An international team of researchers led by National Radio Astronomy Observatory and University of Virginia astrophysicist Sabrina Stierwalt has spotted seven isolated groups of only dwarf galaxies, the building blocks of giant galaxies.
This SDSS image shows a group of dwarf galaxies that are gravitationally bound together. The detection of seven such groups lends support to the idea that large galaxies formed by the collision and merger of smaller galaxies in the early Universe. Image credit: Sloan Digital Sky Survey.
Dwarf galaxies — relatively small collections of stars 100 to 1,000 times smaller than our Milky Way Galaxy — are thought to be the building blocks of massive galaxies.
Evidence for groups of merging dwarf galaxies, however, has been lacking, until now.
Using data from the Sloan Digital Sky Survey (SDSS) and several telescopes, Dr. Stierwalt and her colleagues from Canada, Chile and the United States have discovered seven distinct groups of dwarf galaxies with just the right starting conditions to eventually merge and form larger galaxies, including spiral galaxies like the Milky Way.
“Each group hosts three to five known members, has a baryonic mass of 4.4×109 to 2×1010 solar masses and requires a mass-to-light ratio of
This discovery offers compelling evidence that the mature galaxies we see in the Universe today were formed when smaller galaxies merged many billions of years ago.
“We know that to make a large galaxy, the Universe has to bring together many smaller galaxies,” Dr. Stierwalt said.
“For the first time, we have found examples of the first steps in this process — entire populations of dwarf galaxies that are all bound together in the same general neighborhoods.”
Dr. Stierwalt and co-authors began their search by poring over SDSS data looking for pairs of interacting dwarf galaxies.
They then examined the images to find specific pairs that appeared to be part of even larger assemblages of similar galaxies.
The team then used the Magellan telescope in Chile, the Apache Point Observatory in New Mexico, and the Gemini telescope in Hawaii to confirm that the apparent clusters are not just on the same line of sight but are also approximately the same distance from Earth, indicating they are gravitationally bound together.
“We hope this discovery will enable future studies of groups of dwarf galaxies and offer insights into the formation of galaxies like the Milky Way,” Dr. Stierwalt said.
A paper reporting this discovery is published in the journal Nature Astronomy.
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S. Stierwalt et al. 2017. Direct evidence of hierarchical assembly at low masses from isolated dwarf galaxy groups. Nature Astronomy 1, article number: 0025; doi: 10.1038/s41550-016-0025
