Galactic halo cusp-core: tidal compression in mergers

Abstract

We explain in simple terms how the buildup of dark haloes by merging compact satellites, as in the CDM cosmology, inevitably leads to an inner cusp of density profile $\rho \propto r^{-\alpha}$ with $\alpha \gsim 1$, as seen in cosmological N-body simulations. A flatter halo core with $\alpha <1$ exerts on the satellites tidal compression in all directions, which prevents deposit of stripped satellite material in the core region. This makes the satellite orbits decay from the radius where $\alpha \sim 1$ to the halo centre with no local tidal mass transfer and thus causes a rapid steepening of the inner profile to $\alpha >1$. These tidal effects, the resultant steepening of the profile to a cusp, and the stability of this cusp to tandem mergers with compact satellites, are demonstrated using N-body simulations. The transition at $\alpha \sim 1$ is then addressed using toy models in the limiting cases of impulse and adiabatic approximations and using tidal radii for satellites on radial and circular orbits. In an associated paper we address the subsequent slow convergence from either side to an asymptotic stable cusp with $\alpha \gsim 1$. Our analysis thus implies that an inner cusp is enforced when small haloes are typically more compact than larger haloes, as in the CDM scenario, such that enough satellite material makes it intact into the inner halo and is deposited there. We conclude that a necessary condition for maintaining a flat core, as indicated by observations, is that the inner regions of the CDM satellite haloes be puffed up by about 50% such that when they merge into a bigger halo they would be disrupted outside the halo core. This puffing up could be due to baryonic feedback processes in small haloes, which may be stimulated by the tidal compression in the halo cores.Comment: 19 pages, Latex, mn2e.cls, some revisions, MNRAS in pres

Strong-lensing analysis of MACS J0717.5+3745 fromHubbleFrontier Fields observations: How well can the mass distribution be constrained?

“…Actif galactic nucleus feedback can also contribute to the flattening of the central region (Martizzi et al 2012), but once again this mechanism is efficient only on small scales (up to 10-15 kpc). On scales of 100 kpc, large cores may result from violent interactions related to merging events in this actively evolving cluster, although several studies find that DM profiles are not strongly altered by a collision (Ricker & Sarazin 2001;Dekel et al 2003;Molnar et al 2010). Finally, heating of massive galaxies by dynamical friction against the diffuse dark matter distribution of the cluster can flatten the slope of the DM density profile, and sometimes even dominate over adiabatic contraction (see, e.g.…”

Tides in Colliding Galaxies

“…Remains the cases of two, one or no positive eigenvalues, as mentioned by Dekel et al (2003). For two or one positive λ 's, the tidal field is called (partially) extensive, like e.g.…”

NIHAO IX: the role of gas inflows and outflows in driving the contraction and expansion of cold dark matter haloes

“…This process can increase the density of dark matter haloes by an order of magnitude. In contrast, other processes can cause the dark matter halo to expand: rapid mass-loss and/or time variability of the potential due to feedback from stars (Navarro et al 1996;Read & Gilmore 2005;Mashchenko et al 2006;Macciò et al 2012;El-Zant et al 2016) or Active Galactic Nuclei (AGN; Martizzi et al 2012Martizzi et al , 2013; mergers and stripping of smaller subhaloes that were puffed up by stellar feedback (Dekel et al 2003); and transfer of energy/angular momentum from baryons to the dark matter via dynamical friction due to minor mergers (El-Zant et al 2001;Jardel & Sellwood 2009;Johansson et al 2009;Cole et al 2011), or galactic bars (Weinberg & Katz 2002;Sellwood 2008).…”