Sunday, August 19, 2012

Reionization by Massive Runaway Stars

Between 150 million and one billion years after the Big Bang, the universe underwent a phase of reionization where neutral hydrogen gas was transformed into ionized plasma. Energetic sources likely to be responsible for the reionization of the early universe include quasars, population III stars and quark-novae. Another possible mechanism for reionization in the early universe is massive runaway stars that travel sufficiently far from their birthplaces, allowing most of their ionizing radiation to ionize the neutral hydrogen comprising the intergalactic medium.

NGC 4449 - An irregular galaxy located about 12 million light-years away.

In the Milky Way galaxy, a significant fraction of massive stars travel at velocities exceeding 30 km/s. Their high speeds allow them to travel up to thousands of lights years away from their birthplaces before exploding as supernovae at the end of their several million year lifespans. Such a massive runaway star can be produced through dynamical ejection from a densely packed star cluster or by the explosion of a companion star in a binary star system. In the early universe, galaxies were much smaller than they are today. This enabled runaway massive stars to venture out of their host galaxies into the low-density outer regions. Here, the ionizing radiation they produce can ionize the intergalactic medium without being attenuated by the dense interstellar medium permeating the inner regions of their host galaxies.

The influence of massive runaway stars on the reionization of the universe can be directly tested by observing galaxies in the early universe. If massive runaway stars are common, they should dominate the stellar emission at the outer regions of these galaxies. That will manifest itself as an increase in the proportion of ultraviolet flux at the outskirts of such galaxies. Additionally, observational evidence for a high occurrence rate of supernovae far from the inner regions of these early galaxies will support the prevalence of massive runaway stars.