The capture of halo material by orbiting subhaloes

Abstract

When a dark matter halo falls into a more massive object and becomes a subhalo, it typically loses much of its mass through tidal stripping. The reverse process is also possible in principle. The subhalo may gravitationally capture material from its host. If sufficiently efficient, this process could make an initially starless subhalo visible. We use high-resolution N-body simulations to estimate the efficiency of capture. We find that after an extended period orbiting within its host, at most $\sim 10^{-4}$ of a subhalo's remaining mass has been acquired since infall. This captured material is less concentrated to subhalo centre than material retained from before infall. It is also very much less abundant than host material that is instantaneously passing through the subhalo on almost unperturbed orbits. Captured stars are not sufficiently spatially concentrated to be distinguished from the dominant background of "field" stars, and their concentration in velocity space is no greater than that of typical stellar streams in the halo. Unfortunately, stellar capture is not efficient enough to allow initially starless low-mass subhaloes to be detected.

Figures (9)

• Figure 1: An illustration of our analysis procedure for a randomly selected but relatively massive subhalo with $z_{\rm inf}=1.08$. From left to right, the panels show the projected distribution (in comoving coordinates) of mass and of various specific particle sets at $z=1.17, 0.11$ and 0.0 when sets A, B and C are selected, respectively. The black dashed circles show the virial radius of the main halo in each panel. The solid black circle in the left panel shows $r_{\rm limit}$ for the infalling halo. The red dots in this panel show the 2127 particles that will be considered newly associated to the subhalo at $z=0.11$. The solid black circles in the middle and right panels indicate $2r_{t}$ for the subhalo. Red dots in the middle panel show particles that were within $r_{\rm limit}$ in the left panel. For clarity, we display only a random 10% of these 41638 particles; 5% of them are within $2r_{t}$ at $z=0.11$.The red dots in the right panel show the $z=0$ positions of particles that were newly associated at $z=0.11$ (the same particles plotted in red in the left panel). Each panel is 4cMpc/h on a side.
• Figure 2: Left panel: Density profiles for the subhalo of Fig.\ref{['fig:sketch']} at $z=0.11$ with radius normalised by $r_t$ and density normalised by $\rho_{\rm bkg}$, the mean density in the spherical shell $r_t<r<2r_t$. The black, red and cyan solid lines represent all, retained and newly associated particles, respectively. Additionally, a red dashed line shows the profile for particles identified by SUBFIND as bound to this subhalo. Right panel: The corresponding cumulative particle number profiles. The profile of newly associated particles increases as $r^3$ (the black dashed line) indicating a nearly uniform density from the inner subhalo region out to the surrounding environment.
• Figure 3: Evolution of subhalo mass (left panel, indicated by SUBFIND bound particle number) and of distance to host halo centre (right panel) for the subhalo of Fig.\ref{['fig:sketch']}. In both panels, the black star marks the snapshot when the subhalo is last located outside the $R_{200}$ of host halo, which is also when set $A$ of associated particles is defined. The red star indicates $z_{\rm max}$, when $r_{\rm limit}$ is calculated, while the green star corresponds to $z_{\rm inf}$. We also marked $z=0.11$ with a blue star.
• Figure 4: Stacks of spherically averaged normalized density profiles at $z=0.11$ similar to those shown for an individual subhalo in Fig.\ref{['fig:Randomcase']}. The black, red and cyan solid lines represent straight averages for all, for retained and for newly associated particles, respectively. Error bars give the standard error in the mean estimated from the scatter among the profiles. From left to right, the panels show stacks for different ranges of $n_{\rm inf}$. Additional legends give the number of profiles in each stack. the mean numbers of retained and newly associated particles, the mean number of newly associated particles which are also bound (according to SUBIND), and the mean number of newly associated particles which are still associated at $z=0$. Two final legends give the mean number of orbits executed by $z=0.11$ and the mean halocentric distance of the subhaloes at this time.
• Figure 5: As Fig \ref{['fig:averagez1']}, but for $z_{\rm inf}\approx 2$