Oliver Hahn

Observatoire de la Côte d'Azur
Laboratoire Lagrange
Boulevard de l'Observatoire
CS 34229
06304 NICE

+33 (0)4 92 00 30 62 phone

oliver.hahn AT oca.eu

Research Interests:

My main research focus is numerical simulations of structure formation in the Universe.
Starting from our understanding of the very early universe, I perform simulations of a wide range of scales: how dark matter collapses and forms the seeds of galaxies, how the large-scale filamentary network of galaxies arises from the counteracting forces of the expansion of the universe and the pull of gravity; how the largest collapsed objects in the universe - clusters of hundreds and thousands of galaxies - form and evolve and what we can learn about our universe from those most extreme objects; and how galaxies form as part of the large-scale structure that feeds them with gas. I am also interested in developing numerical simulation techniques that allow us to answer questions about the universe both more precisely and more efficiently.


Starting grant of the European Research Council (2015)
Ambizione fellowship of the Swiss National Science Foundation (2011)

Past and Current Positions:

Observatoire de la Côte d'Azur / Université de Nice, France

University Professor


ETH Zurich, Switzerland

SNSF Ambizione Research Fellow


Stanford University/SLAC, USA

KIPAC Postdoctoral Fellow


ETH Zurich, Switzerland

Graduate Researcher


This section is still under construction, come back for more and improved texts.

arXiv:1509.04289 w/ D. Martizzi, H.-Y. Wu, A. Evrard, R. Teyssier, R. Wechsler

Multi-physics simulations of the most massive objects in our Universe


Galaxy clusters are the most massive collapsed objects in the Universe. They are exquisit astrophysical laboratories and very sensitive cosmological probes. We have performed multi-physics hydrodynamical simulations with the RAMSES code to study their properties and their evolution. In these simulations, we find that the cool-core/non-cool-core dichotomy of observed clusters is reproduced naturally in a cosmological environment. Cool cores are destroyed by low angular momentum major mergers. We also compare in detail the simulations with X-ray, SZ-, lensing results as well as constraints from galaxy properties. Another paper has just been accepted by MNRAS, where we studied the covariance between the stellar and the gaseous content of these massive systems. More to come soon!

arXiv:1501.01959, w/ Raul Angulo

The first alternative method to N-body for simulating dark matter


Dark matter numerical simulations and the N-body method are essential for understanding how structure forms and evolves in the Universe. However, the discrete nature of N-body simulations can affect its accuracy when modelling collisionless systems. We introduce a new approach to simulate the gravitational evolution of cold collisionless fluids by solving the Vlasov-Poisson equations in terms of adaptively refineable "Lagrangian phase space elements". These geometrical elements are piecewise smooth maps between three-dimensional Lagrangian space and six-dimensional Eulerian phase space and approximate the continuum structure of the distribution function. They allow for dynamical adaptive splitting to follow the evolution even in regions of very strong mixing. We discuss various test problems which demonstrate the correctness and performance of our method. We show that it has several advantages compared to standard N-body algorithms by i) explicitly tracking the fine-grained distribution function, ii) naturally representing caustics, iii) providing an arbitrarily regular density field defined everywhere in space, iv) giving a smooth and regular gravitational potential field, thus eliminating the need for any type of ad-hoc force softening. Finally, we illustrate the feasibility of using our method for cosmological studies by simulating structure formation in a warm dark matter cosmology. We show that spurious collisionality and discreteness noise of N-body methods are both strongly suppressed, which eliminates artificial fragmentation of filaments while providing access to the full deterministic evolution of the fluid in phase space. Therefore, we argue that our new approach improves on the N-body method when simulating self-gravitating cold and collisionless fluids, and is the first method that allows to explicitly follow the fine-grained evolution in six-dimensional phase space.

arXiv:1404.2280, w/ Raul Angulo and Tom Abel

New Insights into the Properties of Cosmic Velocity Fields


Understanding the velocity field is very important for modern cosmology: it gives insights to structure formation in general, and also its properties are crucial ingredients in modelling redshift-space distortions and in interpreting measurements of the kinetic Sunyaev-Zeldovich effect. Unfortunately, characterising the velocity field in cosmological N-body simulations is very much complicated by two facts: i) The velocity field becomes manifestly multi-valued after shell-crossing and has discontinuities at caustics. This is due to the collisionless nature of dark matter. ii) N-body simulations sample the velocity field only at a set of discrete locations, with poor resolution in low-density regions. In this paper, we discuss how the associated problems can be circumvented by using a phase-space interpolation technique. This method provides extremely accurate estimates of the cosmic velocity fields and its derivatives, which can be properly defined without the need of the arbitrary "coarse-graining" procedure commonly used. We explore in detail the configuration-space properties of the cosmic velocity field on very large scales and in the highly nonlinear regime. In particular, we characterise the divergence and curl of the velocity field, present their one-point statistics, analyse the Fourier-space properties and provide fitting formulae for the velocity divergence bias relative to the non-linear matter power spectrum. We furthermore contrast some of the interesting differences in the velocity fields of warm and cold dark matter models.

Most movies and images are available at higher resolution and as stereoscopic 3D versions. Publication or reproduction of any images or movies from this website requires permission and proper credits.
Please contact oliver.hahn\\at//oca.eu for more information.

Formation of a very massive galaxy clusters

This movie shows the evolution of the gas during the formation of a very massive galaxy cluster from the RHAPSODY-G simulations. The gas density is shown in gray colors with the gas temperature overlaid in orange. The abundance and properties of these massive systems contain a wealth of information about the physics and cosmology of our Universe. The image is 17 Mpc/h wide, projection depth is 5 Mpc/h.
Visualization by Oliver Hahn
Simulation: Oliver Hahn, Davide Martizzi

Formation of the cosmic web, dark matter streams II

This movie shows the formation of the cosmic web of dark matter from random fluctuations in the early universe. The intricate network of filaments hosts dark matter halos, the most massive of them are rendered in bright yellow hinting at the galaxies they are thought to host in the LCDM cosmological model.
Visualization by Ralf Kaehler, Carter Emmart, Tom Abel for AMNH planetarium show "The Dark Universe"
Simulation: Tom Abel, Oliver Hahn
Read more here

Formation of the cosmic web, dark matter streams I

This movie shows the formation of the in the hierarchical LCDM structure formation scenario. The density field shown in the movie is rendered using the method presented in Kaehler, Hahn & Abel 2012 based on the Lagrangian tesselation method discussed in Abel, Hahn & Kaehler 2012.
Visualization: Ralf Kaehler, Tom Abel, Oliver Hahn
Simulation: Oliver Hahn, Tom Abel
©2011 Kavli Institute for Particle Astrophysics and Cosmology

Formation of a warm dark matter halo

This movie shows the formation of a halo in a warm dark matter cosmology. Unlike in CDM, halos here have no progenitors but form from smooth accretion of dark matter (see also my papers on WDM halo formation, Angulo, Hahn & Abel 2013 and Hahn & Paranjape 2013). The density field shown in the movie is rendered using the method presented in Kaehler, Hahn & Abel 2012 based on the Lagrangian tesselation method discussed in Abel, Hahn & Kaehler 2012.
Visualization: Ralf Kaehler, Tom Abel, Oliver Hahn
Simulation: Oliver Hahn, Tom Abel

Formation of a massive galaxy cluster in an N-body simulation

This movie shows the formation of a massive galaxy cluster in the LCDM cosmological model. The cluster forms hierarchically over time by accreting myriads of smaller clumps, of which the larger ones are expected to host galaxies.
From the RHAPSODY simulation suite, Wu, Hahn, Wechsler, Mao & Behroozi 2013.
Visualization: Ralf Kaehler.
Simulations: Hao-Yi Wu, Oliver Hahn, Risa Wechsler.
Read more here.

More visualizations

More beautiful visualizations can be seen on the web page of my friend and collaborator Ralf Kaehler at SLAC.

Check out a video featuring me about the KIPAC visualization lab.


MUSIC is a program to generate multi-resolution "zoom" "initial conditions for cosmological simulations.

MUSIC currently has the following features:

  • Supports output for many cosmological simulation codes via plugins: RAMSES, ENZO, Gadget-2/3, Arepo, ART, Pkdgrav/Gasoline and NyX are currently supported. New codes can be added easily.
  • Support for first (1LPT) and second order (2LPT) Lagrangian perturbation theory
  • Pluggable transfer functions, currently CAMB, Eisenstein&Hu, BBKS, Warm Dark Matter variants. Distinct baryon+CDM fields are possible.
  • Minimum bounding ellipsoid and convex hull shaped high-res regions supported with most codes optimizing the high-resolution volume. supports refinement mask generation for RAMSES.


This is a C++ header library I started developping several years ago and that allows easy access to AMR and particle data structures from snapshots of the RAMSES code. RAMSESread++ is released under the GPL3.