Although only our neighboring galaxy, Andromeda, is visible with the unaided eye in the Northern hemisphere, there is a wide variety of galaxies in our universe. Grand spirals dominate the covers of many textbooks and irregular galaxies are the subject of many motivational posters, but the universe is dominated by small dwarf galaxies.
To study dwarf galaxies, astronomers first had to develop telescopes that could detect them. Unlike the Milky Way’s 200-400 billion stars, a typical dwarf galaxy only contains about 100 million stars, meaning it is much more dim in the night sky. Most telescopes rely on the visible matter–mainly stars–in a galaxy and the light it emits to detect the presence of a galaxy. This means that the bigger and brighter an object appears, the easier it is to detect; a dwarf galaxy is neither of these. We knew these small galaxies existed, but their abundance wasn’t known until modern powerful telescopes were built.
Thankfully, these galaxies are often found orbiting a large galaxy, which helps astronomers know where to look for them. Telescopes can be aimed at the edges of bright galaxies to search for these dim companions. And we do want to find them, for what they lack in quantity of stars, they make up for in dark matter.
In order to be considered a galaxy, dark matter–an elusive material that does not interact with or emit light–must be present. The ratio of dark matter to stars is highest in dwarf galaxies, making them particularly good targets for studying dark matter.
Dwarf galaxies are also an important laboratory in which we can study what early stars would have looked like. As stars evolve, they create more metals which can then be used in the next generation of stars. If a population of stars has a low metal abundance, such as a population of stars in a dwarf galaxy, then these stars closely resemble stars in the early universe. Astronomers, including some here at IU, are working on many projects involving dwarf galaxies.
Dr. Katherine Rhode and Nick Smith, one of her graduate students, are currently studying a dwarf galaxy, Perseus 1, orbiting the Andromeda Galaxy. They are investigating the stellar population of this galaxy in hopes of learning more about the evolution of galaxies. The ages of stars in the companion dwarf galaxy compared to Andromeda may provide hints about when each formed, and whether other dwarf galaxies have already been consumed by the larger galaxy. A recent IU graduate, Dr. Alec Hirschauer, also discovered an extremely low metallicity galaxy. This is an excellent way to study what stars in the early universe were like and compare them with other dwarf galaxies of a similar age. This galaxy in particular is valuable since it is close enough to earth for astronomers to collect high-resolution data.
The recent interest in dwarf galaxies reminds us that sometimes the small, seemingly mundane things we see all around are far more interesting upon further inspection. The most ordinary objects can turn out to be important clues in unlocking the history of the universe.