Investigating the origin of radio emission in nearby starburst galaxies via high-resolution metre and centimetre observations

Doktorsavhandling, 2016

Star formation and galaxy evolution are intimately linked together. A detailed understanding of the physics of star formation can help us explain how galaxies evolve into, for example, our present Milky Way. Using Very Long Baseline Interferometry (VLBI) techniques it is possible to achieve high enough angular resolution to study radio emission from other galaxies in great detail. This thesis presents observations of three nearby Luminous Infrared Galaxies (LIRGs): NGC 4418, M 82 and Arp 220. While the centres of these galaxies are all heavily obscured in optical wavelengths, radio observations can be used to probe star formation properties in the centres of these galaxies. The galaxy NGC 4418 is radio weak with respect to the far infrared (FIR)-radio correlation for star forming galaxies. We present evidence for a young starburst in the centre, likely fuelled by gas falling in from a recent interaction with the nearby galaxy VV 655. We argue that this scenario can explain the low radio luminosity of this galaxy, and possibly also of other galaxies which appear to be radio weak. M 82 and Arp 220 follow the FIR-radio correlation and are excellent laboratories to study the physics of star formation in extreme environments. In this thesis we report on new groundbreaking subarcsecond resolution observations of M 82 and Arp 220 with the international LOFAR telescope, where we for the first time spatially resolve the radio emission from their nuclei at metre wavelengths. We report on previously unknown steep-spectrum radio structures and study effects of free-free absorption. We conclude that high angular resolution is essential for a correct interpretation of the radio emission from these complex objects. Furthermore, this work demonstrates that LOFAR can be used to obtain subarcsecond resolution images at metre wavelengths, a capability which can be used in multiple areas of astronomy in the future. Finally, we present results from new and archival global VLBI observations of Arp 220 spanning 17 years. We show that a self-consistent approach is essential to understand the nature of the radio emission. We find the data rich in details: we detect more than 80 compact objects, many with luminosities and sizes measured at multiple times and frequencies. We present a first analysis of the data where we discuss the general properties of the source population.