Ralf Klessen

Hello,

I am professor for theoretical astrophysics at Heidelberg University working at theInstitute for Theoretical Astrophysics (ITA) at the Center for Astronomy (ZAH), where I currently serve as Managing Director. My research focuses on star formation in the present-day and early Universe, the structure and dynamics of the interstellar medium and the Milky Way, astrophysical turbulence, and computational astrophysics. I am particularly interested in combining numerical simulations with synthetic observations and advanced machine-learning methods to understand the physical processes that shape galaxies and their observable properties.

My ORCID number is 0000-0002-0560-3172.

For an overview of my curriculum vitae follow this LINK.

For a current list of publications click this LINK to ADS to ADS or this LINK to google scholar.

For my website at the Center for Astronomy at Heidelberg University follow this LINK.

I am co-PI of the ERC Synergy Grant ECOGAL together with Patrick Hennebelle (CEA Saclay), Sergio Molinari (INAF Rome), and Leonardo Testi (Uni Bologna). ECOGAL investigates the emergence of star-forming ecosystems in the Milky Way and the physical processes that regulate star formation. More information can be found here: LINK.The diffuse gamma-ray sky of a Milky Way analog: Local diversity and global constraints

Current Research Highlight

September 2026: The spectrum of magnetized turbulence in the interstellar medium

Karin Kjellgren, Philipp Girichidis, Maria Werhahn, Ralf S. Klessen, Christoph Pfrommer, Juan Soler, Brian Reville, Jim Hinton, Patrick Hennebelle, Noé Brucy, and Simon C. O. Glover:  Astronomy & Astrophysics, 710, A163 (2026) [DOI link]

Diffuse gamma-ray emission is a key tracer of cosmic rays (CRs) in galaxies, encoding information about their transport, energetics, and interaction with the interstellar medium. Interpreting the Milky Way’s gamma-ray sky, however, remains challenging because the observed emission depends jointly on the three-dimensional CR distribution and gas distribution, as well as the position of the observer within the Galaxy. Using the Rhea suite of CR–magnetohydrodynamic (MHD) simulations of a Milky Way analog, we investigated how pion-decay gamma-ray emission varies with galactic environment, local conditions, and CR transport physics. The emission was computed in post-processing under steady-state CR cooling and interaction assumptions, thus enabling us to analyze luminosities, spectra, full-sky emission maps, and angular power spectra (APS) for many observer positions, including those located inside Local Bubble-like cavities. The simulated galaxy naturally reproduces Milky Way-like gamma-ray luminosities and spectral slopes without any parameter tuning. While the total luminosity remains comparatively stable across the galaxy, the detailed morphology of the gamma-ray sky varies strongly with observer location due to the complex distribution of gas in the nearby environment, consistent with longstanding observational results. Across all observers, the APS closely follows the structure of the gas column density rather than the more diffuse CR energy density, in agreement with previous gamma-ray analyses and CR propagation models. Comparisons with Fermi–LAT data show good agreement for both the all-sky spectrum and the APS. A diffusion coefficient energy-scaling with power-law index δ = 0.5 generally matches the observations best. Our results show that these well-established features of Galactic gamma-ray emission arise naturally in fully self-consistent CR–MHD galaxy simulations. Gas density fluctuations are the primary drivers of the morphology of the pion-decay emission, while CR transport parameters govern its spectral and structural details. The Rhea simulations thus provide a physically grounded framework for interpreting diffuse gamma-ray observations and highlight the importance of understanding the observer’s local surroundings when using gamma rays to trace Galactic CR physics.

Earlier Research Highlights

For the all monthly research highlights follow this LINK.

Funding Sources