Formation and evolution of galaxy dark matter halos and their substructure

TL;DR

This paper uses the Via Lactea high-resolution cosmological N-body simulation to dissect how a Milky Way–sized dark matter halo and its subhalo system assemble and evolve. It introduces fixed-mass radial shells to isolate physical growth from artificial virial-radius growth and defines a physical concentration index, $c_V$, to quantify inner densities. The study finds that most physical mass within fixed scales is in place by $z\sim1$, tidal stripping acts from outside in and raises subhalo concentrations—especially near the Galactic center—and environmental effects cause many halos outside the virial radius to be former subhalos with altered assembly histories. Most $z=1$ subhalos survive to $z=0$, though a small fraction are disrupted or merged; these results have implications for interpreting dwarf satellites and the population of field halos around MW-like galaxies.

Abstract

We use the ``Via Lactea'' simulation to study the co-evolution of a Milky Way-size LambdaCDM halo and its subhalo population. While most of the host halo mass is accreted over the first 6 Gyr in a series of major mergers, the physical mass distribution [not M_vir(z)] remains practically constant since z=1. The same is true in a large sample of LambdaCDM galaxy halos. Subhalo mass loss peaks between the turnaround and virialization epochs of a given mass shell, and declines afterwards. 97% of the z=1 subhalos have a surviving bound remnant at the present epoch. The retained mass fraction is larger for initially lighter subhalos: satellites with maximum circular velocities Vmax=10 km/s at z=1 have today about 40% of their mass back then. At the first pericenter passage a larger average mass fraction is lost than during each following orbit. Tides remove mass in substructure from the outside in, leading to higher concentrations compared to field halos of the same mass. This effect, combined with the earlier formation epoch of the inner satellites, results in strongly increasing subhalo concentrations towards the Galactic center. We present individual evolutionary tracks and present-day properties of the likely hosts of the dwarf satellites around the Milky Way. The formation histories of ``field halos'' that lie today beyond the Via Lactea host are found to strongly depend on the density of their environment. This is caused by tidal mass loss that affects many field halos on eccentric orbits.

Figures (18)

• Figure 1: Evolution of radii $r_M$ enclosing a fixed mass versus cosmic time or scale factor $a$. The enclosed mass grows in constant amounts of $0.3\times 10^{12}\,\,\rm M_\odot$ from bottom to top. Shells are numbered from one (inner) to ten (outer). Initially all spheres are growing in the physical (non comoving) units used here. Shells 1 to 6 turn around, collapse and stabilize, while the outermost shells are still expanding today. Solid circles: points of maximum expansion at the turnaround time $t_{\rm ta}$. Open squares: time after turnaround where $r_{\rm M}$ first contracts within $20\%$ of the final value. These mark the approximate epoch of stabilization. The collapse factors $r_M(t_{\rm ta})/r_M(z=0)$ for shells 1 to 6 are 3.29, 2.44, 1.98, 1.70, 1.51 and 1.36, respectively. Thus shells 1 and 2 collapse by more than the factor of 2 derived from spherical top-hat, while shells 4, 5, and 6 collapse by a smaller factor.
• Figure 2: Fraction of material belonging to shell $i$ at epoch $a$ that remains in the same shell today. Shells are same as in Fig. \ref{['LagRadii']}, numbered from one (inner) to ten (outer). Solid circles: time of maximum expansion. Open squares: stabilization epoch. Mass mixing generally decreases with time and towards the halo center.
• Figure 3: Mass accretion history of Via Lactea. Masses within spheres of fixed physical radii centered on the main progenitor are plotted against the cosmological expansion factor $a$. The thick solid lines correspond to spheres with radii given by the labels on the right. The thin solid lines correspond to nine spheres of intermediate radii that are 1.3, 1.6, 2.0, 2.5, 3.2, 4.0, 5.0, 6.3 and 7.9 times larger than the next smaller labeled radius. Dashed line:$M_{200}$. The halo is assembled during a phase of active merging before $a\simeq0.37$ ($z\simeq1.7$) and remains practically stationary at later times.
• Figure 4: Same as Figure \ref{['massaccr_norvcm']}, but using a linear scale in enclosed mass. In addition to $M_{200}$ ( upper dashed line) we now also plot $M_{\rm vir}$ ( lower dashed line) and the mass within the radius of maximal circular velocity ( dotted line). The physical mass accretion is small after the last major merger at $a\simeq0.37$ ($z\simeq1.7$): more than 80% of the present-day material within 400 kpc is already in place at $z=1$. This value is typical for galaxy-size halos. Filled square: median $z=1$ mass fraction ($=0.87$) within 400 kpc for 303 halos of similar mass rescaled to today's Via Lactea mass within 400 kpc. Solid circle: corresponding median $z=1$ value for $M_{200}$. Filled triangle: corresponding median $z=1$ value for the mass within 100 kpc. Error bars indicate the 68% scatter around the median.
• Figure 5: Concentration parameters $c_V$ (solid) and $c_{V/2}$ (dotted) divided by the density contrast $\Delta$ used to define $r_{\rm vir}$ at $z=0$, as a function of $c_{\rm vir} = r_{\rm vir}/r_s$.