A first estimate of the Milky Way dark matter halo spin

Abstract

The spin, $\hat{I}»$, of dark matter (DM) halos in cosmological simulations follows a log normal distribution and has little correlation with galaxy observables. As such, there is currently no way to infer the $\hat{I}»$ parameter of individual halos hosting observed galaxies. We present here a first attempt to measure $\hat{I}»$ starting from the dynamically distinct stellar components identified in high-resolution cosmological simulations with Galactic Structure Finder. In a subsample of NIHAO galaxies, we find tight correlations between the total angular momentum (AM) of the DM halos, $J_h$, and the azimuthal AM, $J_z$, of the stellar components of the form: log($J_h$)=$\hat{I}\pm$+${\hat{I}}^2\cdot$log($J_z$). The stellar halos have the tightest relation with $\hat{I}\pm =9.50\pm 0.42$ and ${\hat{I}}^2=0.46\pm 0.04$. The other tight relation is with the disks: $\hat{I}\pm =6.15\pm 0.92$ and ${\hat{I}}^2=0.68\pm 0.07$. We used Gaia DR2 and APOGEE 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, $v_{\ddot{I}}•(R)$, can be computed. For both disks, $v_{\ddot{I}}•(R)$ decreases with radius with $\sim$2 km s${}^{-1}$ kpc${}^{-1}$ for $R\gtrsim 5$ kpc, resulting in $v_{\ddot{I}•,thin}\backsimeq 221$ km s${}^{-1}$ and $v_{\ddot{I}•,thick}\backsimeq 188$ km s${}^{-1}$ at $R_{\odot }$. These velocity profiles together with the Galaxy mass model of Cautun et al. (2020) result in the AM for the two disks: $J_{z,thin}=(3.26\pm 0.43)\times 10^{13}$ and $J_{z,thick}=(1.20\pm 0.30)\times 10^{13}$ M${}_{\odot }$ kpc km s${}^{-1}$, where the DM halo is assumed to have a contracted NFW profile. Adopting the correlation found in simulations, the spin estimate of the Galaxy's DM halo is $\hat{I}{»}_{MW}=0.061_{-0.016}^{+0.022}$. If the DM halo has a NFW profile instead, the spin becomes $\hat{I}{»}_{MW}=0.088_{-0.020}^{+0.024}$, making the Galaxy a more extreme outlier.

15 pages, 7 figures, accepted for publication in A&A