2022A&A...657A..15O

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, J_h, and the azimuthal angular momentum, J_z, of the dynamical distinct stellar components of the form: log(J_h) = α + β.log(J_z). 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 J_z 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, v_ph(R), can be computed. For both disks, v_ph(R) decreases with radius with ∼2 km/s/kpc for R ≥ 5 kpc, resulting in velocities of v_ph,thin = 221.2 ±0.8 km/s and v_ph,thick = 188 ±3.4 km/s at the solar radius. We use our derived v_ph,thin(R) and v_ph,thick(R) together with the mass model for the Galaxy of Cautun et al. (2020MNRAS.494.4291C) to compute the angular momentum for the two disks: J_z, thin_ = (3.26 ±0.43)x10¹³ and J_z, thick_ = (1.20 ±0.30)x10¹³ M_☉.kpc.km/s, 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 J_h=2.69_–0.32^+0.37*10¹⁵ M_☉.kpc.km/s and the spin estimate is λ_MW = 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 λ_MW = 0.088_–0.020^+0.024, making the Milky Way a more extreme outlier (with a probability of only 0.2%).