PhD positions

Unfortunately, we currently have no explicit openings. PhD positions can however also be funded through the doctoral school of UCA. If you are interested to apply with a project with us, please do get in touch with us well ahead of time.

Postdoc positions

Currently all postdoc positions are filled. But other exciting possibilities exist: If you are interested to apply for a Marie Curie fellowship with us a host, this is always a possibility and we have experience with such grants. Or if you are interested in pursuing an option based on bi-lateral funding between France and your host country, there are usually also multiple options. Please do get in touch.

Master projects/internships

It is in principle always possible to carry out a master project within our group. Do get in touch well ahead of time if you are interested (usually well before X-mas the year before the project is supposed to take place).

Here are some courses that we teach at UCA.

Fluid mechanics for Astrophysicists (O. HAHN)

This is a fundamental course in the first year master program in astrophysics at UCA (MAUCA M1). The course comprises kinetic theory, fluid mechanics with a focus on astrophysical applications, some plasma physics and MHD, as well as a short module on computational fluid mechanics.

The lecture notes can be found here as a PDF file.

If you are an undergrad student and interested in pursuing a master in astrophysics, have a look at MAUCA

Cosmological Simulations (O. HAHN)

This is an optional module in master program in astrophysics at UCA (MAUCA). The course comprises a very concise introduction to cosmology with a very strong focus on numerical aspects and the basics of cosmological simulations: linear perturbation theory, generation of initial conditions, 1d collisionless dynamics, running Gadget simulations, analysing simulations using power spectra and mass functions.

The course notes can be found here as a PDF file.

If you are an undergrad student and interested in pursuing a master in astrophysics, have a look at MAUCA

List of recent publications (since 2017) from the group


  • Ramakrishnan S., Paranjape A., Hahn O., Sheth R.K., 2019, “Cosmic web anisotropy is the primary indicator of halo assembly bias”, MNRAS submitted, arXiv:1903.02007
  • S.S. Tie, D.H. Weinberg, P. Martini, W. Zu, S. Peirani, T. Suarez and S. Colombi, 2019, “UV Background Fluctuations and Three-Point Correlations in the Large Scale Clustering of the Lyman-alpha Forest’’, MNRAS in press
  • C. Gouin , R. Gavazzi , C. Pichon , Y. Dubois , C. Laigle , N. E. Chisari , S. Codis , J. Devriendt & S. Peirani, 2019, “Weak lensing in the Horizon-AGN simulation lightcone - Small scale baryonic effects”, A&A, 626, A72
  • Buehlmann M., Hahn O., 2019, “Large-Scale Velocity Dispersion and the Cosmic Web”, MNRAS, 487, 228, arXiv:1812.07489
  • Uhlemann C., Rampf C., Gosenca M., Hahn O., 2019, “Semiclassical path to cosmic large-scale structure”, PRD, 99, 083524 arXiv:1812.05633
  • Ogiya G., van den Bosch F. C., Hahn O., Green S. B., Miller T. B., Burkert A., 2019, “DASH: a library of dynamical subhalo evolution”, MNRAS, 485, 189 arXiv:1901.08601
  • G. Martin, S. Kaviraj, C. Laigle, J. Devriendt, Y. Dubois, C. Pichon, A. Slyz and S. Peirani, 2019, “The formation and evolution of low-surface-brightness galaxies”, MNRAS, 485, 796,
  • S. Peirani, A. Sonnenfeld, R. Gavazzi, M. Oguri, Y. Dubois, J. Silk, C. Pichon, J. Devriendt and S. Kaviraj, 2019, “Total density profile of massive early-type galaxies in Horizon-AGN simulation: impact of AGN feedback and comparison with observations”, MNRAS, 483, 4615
  • Rampf C., 2019, “Quasi-spherical collapse of matter in ΛCDM”, MNRAS, 484, 5223 arXiv:1712.01878
  • K. Kraljic, C. Pichon, Y. Dubois, S. Codis, C. Cadiou, J. Devriendt, M. Musso, S. Arnouts, H.S. Hwang, C. Laigle, S. Peirani, A. Slyz, M. Treyer & D. Viber, 2019, “Galaxies flowing in the oriented saddle frame of the cosmic web”, MNRAS, 483, 3227
  • A. Halle, S. Colombi and S. Peirani, 2019, “Phase-space structure analysis of self-gravitating collisionless spherical systems”, A&A, 621, A8


  • Ogiya G., 2018, “Tidal stripping as a possible origin of the ultra diffuse galaxy lacking dark matter”, MNRAS, 480, L106 arXiv:1804.06421
  • N.E. Chisari, M.L.A. Richardson, J. Devriendt, Y. Dubois, A. Schneider, A.M.C. le Brun, R.S. Beckmann, S. Peirani, A. Slys and C. Pichon, 2018, “The impact of baryons on the matter power spectrum from the Horizon-AGN cosmological hydrodynamical simulation”, MNRAS, 480, 3962
  • A. Spacek, M. Richardson, E. Scannapieco, J. Devriendt, Y. Dubois, S. Peirani and C. Pichon, 2018, “Using Real and Simulated Measurements of the Thermal Sunyaev-Zel’dovich Effect to Constrain Models of AGN Feedback”, ApJ, 865, 109
  • T. Okabe, T. Nishimichi, M. Oguri, S. Peirani, T. Kitayama, S. Sasaki and Y. Suto, 2018, “Projected alignment of non-sphericities of stellar, gas, and dark matter distributions in galaxy clusters: analysis of the Horizon-AGN simulation”, MNRAS, 478, 1141
  • Paranjape A., Hahn O., Sheth R. K., 2018, “The dependence of galaxy clustering on tidal environment in the Sloan Digital Sky Survey”, MNRAS, 476, 5442 arXiv:1801.04568
  • Adam R., Hahn O., Ruppin F., et al., 2018, “Substructure and merger detection in resolved NIKA Sunyaev-Zel’dovich images of distant clusters”, A&A, 614, A118 arXiv:1712.01836
  • E. Deriaz and S. Peirani, 2018, “Six-dimensional adaptive simulation of the Vlasov equations using a hierarchical basis, Multiscale Modeling and Simulation, 16(2), 583-614
  • van den Bosch F., Ogiya G., 2018, “Dark matter substructure in numerical simulations: a tale of discreteness noise, runaway instabilities, and artificial disruption”, MNRAS, 475, 4066 arXiv:1801.05427
  • Paranjape A., Hahn O., Sheth R.K., 2018, “Halo assembly bias and the tidal anisotropy of the local halo environment”, MNRAS, 476, 3631 arXiv:1706.09906
  • van den Bosch F., Ogiya G., Hahn O., Burkert A., 2018, “Disruption of dark matter substructure: fact or fiction?”, MNRAS, 474, 3043 arXiv:1711.05276
  • Libeskind N. I, …, Hahn, O., …, 2018, “Tracing the cosmic web “, MNRAS 473, 1195, arXiv:1705.03021
  • Ogiya G., Hahn O., 2018, “What sets the central structure of dark matter haloes?”, MNRAS, 473, 4339 arXiv:1707.07693


  • S. Peirani, Y. Dubois, M. Volonteri, J. Devriendt, K. Bundy, J. Silk, C. Pichon, S. Kaviraj, R. Gavazzi and M. Habouzit, 2017, “Density profile of dark matter haloes and galaxies in the Horizon-AGN simulation: the impact of AGN feedback”, MNRAS, 472, 2153
  • N.E. Chisari, N. Koukoufilippas, A. Jindal, S. Peirani, R. S. Beckmann, S. Codis, J. Devriendt, L. Miller, Y. Dubois, C. Laigle, A. Slyz and C. Pichon, 2017, “Galaxy-halo alignments in the Horizon-AGN cosmological hydrodynamical simulation”, MNRAS, 472, 1163
  • Angulo R., Hahn O., Ludlow A., Bonoli S., 2017, “Earth-mass haloes and the emergence of NFW density profiles”, MNRAS, 471, 4687, arXiv:1604.03131
  • R. S. Beckmann, J. Devriendt, A. Slyz, S. Peirani, M.L.A. Richardson, Y. Dubois, C. Pichon, N.E. Chisari, S. Kaviraj , C. Laigle and M. Volonteri, 2017, “Cosmic evolution of stellar quenching by AGN feedback: clues from the Horizon-AGN simulation” MNRAS, 472, 949
  • Hahn O., Martizzi D., Wu. H.-Y., Evrard A.E., Teyssier R., Wechsler R.H., “Rhapsody-G simulations I: the cool cores, hot gas and stellar content of massive galaxy clusters”, MNRAS, 470, 166, arXiv:1509.04289
  • C. Gouin, R. Gavazzi, S. Codis, C. Pichon, S. Peirani and Y. Dubois 2017, “Multipolar moments of weak lensing signal around clusters. Weighing filaments in harmonic space”, A&A, 605, A27
  • Hayashi K., Ishiyama T., Ogiya G., Chiba M., Inoue S., Mori M., 2017, “Universal Dark Halo Scaling Relation for the Dwarf Spheroidal Satellites”, ApJ, 843, 10 arXiv:1703.02604
  • Uhlemann R., Codis S., Hahn O., Pichon C., Bernardeau F., 2017, “Two is better than one: joint statistics of density and velocity in concentric spheres as a cosmological probe”, MNRAS, 469, 2481, arXiv:1612.00019
  • S. Kaviraj, C. Laigle, T. Kimm, J. Devriendt, Y. Dubois, C. Pichon, A. Slyz, E. Chisari and S. Peirani, 2017, “The Horizon-AGN simulation: evolution of galaxy properties over cosmic time, MNRAS, 467, 4739
  • L. Lin, J-H. Lin, C-H. Hsu, H. Fu, S. Huang, S.F. Sanchez, S. Gwyn, J.D. Gelfand, E. Cheung, K. Masters, S. Peirani, et al., 2017, “SDSS-IV MANGA: discovery of an Hα blob associated with a dry galaxy pair - ejected gas or a ‘dark’ galaxy candidate?”, ApJ, 837, 32
  • D. Suto, S. Peirani, Y. Dubois, T. Kitayama, T. Nishimichi, S. Sasaki and Y. Suto, 2017, “Projected Axis Ratios of Galaxy Clusters in the Horizon-AGN Simulation: Impact of Baryon Physics and Comparison with Observations”, PASJ, 69, 14
  • C. Welker, Y. Dubois, J. Devriendt, C. Pichon, S. Kaviraj and S. Peirani, 2017, “The rise and fall of stellar disks across the peak of cosmic star formation history: mergers versus diffuse stellar mass acquisition”, MNRAS, 465, 1241

Some recent highlights of the research in our group

Filaments getting stressed

cosmicweb veldispPhysicists are still puzzled over the exact nature of dark matter. Indirect observations point towards a cold and dark fluid that only interacts gravitationally with the rest of the Universe. Coldness means that in the early Universe, the fluid had a unique velocity at every point in space. During the gravitational collapse of density perturbations, the collisionless fluid starts to overlap and velocity dispersion develops in the collapsed regions forming the cosmic web. In this context, velocity dispersion can be thought of as a direction dependent (anisotropic) temperature which carries information about the formation history.

In Large-scale velocity dispersion and the cosmic web we measure the emergence of this dynamical stress in N-body simulations and use its anisotropic properties to disentangle the individual components (voids, walls, filaments and halos) of the cosmic web.

Never torn apart: a library of satellite halo evolution

N-body simulations are a powerful tool to study the non-linear dynamics of dark matter halos. However, it has been found that in N-body simulations smaller dark matter halos (subhalos) orbiting in a potential field of a larger halo can be unphysically destroyed due to poor numerical resolutions. In order to study the dynamical evolution of subhalos in a more reliable way, we have performed more than 2,000 N-body simulations of halo mergers with high enough numerical resolutions, covering the vast parameter space, and published the DASH library of the simulation data as well as a machine learned model for the mass evolution of subhalos. The DASH library is described in detail in Ogiya et al. 2019 and freely accessible here.

highlights dash orbits

The small meets the large: quantum theory for large-scale structure

Inspired by ideas from quantum mechanics, where a particle can be at many places at the same time, and takes all possible routes between two places, we have developed a new fashion to compute the evolution of the large-scale structure of the Universe. Instead of moving particles, as is done in what we call ‘Lagrangian perturbation theory’, this new theory calculates the probability that matter moved between two places. This approach has several important advantages over the previous way to do these calculations. An interesting analogy between the small and the large is that the locations where matter collapses to form galaxies and clusters corresponds to regions of interference in quantum mechanics.

The paper was published in Physical Review D: Uhlemann, Rampf, Gosenca and Hahn 2019

highlights semiclassicalPT

logoc osmo and simsgroup cosmo and sims umr LagrangeFrom left to right: Rampf, Zjupa, Hahn, Michel, Peirani, Ogiya, Buehlmann. Michaux is unfortunately missing.

 Welcome to the group for Theoretical and Numerical Cosmology at the Observatoire de la Côte d'Azur

We work on numerical simulations and theoretical work surrounding cosmology, galaxy and large-scale structure formation, as well as generally numerical methods in galactic/extragalactic astrophysics. We are part of the `galaxies and cosmology' team of Laboratoire Lagrange. Our work is financed by the European Union (Hahn, PI ERC StGt COSMO-SIMS), (Rampf, PI Marie Curie grant COSMO-BLOWUP), and CNRS (Peirani).

cosmo research cosmovisualization cosmodatasoftware


Oliver Hahn
Observatoire de la Côte d'Azur
Boulevard de l’Observatoire CS 34229
06304 Nice Cedex 4 France

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