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4.3 Time Evolution of the Aquila Simulation

Figures 4.1, 4.2 and 4.3 respectively show the time evolution of the projected dark matter, gas and stellar mass distributions in the central region of the simulation. Each panel is centred on the position of the main subhalo (which is the host of the “Milky Way”), and shows all particles within a cubic volume of side length 1 Mpc At each redshift, theX, Y and Z-coordinates are defined parallel to the principal moments of the inertia tensor of the halo, with eigenvaluesIx> Iy > Iz. Keeping the volume fixed in comoving coordinates corresponds to a zoom-out in physical coordinates by a factor of 7.5 as the universe grows with time from z = 6.5 to

z= 0. The squares indicate the position of all satellite subhaloes belonging to the main FoF halo identified at the time of the snapshot, with the size of the squares in all figures indicative of (but not strictly proportional to) the dark matter mass of the subhalo.

In Figure 4.2, where the blue colour indicates gas density, red boxes denote the subhaloes that contain gas, while white boxes denote subhaloes that are gas-free. It can be seen that even at high redshifts, the majority of subhaloes are gas-free, and only four relatively massive satellites contain gas at z = 0. The smallest of these four satellites, which are the subject of Section 4.5.1, has a total mass of

∼ 5_×109_M

⊙. All lower mass satellites, many of which formed stars, have lost

their gas during their evolution. The different mechanisms of gas loss, internal and environmental, are discussed in Section 4.4.

In Figure 4.3, the green colour shows the stellar density, which is clearly dom- inated by the central object and its halo. Here, red boxes show subhaloes that contain stars, while white boxes show haloes that are essentially dark. At z= 0, there are 90 subhaloes containing stars, including the four which contain gas, as shown in Figure 4.2. The highest mass subhalo that does not contain any stars atz= 0 has a mass of 4.3×108_M

⊙. The lower mass limit for star formation be-

comes difficult to assess, because of the limited resolution of our simulation. The total number of subhaloes with stars is comparable, however, with observational estimates of the number of dwarf satellites around the Milky Way.

The halo reaches a final virial mass of 1.6×1012_M

⊙, comparable to recent obser-

vational estimates of the Milky Way halo, for example 1012_M

⊙ (Xue et al., 2008),

1.4_×1012_M

⊙(Smith, 2007), 1.6×1012M⊙ (Gnedin et al., 2010) and 2.4×1012M⊙

(Li & Helmi, 2008). The corresponding spherical virial radius is∼250 kpc, but we include as satellites all subhaloes within the FoF group. 40% of the satellites are presently located outside ofrvir, with the furthest satellite at a galactocentric dis- tance of 490 kpc. The central galaxy reaches a stellar mass of 10.8×1010_M

⊙, higher

than current observational estimates for the Milky Way, for example 5.5×1010_M

⊙

from (Flynn et al., 2006). Distributions of the different mass components, and the positions of all satellites atz= 0 are shown in three orthogonal projections along

Figure 4.1: Evolution of the dark matter distribution in the central region of the Aquila simulation. Each panel shows a box of sidelength 1 Mpc (co- moving), centred on the central subhalo, and oriented along the major (X) and minor (Z) component of the inertia tensor of the main halo. The squares show the position of identified subhaloes belonging to the FoF-group of the main halo present at each snapshot, with the area proportional to the subhalo mass. Over time, the distribution of mass and of subhaloes changes from an elongated distribution at high redshift to a more rounded distribution at lower redshift (differ- ent projections of the subhaloes at z = 0 are shown in Figure 4.4).

4.3 Time Evolution of the Aquila Simulation

Figure 4.2: Gas distribution in the central region of the Aquila simulation at dif- ferent redshifts, in volumes identical to Figures 4.1 and Figures 4.3. Subhaloes of the main FoF halo with gas are shown as red squares, Gas-free subhaloes are shown in white. While most satellites contain gas at z = 7, this fraction drops significantly, and only four of the most massive satellites are not gas-free atz= 0.

Figure 4.3: Stellar mass distribution in n the central region of the Aquila simu- lation at different redshifts, in volumes identical to Figures 4.1 and Figures 4.2. The central, Milky Way type galaxy dominates the total stellar mass at every redshift. The positions of satellite galaxies are shown in red, while dark subhaloes are shown as white squares. The size of each square corresponds to the dark matter mass of each sub- halo. While nearly all subhaloes present atz = 7 also contain stars, the fraction drops to ∼45% at z = 0, with more massive subhaloes more likely to contain stars.

4.3 Time Evolution of the Aquila Simulation

Figure 4.4: Projections of the dark matter (top), gas mass (middle) and stellar mass (bottom) distributions at z= 0, with the location of subhaloes overplotted as squares. As in Figures 4.1 – 4.3, the size of the squares indicate the dark matter mass of each subhalo. Analogous to Fig- ure 4.2 and 4.3, in the middle row, red and white squares distinguish satellites which have gas from those that are gas-free, while in the bottom row, the distinction is between subhaloes with and without stars. Notable from the middle row is the pair of late-infalling, gas- rich satellites 1 and 7, as described in Section 4.5.1 .

the principal axes of the inertia tensor of the halo in Figure 4.4. By comparison with the flattened distribution seen at high redshift, the final shape of the halo appears round and largely featureless. This transformation from a triaxial mass distribution, expected from purely gravitational assembly, to an oblate halo sim- ilar to that observed, is also studied in Tissera et al. (2010), who attribute the difference to baryonic effects.