Research projects

Influence of the cluster environment on the galaxy-halo connection

Galaxy clusters are large structures in the Universe, composed of tens or hundreds of galaxies bound by gravity. In the hierarchical formation model, they form and grow by accretion of smaller groups or isolated galaxies. In this scenario, understanding how these accreted galaxies interact with the very dense cluster environnement is an important step towards explaining the global picture of galaxy evolution and structure formation. Indeed, during infall, galaxies are subject to numerous interactions with the host cluster, both at the level of the baryonic and dark matter component, and these interactions influence their properties. In particular, both observations and numerical simulations suggest that its dark matter halo is stripped by the tidal forces of the host.

Part of my work is focussed on getting a better understanding on the evolution of cluster galaxies and their associated subhaloes, using weak gravitational lensing and numerical simulations.


Mapping the mass distribution in galaxy clusters with strong and weak gravitational lensing

In the cluster core, the geometry of the strong lensing constraints and the light distribution makes it possible to physically motivate the position of mass clumps. It is therefore more advantageous to use so-called parametric models in this region, which are described by physical quantities that allow a direct interpretation of the results. On the contrary, in the cluster outskirts a more flexible model is necessary, to account for the potentially irregular shape of the cluster, and allow for substructure detection. This can be achieved by using a non-parametric model where the mass distribution is reconstructed as a grid of mass pixels. I am developing a new version of the Lenstool software where the core and the outskirts of galaxy clusters can be modeled jointly, combining parametric and non-parametric modeling. Both components are optimized using strong and weak lensing constraints, producing a self-consistent cluster mass model at all scales.

I am part of the BUFFALO collaboration, an HST survey that will extend the HST coverage of the 6 frontier fields clusters by a factor of almost 4. This extended coverage will allow to map the dark matter distribution in these clusters with great precision using both strong and weak lensing constraints, and to probe the high-z Universe that they magnify.

The new composite image of Abell 370 made for the BUFFALO project together with previous observations from the Frontier Fields program showing how BUFFALO observations extend the field of view around the galaxy cluster. Credit: NASA, ESA, A. Koekemoer, M. Jauzac, C. Steinhardt, the BUFFALO team and HST Frontier Fields.



  • Multiscale grid for Lenstool: python scripts to prepare a Lenstool input file with a multiscale grid.
  • SL+WL Lenstool: still in test phase, should be publicly available at some point in the future.

Galaxy assembly bias

Results from cosmological simulations show that halo clustering depends not only on halo mass, but also on their accretion history. At fixed halo mass, haloes that assemble earlier are found to be more tightly clustered than those that assemble at later times, an effect known as halo assembly bias. On the observational side, some studies have used galaxy distributions as tracers for the halo properties to detect halo assembly bias, but the results are still highly debated. A complementary question is whether such dependence of the galaxy clustering signal on a secondary galaxy property, not related to halo mass, can be found. This effect, which we refer to as galaxy assembly bias, has not been solved either. If the secondary galaxy property can be shown to correlate with the halo formation history, the galaxy assembly bias would be a manifestation of the halo assembly bias.


  • Probing galaxy assembly bias with LRG weak lensing observations. A. Niemiec, E. Jullo, A. Montero-Dorta, F. Prada, S. Rodriguez-Torres, E. Perez and A. Klypin. Monthly Notices of the Royal Astronomical Society: Letters, Volume 477, Issue 1, p.L1-L5, June 2018.
  • The Dependence of Galaxy Clustering on Stellar-mass Assembly History for LRGs. A. D. Montero-Dorta, E. Pérez, F. Prada, S. Rodríguez-Torres, G. Favole, A. Klypin, R. Cid Fernandes, R. M. González Delgado, A. Domínguez, A. S. Bolton, R. García-Benito, E. Jullo and A. Niemiec. The Astrophysical Journal Letters, Volume 848, Issue 1, article id. L2, 6 pp. October 2017 PDF

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