Reionization and Satellite Galaxies to the Milky Way

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Milky Way is a spiral galaxy: Image by NASA

The Milky Way galaxy has a number of small satellite, or companion, dwarf elliptical galaxies orbiting it. Many astronomers, however, think that the Milky Way should have more satellite galaxies than it does. A theory recently proposed by Pierre Ocvirk and Dominique Aubert of the Strasbourg Observatory in France tries to explain this shortage. They propose that ultraviolet light from the first very hot massive stars to form in the Milky Way reionized the hydrogen gas surrounding the Milky Way, and thereby prevented more satellite galaxies from forming.

The Milky Way and Satellite Galaxies

On a clear dark night, stargazers can see a milky band that stretches across the sky. This band, the Milky Way, is the galaxy in which our solar system is located. The Milky Way is a giant spiral galaxy that contains a few hundred billion stars in a disk about 100,000 light years across. The stars in the disk of the Milky Way are arranged in a spiral arm pattern spiraling out from the central nucleus. The oldest stars in the Milky Way are distributed in a spherical halo, indicating that the galaxy originally had a spherical shape then flattened into a disk shape.

The large magellanic cloud is a satellite galaxy to the Milky Way. Image by NASA

The Milky Way has a number of companion or satellite galaxies. Perhaps the best known are the irregular Large Magellanic Cloud and Small Magellanic Cloud. Most of the Milky Way satellite galaxies are however dwarf elliptical galaxies, which typically contain only a few million very old stars.

What is Reionization?

Normal atoms have the same number of electrons as protons. Atoms with either extra or missing electrons are ions. In astronomical contexts, most ions result from missing rather than extra electrons.

When the universe formed in the big bang, the matter in the universe was ionized. The universe’s temperature, pressure, and density were too high for electrons to combine with nuclei to form neutral atoms. As the universe cooled, electrons could combine with hydrogen and helium (the only elements made in significant quantities in the big bang) nuclei to form neutral atoms.

Most of the atoms in the early and modern universe are hydrogen. Ultraviolet light, which the hottest most massive stars emit, has enough energy to reionize hydrogen atoms. An electron in a hydrogen atom can absorb the energy from ultraviolet light and escape the proton. Massive hot stars therefore reionize nearby neutral interstellar hydrogen atoms.

Reionization and Satellite Galaxies

According to Ocvirk and Aubert, the first generation of massive stars to form in the Milky Way reionized the interstellar hydrogen gas in and around the Milky Way. The newly reionized hydrogen could not as readily collapse to form new stars. Hence, the reionization inhibited the formation of satellite galaxies near the Milky Way.

The “hub” of the Milky Way: Image by NASA

Ocvirk and Aubert used a numerical computer simulation to calculate the effect of the reionization on the formation of satellite galaxies. Previous calculations assumed that the amount of ultraviolet light from hot stars was distributed uniformly around the Milky Way. Ocvirk and Aubert’s calculations more realistically take into account how the ultraviolet light from the first hot massive stars to form near the core of the Milky Way takes longer to reionize the gas further from the Milky Way. They compared their calculations to the actual distribution of satellite galaxies. The calculations match the data fairly well. Hence, they conclude that the reionization explains why there are fewer than expected satellite galaxies orbiting the Milky Way.

The reionization also explains why the dwarf elliptical galaxies only contain old stars. After the interstellar hydrogen in these galaxies reionized, it dissipated, rather than collapsing to form additional stars.

This theory is not yet a complete answer, but it represents progress in understanding the Milky Way’s satellite galaxies. Further, more accurate, calculations will both help astronomers understand the distribution of satellite galaxies with distance from the Milky Way’s core, and further test the theory.

Further Reading

Ocvirk, P. and Aubert, D. “A Signature of the Internal Reionization of the Milky Way.” (2011). Monthly Notices of the Royal Astronomical Society, vol.417, p. L93-L97.

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