A first estimate of the Milky Way dark matter halo spin

Abstract

The spin, or normalized angular momentumλ, of dark matter halos in cosmological simulations follows a log normal distribution and has little correlation with galaxy observables such as stellar masses or sizes. There is currently no way to infer theλparameter of individual halos hosting observed galaxies. Here, we present a first attempt to measureλstarting from the dynamically distinct disks and stellar halos identified in high-resolution cosmological simulations with theGalactic Structure Finder (gsf). In a subsample of NIHAO galaxies analyzed withgsf, we find tight correlations between the total angular momentum of the dark matter halos,Jh, and the azimuthal angular momentum,Jz, of the dynamical distinct stellar components of the form: log(Jh) =α+β⋅log(Jz). The stellar halos have the tightest relation withα = 9.50 ± 0.42 andβ = 0.46 ± 0.04. The other tight relation is with the disks, for whichα = 6.15 ± 0.92 andβ = 0.68 ± 0.07. While the angular momentum is difficult to estimate for stellar halos, there are various studies that calculatedJzfor disks. In application to the observations, we usedGaiaDR2 and APOGEE data to generate a combined kinematics-abundance space, where the Galaxy’s thin and thick stellar disks stars can be neatly separated and their rotational velocity profiles,vϕ(R), can be computed. For both disks,vϕ(R) decreases with radius with ∼2 km s−1kpc−1forR ≳ 5 kpc, resulting in velocities ofvϕ,thin= 221.2 ± 0.8 km s−1andvϕ,thick= 188 ± 3.4 km s−1at the solar radius. We use our derivedvϕ,thin(R) andvϕ,thick(R) together with the mass model for the Galaxy of Cautun et al. (2020, MNRAS, 494, 4291) to compute the angular momentum for the two disks:Jz, thin = (3.26 ± 0.43)×1013andJz, thick = (1.20 ± 0.30)×1013 M⊙kpc km s−1, where the dark halo is assumed to follow a contracted NFW profile. Adopting the correlation found in simulations, the total angular momentum of the Galaxy’s dark halo is estimated to beJh= 2.69−0.32+0.371015 M⊙kpc km s−1and the spin estimate isλMW= 0.061−0.016+0.022, which translates into a probability of 21% using the universal log normal distribution function ofλ. If the Galaxy’s dark halo is assumed to follow a NFW profile instead, the spin becomesλMW= 0.088−0.020+0.024, making the Milky Way a more extreme outlier (with a probability of only 0.2%).