As Associate Astronomy Prof. Eric Bell tried to get his restless 2 year old to sleep late one night in July, he came to a realization — one that would lead to the discovery of two new dwarf galaxies.
Bell realized the latest data released from the Sloan Digital Sky Survey was not yet analyzed by any group to find dwarf galaxies around the galaxy Andromeda. He quickly downloaded the data and began his search. Within a week, he and Rackham student Colin Slater each found a galaxy: Andromeda XXVIII and Andromeda XXIX.
“We know there are some around Andromeda but we don’t know how many, (and) we don’t really know the properties of them,” Slater said. “We don’t know this because they’re hard to detect. They’re faint, they’re small, (and) you need to look through a lot of sky to find them.”
The two galaxies are the farthest away from Andromeda, than any dwarf galaxy previously discovered.
Bell and Slater’s findings are published in the current edition of the Astrophysical Journal. Bell said the galaxies didn’t take long to find, but writing the paper and verifying their findings took several months.
“The actual finding of them is quite fast,” Bell said. “We had some other candidates, too, that were a lot less obvious, and we’re still actually trying to chew on those and figure out what’s going on.”
The discovery of the galaxies is aiding the analysis of dark matter in the night sky. Dark matter is used to explain unaccountable mass in galaxies. It’s called “dark” matter because it must have mass, but is not detectable with our eyes because it is not influenced by electromagnetic radiation, thus it does not emit or scatter light.
Models indicate that there should be thousands of dark matter halos surrounding Andromeda, and dwarf galaxies are indicators of those halos. With Bell and Slater’s discoveries, only 29 halos have been found — suggesting that some of the halos don’t have stars in them or that the models are wrong.
“This is one of the big reasons why one does this,” Bell said. “If none of those models can fit the observations, you may be backed into the corner where you have to accept that there are just a lot less dark matter halos than you imagined, and then you’ve learned something very exciting about dark matter halos.”
To find the galaxies, Bell and Slater looked at the sky survey data to locate clusters of Red Giant stars. Scientists predict that the sun will become a Red Giant at the end of its life when it no longer has a hydrogen-burning core.
Once a cluster of stars is found it must be verified as a dwarf galaxy with images from a telescope unless the data is strong enough that no image is necessary.
“They’re reasonably bright things for us to have missed,” Bell said. “So it’s good that we found them.”