A first estimate of the Milky Way dark matter halo spin

The spin, or normalized angular momentum λ, 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 the λ parameter of individual halos hosting observed galaxies. Here, we present a first attempt to measure λ starting from the dynamically distinct disks and stellar halos identified in high-resolution cosmological simulations with the Galactic Structure Finder (gsf). In a subsample of NIHAO galaxies analyzed with gsf, 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 with α = 9.50 ± 0.42 and β = 0.46 ± 0.04. The other tight relation is with the disks, for which α = 6.15 ± 0.92 and β = 0.68 ± 0.07. While the angular momentum is difficult to estimate for stellar halos, there are various studies that calculated Jz for disks. In application to the observations, we used Gaia DR2 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-1 kpc-1 for R 5 kpc, resulting in velocities of vφ,thin = 221.2 ± 0.8 km s-1 and vφ,thick = 188 ± 3.4 km s-1 at the solar radius. We use our derived vφ,thin(R) and vφ,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)×1013 and Jz 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 be Jh = 2.69-0.32+0.37 1015 M· kpc km s-1 and the spin estimate is λMW = 0.061-0.016+0.022, which translates into a probability of 21% using the universal log normal distribution function of λ. If the Galaxy's dark halo is assumed to follow a NFW profile instead, the spin becomes λMW = 0.088-0.020+0.024, making the Milky Way a more extreme outlier (with a probability of only 0.2%).