bright green. The properties of this object are unlike any galaxy ever seen before, something totally unexpected. Schirmer quickly applied and got time to use ESO’s Very Large Telescope to find out what was creating the unusual green glow.
The new object, labeled J224024.1−092748 or J2240, lies in 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 color and because they are superficially similar to, but larger than, green pea galaxies.
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 Schirmer’s attention. The team’s further analysis of the data 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 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 center, 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 centers 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.
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