High-resolution radio imaging of galaxy nuclei

Licentiate thesis, 2014

Star formation and galaxy evolution are intimately linked together. A detailed understanding of the environment where stars form may help us explain the differences we see between galaxies today and at earlier times in the history of the Universe. Galaxy nuclei are often heavily obscured by dust and cannot be observed in detail at optical wavelengths. Since radio waves are almost unaffected by dust, they can tell us about the physics in dense star forming regions. To achieve high enough image resolution at radio wavelengths, we use interferometry to synthesize a large aperture using data from multiple radio telescopes. We obtain detailed images of radio sources associated with star formation, such as core-collapse supernovae and their corresponding remnants. Using series of images at different observing wavelengths and different times, we can model the evolution of the radio sources and estimate densities, magnetic fields and star formation rates. We have studied the luminous nuclei of three different galaxies; M82, NGC 4418 and Arp 220. M82 and Arp 220 are well known starburst galaxies, meaning they have rates of star formation so high that they cannot be sustained over the history of the Universe. Both galaxies also follow the well-known FIR/radio correlation. In this thesis, I present ongoing work to model in detail the environment of Arp 220 and M82, based on high-resolution imaging of their nuclei. Our image of M82 is a new record in terms of image resolution at low frequencies. NGC 4418 is different; it does not follow the FIR/radio correlation. The high observed excess infrared luminosity of this galaxy has been suggested to come instead from an accreting black hole. We argue that although there might be an accreting black hole in NGC 4418, it cannot account for all the complex structure revealed by high-resolution radio imaging, indicating a significant starburst component.