Constraints of the Clumpiness of Dark Matter Halos through Heating of the Disk Galaxies

Abstract

Motivated by the presence of numerous dark matter clumps in the Milky Way's halo, as expected from the cold dark matter cosmological model, we conduct numerical simulations to examine the heating of the disk. We construct a fairly realistic initial Galaxy model with a stable thin disk. The disk interacts with dark matter clumps for about 5 Gyr. Three physical effects are examined: the mass spectrum of the dark matter clumps, the initial thickness of the galactic disk, and the spatial distribution of the clumps. We find that the massive end of the mass spectrum determines the amount of disk heating. Thicker disks suffer less heating. There is a certain thickness at which the heating due to the interaction with the clumps is saturated. The spatial distribution of the clumps plays an important role in disk heating. We adopt two different spatial distributions of the clumps. The first, which mimics the primordial distribution of the clumps at the epoch of the collapse of the halo, is proportional to the underlying halo density distribution. Recent cosmological simulations, however, yield depletion of the clumps within the extent of the disk in the present-day galaxies. Therefore, we construct the second distribution, which has the same number density of the clumps as the prediction for the cosmological simulations within the disk region. Our numerical simulations show that the first distribution produces considerable disk heating, while the latter does not. These results suggest that at early epochs, or in cases where many clumps are surviving until the present, the disk should have suffered considerable heating in the earlier epochs of their evolution.

The Radical Consequences of Realistic Satellite Orbits for the Heating and Implied Merger Histories of Galactic Disks

“…This provides a physical reason why simulations increasingly find, contrary to historical expectations, that systems can undergo cosmologically expected merger histories and still yield relatively thin disks at z ¼ 0 in line with observations (see, e.g., Velazquez & White 1999;Ardi et al 2003;Abadi et al 2003;Benson et al 2004;Robertson et al 2004Robertson et al , 2006Okamoto et al 2005;Hayashi & Chiba 2006;Governato et al 2007;Kazantzidis et al 2008;Younger et al 2008;Villalobos & Helmi 2008). Many of these studies found that idealized numerical experiments yield less efficient disk heating than that predicted by Toth & Ostriker (1992); the key contribution of our work is to outline the physical reasons for this difference and to demonstrate that these differences are not a matter of, e.g., the normalization or numerical prefactors involved in an estimate of disk heating: heating is second order in mass ratio ().…”

“…Hayashi & Chiba (2006) investigate dissipationless mergers of a disk + bulge + halo primary (including both rigid halos + bulges and restricted halos free to move but not realized in N-body fashion, with an N-body disk; note that Velazquez & White [1999] argue that these restricted halo models may artificially inflate the resulting scale heights by a factor $2, but this is comparable to the scatter in any case). The halo is merged with a large number of satellites (subhalos), drawn in Monte Carlo fashion from a mass spectrum with power-law index dN/dM / M À2 , similar to the subYMilky Way end of the halo mass function (see also Ardi et al [2003], who perform a similar exercise and reach similar conclusions). A mass fraction equal to 0.1 of the primary is placed into satellites according to that mass distribution, with a their spatial distribution varied systematically (following a Hernquist 1990 profile), and their initial velocity distribution determined assuming they follow the local halo velocity ellipsoid with either an isotropic dispersion tensor […”

Satellite Survival in Cold Dark Matter Cosmology

“…The initial conditions and subsequent evolution of N-body computer simulations still plague their interpretation and application to observed galaxies in terms of simple estimates of satellite disruption times. For instance, Font et al (2003) and Ardi et al (2003) have questioned the rate of disk heating by infalling dark satellites. These authors find that thin disks may yet remain stable despite a high count of bound dark matter clumps, provided the clumps do not follow near-radial orbits.…”

Dark matter electron anisotropy: A universal upper limit

“…Finally, we chose a clump mass range 10 −6 ÷ 10 10 M , with the upper limit coming from constraints due to disk stability [35]. The lower limit is set instead following the most common choice in the literature.…”