Molecular gas and ionizing radiation in star-forming galaxies

I present the main findings of my PhD work. Using panchromatic observations ranging from the 21cm H I line in the radio regime to the extreme ultraviolet (UV), we studied ionizing radiation from massive stars (direct and through hydrogen recombination lines) as well as the atomic and molecular gas content in 15 highly star-forming local galaxies. The results are brought into cosmological context, taking a step forward towards finding answers to the following open questions in galaxy evolution: Which physical conditions enable galaxies to leak ionizing radiation (and power reionization)? What drives the high Lyman-alpha escape fractions observed in the early Universe? How did the massive stellar clumps found in high redshift galaxies have possibly formed?

One of the galaxies we studied is Tololo 1247-232, a galaxy that leaks ionizing radiation. We argue that the Lyman continuum escape in Tololo 1247-232 is facilitated by the large amount of ionizing radiation that is produced in the central region and then escapes from clumpy, density bounded regions. This scenario may also explain how early galaxies at z>6 have powered cosmic reionization. Additionally, we performed infrared and molecular gas (traced by CO) observations of galaxies drawn from the “Lyman Alpha Reference Sample” (LARS). The galaxies were selected as analogues of high-redshift galaxies. Our main discovery is a roughly linear trend between the Lyman-alpha escape fraction and the total gas depletion time. This finding is counter-intuitive, because given the resonant scattering nature of Lyman-alpha photons, an increase in atomic gas should result in longer path lengths out of the galaxy, making photons more prone to absorption. Some other process seems to facilitate Lyman-alpha escape. We speculate that gas accretion enhances the turbulence of the cold gas and shifts the Lyman-alpha photons out of resonance. This  scenario would naturally explain elevated Lyman-alpha escape fractions during the phases in cosmic history when galaxies were still accretion-dominated (at high-z) rather than defined by gas depletion. Finally, we present high-resolution interferometric observations of a single galaxy, LARS 8. The galaxy is a proto-typical analogue of normal star-forming galaxies at z~1-2, i.e. it is massive, has a large gas fraction, is rotationally supported and its morphology is dominated by massive clumps. We show that these clumps are the result of an extremely gravitationally unstable gas disc. Large scale instabilities are found across the whole extent of the rotating disc, with only the innermost 500pc being stabilized by its bulgelike structure. Our findings prove that gravitational instabilities may play a significant role in galaxy evolution, in particular at z ≃ 1-3, when galaxies are characterized by massive clumps.