In the middle of a giant galaxy cluster, extremely large and yet faint galaxies have been expected. Heidelberg Astronomers discovered the extremely-low-density galaxies called ultra-diffuse galaxies, in a place where no one would have expected them to be present.
This discovery is considered to be,
"both remarkable and puzzling," states Dr Thorsten Lisker. The research work was performed by Carolin Wittmann in Dr Lisker’s team at the Centre for Astronomy of Heidelberg University (ZAH). The findings have been published in the “Monthly Notices of the Royal Astronomical Society”.
The central region of the Perseus galaxy cluster. The mosaic image is composed of many "tiles" of long-exposure images taken with the 4.2 metre William Herschel Telescope on the island of La Palma. Ultra-diffuse galaxies are difficult to spot, as the enlargement shows. The diameter of the whole image area is about that of the full moon on the sky background. (Source: Carolin Wittmann, ZAH/ARI)
The solar system is located in the midst of an enormous galaxy made up of billions of stars, the Milky Way. The naked eye can see about 3,000 stars. However, only a "trace" of a galaxy and a few dozen stars would be visible in the night sky if Earth was located in an ultra-diffuse galaxy. These special classes of galaxies, so named for their majorly diffuse appearance, seemingly developed far fewer stars than other galaxies or else were earlier stripped of them by galactic tidal forces.
Only three years ago, Astronomers started to systematically search the universe for such ultra-diffuse galaxies. With the help of new technologies and large telescopes, they discovered the galaxies, particularly in huge galaxy clusters. Much to their surprise, the Researchers detected about 90 such galaxies in the core of the Perseus galaxy cluster.
The Perseus cluster is considered to be a dense collection of hundreds of small and large galaxies located 240 million light years away. Most of the ultra-diffuse galaxies surprisingly appear intact, with just few candidates displaying signs of ongoing disruption in spite of the sturdy tidal field.
"We asked ourselves how the sensitive ultra-diffuse galaxies could survive at all in an environment as unsettled as a galaxy cluster," explains Carolin Wittmann, First Author of the study and PhD Student at the Institute for Astronomical Computing (ARI) at the ZAH. He added, " Perhaps the stars in ultra-diffuse galaxies are gravitationally bound due to an especially high dark matter content." That would make sense, according to the researcher, since practically no signs of interactions with bigger galaxies were found.
The Heidelberg research was centered on long-exposure images of the Perseus galaxy cluster attained in 2012 with the 4.2 m William Herschel Telescope on the Canary Island of La Palma.
We originally wanted to study the effects of tidal forces on small known galaxies and any attending structural disruptions. But the quality of the data was so exceptional that we were able to find numerous ultra-diffuse galaxies.
Dr Thorsten Lisker, Project Initiator and Co-ordinator
His research group, in collaboration with international partners, is currently hoping to attain data of similar quality on the outskirts of the Perseus cluster, where, Thorsten Lisker adds, the influence of the environment would have been less strong, preserving much of the original appearance of the galaxies.