A new galaxy class, named ‘green bean galaxies’, has been discovered by astronomers.
The discovery was made using observations from ESO’s Very Large Telescope (VLT), the Gemini South telescope, and the Canada-France-Hawaii Telescope (CFHT). The galaxy class, nicknamed green bean galaxies because of their colour and because they are superficially similar to, but larger than, green pea galaxies, are amongst the rarest objects in the Universe, astronomers said.
Gemini Observatory astronomer Mischa Schirmer came across the galaxy class while looking through the Canada-France-Hawaii Telescope. He had been searching for clusters of galaxies, but came across one object in an image from the telescope that caught his attention - 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 J2240. It 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.
Many galaxies have a giant black hole at their centre that causes the gas around it to glow, however with green bean galaxies the entire galaxy is glowing, not just the centre. The team’s observations showed that with J2240, the glowing region spanned the entire object. J2240 displays one of the biggest and brightest such regions ever found, the astronomers said.
"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 also showed that 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," said Schirmer. "It's very inspiring!"