06/05/2025
Today my colleague Christian ( ) successfully defended his PhD thesis, "Galaxy Clusters in the Decameter Sky"! In it, he mapped the sky at a largely unexplored range of radio frequencies—10–30 MHz—using the LOFAR telescope. Studying space at such low frequencies is incredibly challenging because the Earth's ionosphere distorts the incoming radio waves. Christian helped develop a new calibration technique that enables higher sensitivity and resolution imaging than ever before at these wavelengths.
I had the honour of designing Christian’s thesis cover, which features real decameter radio data in the background and four active galactic nuclei on the back cover (image 2). I also created an original digital artwork to represent the Perseus cluster on the front cover (images 1 & 3).
Christian’s research focused on the Perseus galaxy cluster—a massive system over 240 million light-years away—at 30–50 MHz using LOFAR’s low-frequency capabilities. His team confirmed the presence of two giant structures: a known radio "mini-halo" around the central galaxy, and a newly revealed, much larger "giant radio halo" extending over a million light-years. These halos are thought to arise from turbulence caused by ancient cosmic mergers, which re-energize particles and make them glow in radio light—something I tried to capture in my artwork.
They also detected faint, aged radio emissions from "ghost cavities," relics of past outbursts from the cluster’s central black hole. These findings offer new insight into how black holes and galaxy collisions shape the structures of the universe.
Congratulations, Dr. Groeneveld 👏🎓🌌