Dr Oey will investigate how ultraviolet light from hot young stars, with photon energies high enough to ionize the hydrogen gas in and around galaxies, can escape from the stars' natal region and spread through the galaxy and beyond. She will use the red light of H alpha to map the ionized hydrogen in galaxies undergoing a burst of star formation, and use radiative transfer models to predict how much of the ultraviolet light escapes to intergalactic space. Studying the spectra of ionized gas around single hot stars in very nearby galaxies (the Magellanic Clouds and M33), she will test stellar-atmosphere models that predict how much ultraviolet light each star should supply. With spectroscopic measurements of star-bursting dwarf galaxies that trace emission from several atomic species, she will probe chemical inhomogeneity and ionization in the gas.
A woman graduate student and a woman postdoc will take part in the research. Dr Oey is a woman with a strong track record of working with undergraduates, and is involved in undergraduate outreach. At U. Michigan she is the lead organizer for a Theme Semester for the 2009 International Year of Astronomy, which includes special undergraduate classes, a production of Brecht's 'Galileo', distinguished lectures, exhibitions and star parties.
Massive stars are the source of high-energy, ultraviolet radiation in the universe, and this radiation evaporates gas. This radiation is a fundamental component of the energy budget in galaxies, and its interaction with gas determines whether and when new stars can form. On the largest scales, we want to know whether this radiation can escape from galaxies and evaporate the gas of the intergalactic medium, as it once did in the dawn of cosmic time. We have made major strides in understanding the origin and fate of this ultraviolet, ionizing radiation. We have used the emission-line spectra of the glowing nebulae heated by these massive stars, to test our quantitative understanding of the radiation energy emitted by stars of different masses and luminosities. This establishes which stellar atmosphere models are the most accurate, and in what areas they still need improvement. We also developed a new technique that uses nebular gas to map out the extent to which these clouds are transparent to this ionizing radiation. This allowed us to compile statistics on cloud transparency in two of the nearest star-forming galaxies, the Magellanic Clouds. We found that at least half of the nebulae are at least partially transparent, implying that much of the stellar radiation travels far beyond their birth clouds, and explains the heating of the interstellar medium. We also applied the technique to very active, starburst galaxies, and discovered one, NGC 5253, that clearly exhibits an ionization cone, which is a narrow region apparently evaporated by the ultraviolet radiation. This strongly suggests that the radiation is escaping from the galaxy altogether. Thus the reason for the failure of previous studies searching for escaping UV radiation from such galaxies may be that in such galaxies, the ionization cone must be oriented toward our line of sight. Finally, we examined a more distant sample of starburst galaxies with extreme properties. Their gas emission strongly suggests that they also are transparent to UV radiation, but we cannot definitively demonstrate that. However, our results show that if we can positively identify the presence of fast shocks in these objects, then we can conclude that the objects are transparent. We are now obtaining further observations of these objects to follow up on this possibility.
