Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2012
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012acasn..53...92g&link_type=abstract
Acta Astronomica Sinica, vol. 53, no. 1, p. 92-93
Astronomy and Astrophysics
Astronomy
Methods: Numerical, Galaxies: Haloes, Galaxies: Evolution, Galaxies: Interactions, Cosmology: Dark Matter
Scientific paper
In the popular cold dark matter (CDM) model, the structure is formed during the collapse of dark matter due to gravitational instability,and the virial structure is seen as dark matter halo. The baryonic gases condense in the radiative cooling, fall into the halo, and finally form a galaxy. The halo formation is processed in a hierarchical manner that small haloes form first, and they subsequently merge to form bigger haloes. After the merger, the small haloes survive as self-bound entities and become the substructures (subhaloes) of the bigger halo (host halo). The galaxy in the subhalo is the satellite galaxy of the central galaxy formed in the host halo. The satellite galaxies usually have low masses and luminosities, and are observed as dwarf galaxies. When the subhalo falls into the host halo, it will merge with the host halo center due to dynamical friction. The time interval needed for the merger is called the dynamical friction timescale Tdf. The dynamical evolution model for subhalo includes some important physical processes such as the dynamical friction, tidal mass stripping, tidal heating and tidal disruption.We find that the Tdf depends strongly on the efficiency of tidal stripping. The Tdf increases with the increasing tidal strippingefficiency, and this is the main reason for the discrepancies among the previous results. We also find that the dependence of Tdf on orbital circularity is determined by the merging samples, in which the distribution of mass ratio between the subhalo and host halo is crucial. With respect to the late stage of subhalo evolution, we find that the tidal mass stripping is only efficient in the early stage, but this effect can be ignored in the late evolution. The Coulomb logarithm in the dynamical friction has not been well stated. By comparing the predicted Tdf with simulation, we give a fitting formula of Coulomb logarithm. Combining the dynamical model with the halo formation histories (merger trees) predicted by the Extended Press-Schechter (EPS) formalism, we have followed the evolution of the accreted subhaloes in the host haloes. The predicted subhalo population, including the spatial distribution and subhalo mass function (SHMF), can be compared with the results of the simulation or observation. In the application to a Milky-Way sized halo, we find that the SHMF is mainly determined by the tidal stripping efficiency with the trend that a higher efficiency of tidal stripping leads to a lower amplitude of SHMF. The SHMF is insensitive to the Coulomb logarithm and definition of tidal disruption. The predicted radial distribution of subhaloes is more concentrated than that of simulation, but is consistent with the distribution of the satellite galaxies in Milky Way galaxy. The radial distribution of subhaloes depends weakly on the masses of subhaloes at both present day and the accretion time, but strongly depends on the accretion time.
Applying the model to the formation histories of the host haloes with different masses, we find that the masses of host haloes influence strongly on SHMF, but weakly on the radial distribution of subhaloes. In different host haloes, the tidal field has the same effect on SHMF. The predicted halo occupation distribution of subhaloes follows the Poisson distribution well, and this distribution is independent of the masses of host haloes and subhaloes
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