As galaxies go, Andromeda IX is a mighty dim bulb.
In fact, it is dimmest galaxy ever detected, which means it could give clues to the mysterious dark matter that appears to be pushing regular matter around. And it’s right in our cosmic back yard.
Andromeda IX is a small satellite of the Andromeda galaxy, the Milky Way’s closest galactic neighbor at a distance of about 2 million light-years from Earth. A light-year is about 6 trillion miles (10 trillion kilometers), the distance light travels in a year.
Astronomers making a map of one-quarter of the sky found it by concentrating on a diffuse clump of stars that turned out to be the tiny galaxy. The discovery was the subject of a presentation on Monday at a meeting of the American Astronomical Society in Denver.
How dim is Andromeda IX? At least twice as faint as the previous record holder, and so diffuse that it appears 100 times dimmer than the night sky. Astronomers spotted it with instruments involved in the Sloan Digital Sky Survey, the sky-mapping project.
Astronomer Daniel Zucker said Andromeda IX fits the profile for the small, dim galaxies that cosmic theorists predict should exist as leftovers from the formation of big galaxies.
How small can a galaxy be?
These little galaxies have never been detected before, but Andromeda IX might be one, and could help uncover more information about how dark matter behaves, Zucker said by telephone from the Max Planck Institute for Astronomy in Heidelberg, Germany.
These small galaxies have has such scanty supplies of stars that they are extremely hard to spot, Zucker said.
“We found Andromeda IX from a couple dozen of its brightest stars,” he wrote in an e-mail. “This raises the question, ‘How small can a galaxy actually be?”’
Dark matter seeds
Theorists believe dark matter clumped together quicker than regular matter — the stuff that makes up stars, planets, human beings and everything else on Earth — after the theoretical Big Bang explosion 13 billion years ago.
These clumps of dark matter are the seeds for the formation of galaxies, but current theories indicate there should be a large number of these dark matter seeds — from about 50 to several hundred of them — floating around in our cosmic neighborhood without having been absorbed into larger galaxies, Zucker said.
“If you have the seed of dark matter with maybe some stars associated with it, you would probably have a little galaxy,” he said. “But the problem is that we haven’t found these.”
Andromeda IX might be the first of these mini-galaxies associated with free-floating dark matter, but this has not been confirmed. Astronomers may know more when they train their instruments on several other patches of faint stars in that area of the sky in a few months.
