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Papers by Fidèle Robichaud

The Astrophysical Journal, 2014

Using a large N-body cosmological simulation combined with a subgrid treatment of galaxy formatio... more Using a large N-body cosmological simulation combined with a subgrid treatment of galaxy formation, merging, and tidal destruction, we study the formation and evolution of the galaxy and cluster population in a comoving volume (100 Mpc) 3 in a ΛCDM universe. At z = 0, our computational volume contains 1788 clusters with mass M cl > 1.1 × 10 12 M ⊙ , including 18 massive clusters with M cl > 10 14 M ⊙ . It also contains 1 088 797 galaxies with mass M gal ≥ 2 × 10 9 M ⊙ and luminosity L > 9.5 × 10 5 L ⊙ . For each cluster, we identified the brightest cluster galaxy (BCG). We then computed two separate statistics: the fraction f BNC of clusters in which the BCG is not the closest galaxy to the center of the cluster in projection, and the ratio ∆v/σ, where ∆v is the difference in radial velocity between the BCG and the whole cluster, and σ is the radial velocity dispersion of the cluster. We found that f BNC increases from 0.05 for low-mass clusters (M cl ∼ 10 12 M ⊙ ) to 0.5 for high-mass ones (M cl > 10 14 M ⊙ ), with very little dependence on cluster redshift. Most of this turns out to be a projection effect, and when we consider 3D distances instead of projected distances, f BNC increases only to 0.2 at high cluster mass. The values of ∆v/σ vary from 0 to 1.8, with median values of in the range 0.03-0.15 when considering all clusters, and 0.12-0.31 when considering only massive clusters. These results are consistent with previous observational studies, and indicate that the central galaxy paradigm, which states that the BCG should be at rest at the center of the cluster, is usually valid, but exceptions are too common to be ignored. We built merger trees for the 18 most massive clusters in the simulation. Analysis of these trees reveal that 16 of these clusters have experienced one or several major or semi-major mergers in the past. These mergers leave each cluster in a nonequilibrium state, but eventually the cluster settles into an equilibrium configuration, unless it is disturbed by another major or semi-major merger. We found evidence that these mergers are responsible for the off-center positions and peculiar velocities of some BCGs. Our results thus support the merging-group scenario, in which some clusters form by the merger of smaller groups in which the galaxies have already formed, including the galaxy destined to become the BCG. Finally, we argue that f BNC is not a very robust statistics, being very sensitive to projection and selection effect, but that -2 -∆v/σ is a more robust one. Still, both statistics exhibit a signature of major mergers between clusters of galaxies.

The Astrophysical Journal, 2014

Using a large N-body cosmological simulation combined with a subgrid treatment of galaxy formatio... more Using a large N-body cosmological simulation combined with a subgrid treatment of galaxy formation, merging, and tidal destruction, we study the formation and evolution of the galaxy and cluster population in a comoving volume (100 Mpc) 3 in a ΛCDM universe. At z = 0, our computational volume contains 1788 clusters with mass M cl > 1.1 × 10 12 M ⊙ , including 18 massive clusters with M cl > 10 14 M ⊙ . It also contains 1 088 797 galaxies with mass M gal ≥ 2 × 10 9 M ⊙ and luminosity L > 9.5 × 10 5 L ⊙ . For each cluster, we identified the brightest cluster galaxy (BCG). We then computed two separate statistics: the fraction f BNC of clusters in which the BCG is not the closest galaxy to the center of the cluster in projection, and the ratio ∆v/σ, where ∆v is the difference in radial velocity between the BCG and the whole cluster, and σ is the radial velocity dispersion of the cluster. We found that f BNC increases from 0.05 for low-mass clusters (M cl ∼ 10 12 M ⊙ ) to 0.5 for high-mass ones (M cl > 10 14 M ⊙ ), with very little dependence on cluster redshift. Most of this turns out to be a projection effect, and when we consider 3D distances instead of projected distances, f BNC increases only to 0.2 at high cluster mass. The values of ∆v/σ vary from 0 to 1.8, with median values of in the range 0.03-0.15 when considering all clusters, and 0.12-0.31 when considering only massive clusters. These results are consistent with previous observational studies, and indicate that the central galaxy paradigm, which states that the BCG should be at rest at the center of the cluster, is usually valid, but exceptions are too common to be ignored. We built merger trees for the 18 most massive clusters in the simulation. Analysis of these trees reveal that 16 of these clusters have experienced one or several major or semi-major mergers in the past. These mergers leave each cluster in a nonequilibrium state, but eventually the cluster settles into an equilibrium configuration, unless it is disturbed by another major or semi-major merger. We found evidence that these mergers are responsible for the off-center positions and peculiar velocities of some BCGs. Our results thus support the merging-group scenario, in which some clusters form by the merger of smaller groups in which the galaxies have already formed, including the galaxy destined to become the BCG. Finally, we argue that f BNC is not a very robust statistics, being very sensitive to projection and selection effect, but that -2 -∆v/σ is a more robust one. Still, both statistics exhibit a signature of major mergers between clusters of galaxies.