A new type of galaxy has been discovered thanks to observations made with the ESO’s VLT giant telescope together with the Gemini South and Canada-France-Hawaii telescopes. Because of their strange appearance, the galaxies have been nicknamed “green bean galaxies”. Behind the galaxies’ luminosity lies the intense radiation emitted from the regions around giant black holes. These galaxies are among the rarest in the Universe.
Many galaxies have giant black holes at their centers that make the gas around them glow. In a green bean galaxy, it is the whole galaxy that glows, not just its center. These new observations show the largest and brightest such regions ever found. The source of the light is believed to be the central black hole, which now appears to have shut down after previously being very active.
Mischa Schirmer’s Discovery
Astronomer Mischa Schirmer at the Gemini Observatory had been looking at many images of the distant universe in search of galaxy clusters. But when he happened to see an object in an image from the Canada-France-Hawaii Telescope (CFHT), he was stunned. It looked like a galaxy, but glowed brightly in a green color, and didn’t look like any galaxy he had seen before. This was something completely unexpected. He immediately applied to use ESO’s Very Large Telescope (VLT) to find out what was causing the unusual green color.
“ESO granted me special observing time and just a few days after I sent my application, this bizarre object was observed by the VLT. Ten minutes after the observations were made in Chile, I had the measurements on my computer in Germany. I quickly changed the focus of my research activities because it became clear that I had encountered something really new,” says Schirmer.
The New Object: J2240
The new object has been given the catalog name J224024.1-092748 or just J2240. It is located in the constellation Aquarius and it has taken 3.7 billion years for its light to reach Earth.
After the discovery, Schirmer’s team searched through a list of nearly a billion other galaxies and found 16 others with similar properties. These were then confirmed by observations with the Gemini South telescope. These galaxies are so rare that on average there is only one in the volume occupied by a cube with sides 1.3 billion light years across. This new class of galaxies has been nicknamed green bean galaxies because of their color. They are also reminiscent of the smaller objects known as green pea galaxies.
The Luminous Green Regions
In many galaxies, the material around the supermassive black hole at the centre of the galaxy glows so brightly that the surrounding gas is ionized and starts to shine brightly. Usually, these luminous areas in active galaxies are still small, at most 10 percent of the galaxy’s diameter. But in the case of J2240, the team’s observations show that the luminous region in this galaxy is huge. It covers the entire galaxy, making it one of the largest such areas ever found. Ionized oxygen glows with a bright green hue, which explains the strange color that first caught Schirmer’s attention.
“These luminous regions serve as amazing samples for those who want to understand the physics of galaxies. It’s like taking the temperature of a distant galaxy. Normally, these regions are neither very large nor bright, and are only visible in nearby galaxies. But in these newly discovered galaxies, they are huge and so bright that they can be studied in detail even though they are far away,” says Schirmer.
The Mystery of the Black Hole
After further analysis of the measurements, another mystery emerged. The black hole at the center of J2240 is far less active than expected based on the size of the luminous objects around it. The team suspects that what they are seeing is the echo of a time when the black hole was much more active than it is now. It is likely that these regions will gradually fade as what remains of the black hole’s radiation passes through them and out into space.
Galactic Evolution
These galaxies point to a galactic nucleus whose luminosity is fading, and thus also to a very short-lived phase in the life history of a galaxy. When the Universe was young, galaxies were far more active than they are today. Heavy black holes grew in their midst, swallowing up the stars and gas surrounding them so that they could shine with up to 100 times more luminosity than all the stars in the galaxy combined. Echoes like those seen in J2240 allow astronomers to study what happens when these active celestial bodies shut down. Not only that, but also how, when and why they shut down and why they are so rare in younger galaxies. This is exactly what the team plans to do next, by following up this study with observations in X-rays and with spectroscopy.
“Discovering something genuinely new is an astronomer’s dream come true, a once-in-a-lifetime event. It’s very inspiring!” concludes Schirmer.
