A first estimate of the Milky Way dark matter halo spin

¹ Universitäts-Sternwarte München, Scheinerstraße 1, 81679 München, Germany
e-mail: obreja@usm.lmu.de
² Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
³ New York University Abu Dhabi, PO Box 129188 Saadiyat Island, Abu Dhabi, UAE
⁴ Center for Astro, Particle and Planetary Physics (CAP 3), New York University Abu Dhabi, Abu Dhabi, UAE
⁵ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

Received: 1 April 2021
Accepted: 27 September 2021

Abstract

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_ϕ(R), can be computed. For both disks, v_ϕ(R) decreases with radius with ∼2 km s⁻¹ kpc⁻¹ for R ≳ 5 kpc, resulting in velocities of v_ϕ,thin = 221.2 ± 0.8 km s⁻¹ and v_ϕ,thick = 188 ± 3.4 km s⁻¹ at the solar radius. We use our derived v_ϕ,thin(R) and v_ϕ,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: J_z, thin = (3.26 ± 0.43)×10¹³ and J_z, 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 J_h = 2.69_−0.32^+0.37 10¹⁵ M_⊙ kpc km s⁻¹ 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%).