Backtracking

We can have a closer look on the change of the dynamic structure of the subhaloes by tracing them back to the redshift just before they merged with the main halo and compare the structural parameters rmax and vmax as well as the masses pre and post infall. We restricted to subhaloes with more than 100 particles.

The distribution of the infall-redshifts for GA3n is shown in the upper panel of Figure

3.11. The lower panel shows the cumulative distribution of these redshifts normalised to the total subhalo number, i.e. 412. It is striking that most of the subhaloes that can still be detected today were accreted only very recently. Half of the subhaloes were accreted later thanz_≈0.4. Only 10% of the haloes were accreted beforez_≈1.6; only 1% before z _≈ 3.8. The oldest remnant present in GA3n at z = 0 was accreted at z _≈ 7.2. All this implies that tidal disruption is very strong which is in agreement with results from Gao Liang (2003) (private communication). The outer regions of the infalling haloes get stripped away and the material is lost to the halo of the MW.

3.2 The simulated Milky Way

Figure 3.10: Upper panel: vmax-values for the subhaloes with more than 300 particles in the simulations GA1n (dots), GA2n (open circles) and GA3n (open

Figure 3.11: Accretion of the subhaloes with more than 100 particles. The upper panel shows the number of accreted subhaloes that survived to z= 0 as a function of their accretion redshift. The lower panel shows the cumulative

3.2 The simulated Milky Way

The fit (dotted line) corresponds to

f(z)_≈(1 +z)−2.65. (3.12)

The line in the plot is shifted to match the value of 100% atz= 0.06 which is the redshift of our penultimate output. f(z) is the relative number of surviving haloes which are accreted onto the main halo before z.

However, not only the outer regions of the haloes change upon infall, also their internal structure is altered. The change in the circular velocity profiles is discussed in the next section.

In this section we study the changes on an object-by-object basis for the subhaloes of one simulation. Figure 3.12 shows the rmax and vmax values before the infall and at redshift z = 0 for all four simulations. For all subhaloes the peak rotation velocity decreases and for most of them the position of the peak moves inwards. This change is not a step function at the redshift of accretion. It gradually increases with the time the subhalo is orbiting in the main halo. This can be seen from Figure 3.13. Although the scatter is huge, the average relative change inrmax orvmax is a rather smooth function of the accretion redshift.

Figure 3.14 shows the mass-loss that the subhaloes suffer upon infall into the main halo as a function of the distance from the centre of the main halo at redshift z = 0. In the upper panel, the mass of the pre-infall halo is the FOF mass whereas for the post-infall mass thesubfindmass is used. As expected, all sub haloes suffer from tidal stripping. This effect is stronger in regions of higher density. It is interesting to observe, that this trend is clearly visible (solid histogram line for GA3n subhaloes) although only a snapshot of the subhalo distribution is analysed. The fact that this trend is observable in a snapshot means that the disruption-time of a subhalo - or the time where significant mass loss is occurring -, is comparable to the time it takes a subhalo to cross the main halo.

subfinddetermines the mass of the subhaloes by finding the particles that are gravita- tionally bound to the substructure. This naturally results in smaller masses for subhaloes that are closer to the centre of the main halo being deeper in the potential well. In order to assure that the observed effect is neither due to the different algorithms used for the mass determination (FOF for the pre-infall haloes and subfind for the subhaloes) nor to the subfind itself, we redid the above analysis, this time only using the structural parameters rmax andvmax. Assuming that the pre-infall haloes and the subhaloes have NFW density profiles we can compute masses “m200” for all of them and determine the

mass loss as before. Some haloes seem to have gained mass which is an artefact from the approximation with the NFW profile. The result of this test is shown in the lower

3.2 The simulated Milky Way

Figure 3.13: Relative change of vmax and rmax of the haloes upon infall into the main object as a function of its accretion redshift: The peaks of the subhalo

Figure 3.14: Mass-loss fraction of haloes due to their infall into the main halo as a function of their distance to the MW centre at z= 0. Although only one “snapshot” of the evolution is analysed, the trend is clearly visible. The upper

3.2 The simulated Milky Way

The resolution of the simulations, especially GA3n, is so high, that the orbits of the subhaloes in the Milky Way potential can be followed up to high redshifts. An interesting follow-up project would be to determine the full path of the subhalo and track the evolution of structural parameters, correlating them with the initial orbital parameters and the initial mass. This together with the infall distribution would make it possible to better understand the buildup of DM haloes and would probably allow improvement of semi-analytic modelling techniques (Kauffmann, White & Guiderdoni (1993);Cole et al.(2000); Taylor & Silk (2003)).