The Relation Between Halo Shape, Velocity Dispersion and Formation Time

TL;DR

This study reveals a strong correlation between halo shape and dynamics, driven by formation time, consistent across simulations and galaxy groups, with early-forming systems being more spherical and dynamically hotter.

Contribution

It demonstrates that halo shape and velocity dispersion are linked to formation time, confirmed in both simulations and observational galaxy groups, highlighting the role of formation history.

Findings

Early formation haloes are more spherical and have higher velocity dispersions.

Less concentrated groups, indicating later formation, are more elongated with lower velocity dispersions.

The shape-dynamics correlation persists even when projected in 2D.

Abstract

We use dark matter haloes identified in the MareNostrum Universe and galaxy groups identified in the Sloan Data Release 7 galaxy catalogue, to study the relation between halo shape and halo dynamics, parametrizing out the mass of the systems. A strong shape-dynamics, independent of mass, correlation is present in the simulation data, which we find it to be due to different halo formation times. Early formation time haloes are, at the present epoch, more spherical and have higher velocity dispersions than late forming-time haloes. The halo shape-dynamics correlation, albeit weaker, survives the projection in 2D (ie., among projected shape and 1-D velocity dispersion). A similar shape-dynamics correlation, independent of mass, is also found in the SDSS DR7 groups of galaxies and in order to investigate its cause we have tested and used, as a proxy of the group formation time, a concentration parameter. We have found, as in the case of the simulated haloes, that less concentrated groups, corresponding to late formation times, have lower velocity dispersions and higher elongations than groups with higher values of concentration, corresponding to early formation times.