An international team of astronomers has found tails of gas and/or stars trailing behind a sample of young galaxies without current star formation. Based on this result, the team concludes that about half of the ultra-diffuse galaxies in the Coma cluster are likely to have evolved through collisions with external gas. Ultra-diffuse galaxies together with similar dwarf elliptical galaxies account for about 80% of the members of galaxy clusters, so understanding their evolution is an important part of modeling the evolution of the Universe.
Figure: Conceptual image of the evolutionary path from a normal dwarf galaxy to an UDG/dE in a cluster. (a) An unperturbed galaxy falls near, but not directly through, the cluster. (b) Collision with intercluster gas triggers star formation and gas stripping, creating a “jellyfish” galaxy. (c) Star formation and stripping remove all of the gas, quenching further star formation. (d) The galaxy evolves into an UDG or dwarf elliptical (dE). (Credit: Kirill Grishin, Legacy Surveys / D. Lang (Perimeter Institute), NAOJ, CFHT, ESO )
Extended galaxies sparely populated by stars and exhibiting little current star formation are commonly found in galaxy clusters. It is thought that these ultra-diffuse galaxies (UDG) started as more normal dwarf galaxies, but some event removed most of the gas from the galaxies, preventing them from forming new stars, and causing them to puff up in size. But precisely because these ultra-diffuse galaxies are faint and diffuse, they are difficult to study, so their evolution remains poorly understood.
To work around this problem, an international team of astronomers from Russia, the USA, Japan, France, and the UAE, used archive data from the 8.2 m Subaru Telescope and new observations with the 6.5 m MMT to study galaxies which are currently bright, but expected to evolve into UDGs. The sample includes 9 galaxies in the Coma cluster (320 million light-years away in the direction of the constellation Coma Berenices) and 2 galaxies in the Abell 2147 cluster (510 million light-years away in the direction of the constellation Hercules). The team found that every galaxy in the sample exhibits a tail of gas and/or stars, indicating that they have recently collided with outside gas.
The space between galaxies in a cluster is not a perfect vacuum; there is very hot, thin intracluster gas. When a small galaxy passes through it, the gas inside the galaxy collides with this intracluster gas. This triggers a burst of rapid star formation, and the pressure from the intracluster gas pushes the original gas out of the galaxy. During this phase, the galaxy exhibits a bright tail or tails of gas streaming behind it, earning it the nickname “jellyfish galaxy.” The loss of gas prevents further star formation and changes the dynamics of the galaxy, causing it to puff up in size. In this way, collision with intracluster gas provides an all-in-one explanation for the evolution of UDGs. From the number of galaxies studied in this sample, the team estimates that approximately half of the UDGs in the Coma cluster have experienced this kind of gas stripping.
These results appeared as Grishin et al. "Transforming gas-rich low-mass disky galaxies into ultra-diffuse galaxies by ram pressure" in Nature Astronomy on November 1, 2021.
The Subaru Telescope is a large optical-infrared telescope operated by the National Astronomical Observatory of Japan, National Institutes of Natural Sciences with the support of the MEXT Project to Promote Large Scientific Frontiers. We are honored and grateful for the opportunity of observing the Universe from Maunakea, which has cultural, historical, and natural significance in Hawai`i.
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