Collapse thresholds for the formation of CDM haloes in the ellipsoidal collapse model

Details Collapse thresholds for the formation of CDM haloes in the ellipsoidal collapse model Collapse thresholds for the formation of CDM haloes in the ellipsoidal collapse model

2011-07-28

arxiv.org/abs/1107.5808v1

Astronomy and Astrophysics

Astrophysics

Cosmology and Extragalactic Astrophysics

15 pages, 11 figures, submitted to MNRAS. Comments welcome

Scientific paper

In the excursion set approach to structure formation initially spherical regions of the linear density field collapse if the average density contrast within them exceeds some critical value, \delta_c. This model allows one to make predictions for several fundamental aspects of structure formation such as the mass function of dark matter halos, their merger histories and clustering properties. The value of \delta_c is often calculated from the spherical or ellipsoidal collapse model, which provide well-defined predictions given auxiliary properties of the linear tidal field at a given point. We use two cosmological simulations of structure growth in a LCDM cosmology to test a key assumption used in calculating \delta_c: that the shapes of the initial Lagrangian patches that eventually collapse (or proto-haloes) are spherical. Our results indicate that the vast majority of dark matter proto-haloes are non-spherical, and have minor-to-major axis ratios that vary from ~0.4 at the galaxy mass scale to ~0.65 for rich galaxy clusters. We show that this non-sphericity likely originates from the asymmetry of the linear tidal field which pushes material onto, or away from, local density maxima in the linear density field. We study the implications of these results for the collapse barriers for CDM halo formation inferred from the classic ellipsoidal collapse model. Our results indicate that the "standard" ellipsoidal collapse model commonly adopted in the literature does not provide a full account of the possible collapse thresholds for halo formation, since the model predictions depend sensitively on the assumed shape of the primordial perturbation. We show that an improved model, which accounts for the intrinsic shapes of proto-haloes, provides a much more accurate description of their measured minimum overdensities.

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