Effects of merging histories on angular momentum distribution of dark matter haloes

Abstract

The effects of merging histories of proto-objects on the angular momentum distributions of the present-time dark matter haloes are analysed. An analytical approach to the analysis of the angular momentum distributions assumes that the haloes are initially homogeneous ellipsoids and that the growth of the angular momentum of the haloes halts at their maximum expansion time. However, the maximum expansion time cannot be determined uniquely, because in the hierarchical clustering scenario each progenitor, or subunit, of the halo has its own maximum expansion time. Therefore the merging history of the halo may be important in estimating its angular momentum. Using the merger tree model by Rodrigues & Thomas, which takes into account the spatial correlations of the density fluctuations, we have investigated the effects of the merging histories on the angular momentum distributions of dark matter haloes. It was found that the merger effects, that is, the effects of the inhomogeneity of the maximum expansion times of the progenitors which finally merge together into a halo, do not strongly affect the final angular momentum distributions, so that the homogeneous ellipsoid approximation happens to be good for the estimation of the angular momentum distribution of dark matter haloes. This is because the effect of the different directions of the angular momenta of the progenitors cancels out the effect of the inhomogeneity of the maximum expansion times of the progenitors.The contribution of the orbital angular momentum to the total angular momentum when two or more pre-existing haloes merge together, was also investigated. It is shown that this contribution is more important than that of the angular momentum of diffuse accreting matter to the total angular momentum, especially when the mergers occur many times.

The role of spin in the formation and evolution of galaxies

“…This is what is predicted in analytical and numerical results (Heavens & Peacock 1988; Nagashima & Gouda 1998; Bett et al 2007) but in contrast with the results of Faltenbacher et al (2002) who explicitly computed the marked correlation of the spin parameter λ in their simulations and found an environmental dependence. However, Faltenbacher et al (2002) argue that large samples are needed to measure this signal, it therefore remains to be seen if much larger sample can confirm our result.…”

Dependence of Halo Properties on Interaction History, Environment, and Cosmology

“…Angular momentum has been found to be relatively insensitive to cosmology and collapse anisotropy for halos formed by less violent collapse, with the spread inλ arising mainly from tidal effects (Huss, Jain, & Steinmetz 1999). This view is supported by Nagashima & Gouda (1998) who, using semi-analytics (the "merging-cell model"), find that the orbital angular momentum of major mergers is not important for the remnant, and that major merging activity as a whole has little effect on the halo angular momentum distribution. Given the large spread inλ in my results, it seems tempting to conclude that on a halo-by-halo basis, tidal effects are more important than merger history in determining the ultimate angular momentum of a halo.…”

“…Orbital angular momentum of interaction halos would seem to be the cause most consistent my data. However, on galaxy scales, the angular momentum of ellipticals is generally much lower than spirals (Nagashima & Gouda 1998). This is troublesome if ellipticals are to be considered merger remnants and spirals the results of long periods of quiescence.…”

Dynamical response to supernova-induced gas removal in two-component spherical galaxies

“…The size of disc galaxies is usually considered to be the radius at which the discs are supported by their rotation under the assumption that their specific angular momenta are conserved while the gas shrinks owing to the cooling. The initial value of the specific angular momentum before the cooling is given by the so‐called dimensionless spin parameter, λ, which is distributed log‐normally around 0.05, derived by the tidal torque due to the large‐scale density inhomogeneity in the Universe (White 1984; Catelan & Theuns 1996a,b; Nagashima & Gouda 1997). Because the effective radius of the disc is ≃λ r vir (Fall 1983) and the size of elliptical galaxies is not significantly different from the disc size, the values z i = 0.05 and 0.2 are realistic.…”