LCDM-based models for the Milky Way and M31 I: Dynamical Models

TL;DR

This work tests the compatibility of LCDM-based cuspy halos with the Milky Way and M31 by incorporating adiabatic contraction from baryon infall and two angular-momentum scenarios. Using a two-component mass model and a physically motivated halo prescription, the authors constrain $M_{ m vir}$ to about $(1-2)\times10^{12}\,M_\odot$ and $C$ to $12-17$, finding that central regions are not DM-dominated and that angular-momentum transfer can allow fast bars. The results show that cuspy halos can reproduce the rotation curves and dynamical properties of normal high-surface-brightness spirals, provided a substantial fraction of baryons lie outside the disk/bulge and baryonic physics are properly accounted for. They also examine the implications for the global luminosity function, microlensing observations, and SMBH-induced inner density changes, arguing for LCDM consistency in this regime and setting up a statistical follow-up study (Paper II).

Abstract

We apply standard disk formation theory with adiabatic contraction within cuspy halo models predicted by the standard LCDM cosmology. The resulting models score remarkably well when confronted with the broad range of observational data available for the Milky Way and M31 galaxies, giving a Milky Way virial mass of 1-2x10^12Msun and concentration C=12-17. We consider two types of models, in which: (A) baryons conserve angular momentum and (B) some of the angular momentum of the baryons is transferred to the dark matter. Type-A models produce good agreement with observed rotation curves and obey constraints in the solar neighborhood, but may have too much dark matter in the center to allow a fast rotating bar. The type-B models with angular momentum transport have a slightly more massive disk and less dark matter in the central part, allowing a fast rotating bar to persist. Both classes of models probably have sufficient baryonic mass in the central 3.5kpc to reproduce recent observational values of the optical depth to microlensing events towards the Galactic center. All models require that about 1/2 of all baryons expected inside the virial radius must not be in the disk or bulge. We investigate whether the range of virial masses allowed by our dynamical models is compatible with constraints from the galaxy luminosity function, and find a range of parameter space that is allowed by this constraint. We conclude that rotation curves and dynamical properties of ``normal'' high surface brightness spiral galaxies appear to be consistent with standard LCDM.

Figures (9)

• Figure 1: Dependence of terminal velocities on galactic longitude $l$. The full curve is for model $A_1$. Symbols show observational data from the HI measurements of Knapp et al. (1985) (circles) and Kerr et al.(1986) (triangles). At small angles the deviations from circular velocities are expected to be large due to the central bar. This is clearly seen at $l < 20^o$.
• Figure 2: Rotation curve for our favorite models $A_1$ (no exchange of angular momentum) and $B_1$ (with the exchange). Note that the dark matter dominates only in the outer part of the Milky Way. Symbols show observational data from HI measurements of Knapp et al. (1985) (circles) and Kerr et al.(1986) (triangles).
• Figure 3: Mass distribution of the MW galaxy for Model $A_1$ (full curve) and model $B_1$ (dashed curve). The large dots with error bars are observational constraints. From small to large radii the constraints are based on: stellar radial velocities and proper motions in the galactic center; radial velocities of OH/IR stars; modeling of the bar using DIRBE and stellar velocities; rotational velocity at the solar radius; dynamics of satellites.
• Figure 4: Mass distribution ( top panel) and rotation velocity ( bottom panel) of the M31 galaxy for Model $C_1$. The large dots with error bars are observational constraints. In the bottom panel the circles show results of CO ($r< 10~\hbox{kpc}$) and HI ($r> 10~\hbox{kpc}$) observations. The large dip at around 5 kpc is likely due to non-circular motions induced by the bar. Observational data points in the top panel at small radii are from stellar motions in the nucleus. Vertical bars correspond to 20% errors in mass. Data points at 7, 15, and 30 kpc correspond to circular velocities of $240\pm 10, 270\pm 20,$ and $240\pm 10$km/s.
• Figure 5: Surface brightness of M31 in the R-band on linear ( bottom panel) and logarithmic ( top panel) scales. Deviations from the observational results are less than 0.2 mag.