Our theories of how galaxies form are firming up. A study of stars at the edge of the large spiral Galaxy M81 and its brightest neighbors confirms what we have seen in our own local group of galaxies, strengthening the theory that large galaxies are built from the accumulation of smaller ones.
Over recent years astronomers have come to favor what is known as the hierarchical model of galaxy formation, rather than the monolithic model whereby galaxies form largely independently. Nevertheless, the question is so important for our understanding of the cosmos that research continues.
The hierarchical model proposes that small “over-densities” of matter led to the formation of dwarf galaxies that gradually amalgamated or attracted material from neighbors until they formed large galaxies such as the one in which we live. One of the model’s predictions is that galaxies should lie inside huge envelopes of diffuse gas peppered with stars. This has been observed in the local group of galaxies, but naturally it is harder to detect such dispersed stars as we look further away.
A team led by Dr Sakurako Okamoto of the Shanghai Astronomical Observatory studied the area around the galaxy M81 and its near neighbors M82 and NGC 3077. This trio, which are all part of the M81 Group, and their accompanying dwarf galaxies lie 12 million light years away in Ursa Major, making them favorite objects for northern hemisphere amateur astronomers.
Previous observations had shown patterns in the gas suggestive of galaxies interacting. For example, Okamoto and her colleagues note in the Astrophysical Journal Letters that, “In NGC3077, 90% of the atomic hydrogen is located eastward of the center, in the tidal arm called ‘the Garland’”. Using the Subaru Telescope, Okamoto found “Young stars (~30–160 million years old) closely follow the neutral hydrogen distribution” for this and other structures. Similar-aged stars are also found in M81’s outer disk. Okamoto admits that no one knows why star formation ceased in these regions 30 million years ago.
Although Okamoto says that the team was not surprised at the neutral hydrogen-young star match, she told IFLScience that “The location of the M81 Group is an area of the sky [with large amounts of hydrogen], and it is difficult to know whether observed material is [from our galaxy] or pulled out of M81.” By identifying the stars as having the same distribution, the team confirmed that these structures are evidence of galaxies stripping material from each other.
Okamoto told IFLScience that the pattern observed between the three galaxies is similar to what we see between the Andromeda Galaxy (the largest member of the local group) and one of its companions, as well as what occurs between the Milky Way and the Magellanic Clouds. However, “these galaxies are less massive than M82 and NGC3077. In the M81 group, we could see more active galaxy assembly.”
The stars and gas stretching between the galaxies are the result of interactions within the last 300 million years, but Okamoto hopes to conduct “galactic archeology,” reconstructing the entire history of the group’s formation.