University of Minnesota
School of Physics & Astronomy

Spotlight

Dwarf Galaxies Provide Clues to Early Universe

Evan Skillman
Evan Skillman
Alex Schumann
                                                       

Evan Skillman is an astrophysicist who studies helium abundances in dwarf galaxies to learn more about the very early universe. In the Big Bang theory, within the first three minutes, the material cools sufficiently to form the lightest elements—hydrogen, helium, and a tiny bit of lithium. All of the heavier elements are made later by stars, so the amount of helium relative to hydrogen is a prediction and therefore a strong constraint on the Big Bang theory.

The prediction is that helium accounts for roughly ¼ of all normal (baryonic) mass in the universe, with a three to one ratio of mass of hydrogen to helium.

This formation of elements as the universe cools is called big bang nucleosynthesis. There are predictions for how much helium was created in the early universe and Skillman and his collaborators are trying to use observations to measure the abundance of primordial helium. Skillman says that their measurement of the primordial helium abundance agrees very well with recent result released by the Planck space telescope mission which is currently surveying the remnant light from the Big Bang. “Our measurement of the primordial helium abundance was the only number they compared it to,” Skillman said.

Skillman and his group measure helium abundances by looking at galaxies with very little enrichment from the heavy elements. These are dwarf galaxies, so-called because they are less than one percent of the mass of our galaxy. Because these galaxies have been very inefficient at star formation, there are relatively few heavy elements enriching the interstellar medium around them. “When we measure the helium abundance in those pristine environments, it is a good measurement of the primordial helium abundance.”

Skillman’s group uses the Large Binocular Telescope (LBT) in Arizona to obtain spectra of the ionized gas in star forming regions. His group recently discovered an object whose heavy element abundances are so low that it is a record-holder, tied with the lowest ever measured. This ultra-pristine dwarf galaxy is just on the edge of our “Local Group,” a term astrophysicists use to describe the Milky Way, Andromeda, and more than 70 other smaller galaxies.

Skillman and his group are planning to use the LBT to do more measurements of helium and a comprehensive abundance measurement for all the elements in other types of galaxies. This comprehensive study will help them further understand the chemical evolution of galaxies, to see how galaxies change from when they form after the Big Bang into the galaxies that we see today.