A first estimate of the Milky Way dark matter halo spin

Obreja, Aura; Buck, Tobias und Maccio, Andrea V. (2021): A first estimate of the Milky Way dark matter halo spin. In: Astronomy & Astrophysics, Bd. 657, A15

Abstract

The spin, or normalized angular momentum lambda, 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 lambda parameter of individual halos hosting observed galaxies. Here, we present a first attempt to measure lambda 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)) = alpha + beta.log(J(z)). The stellar halos have the tightest relation with alpha = 9.50 +/- 0.42 and beta = 0.46 +/- 0.04. The other tight relation is with the disks, for which alpha = 6.15 +/- 0.92 and beta = 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(phi)(R), can be computed. For both disks, v(phi)(R) decreases with radius with similar to 2 km s(-1) kpc(-1) for R greater than or similar to 5 kpc, resulting in velocities of v(phi,thin) = 221.2 +/- 0.8 km s(-1) and v(phi,thin) = 188 +/- 3.4 km s(-1) at the solar radius. We use our derived v(phi,thin)(R) and v(phi,thin)(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) x 10(13) and J(z,thick) = (1.20 +/- 0.30) x 10(13) M-circle dot 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)10(15) M-circle dot kpc km s(-1) and the spin estimate is lambda(W)(M) = 0.061(-0.016)(+0.022), which translates into a probability of 21% using the universal log normal distribution function of lambda. If the Galaxy's dark halo is assumed to follow a NFW profile instead, the spin becomes lambda(W)(M) = 0.0881(-0.0)(20)(+)(0.)(02)(4), making the Milky Way a more extreme outlier (with a probability of only 0.2%).

• Dokumententyp: Zeitschriftenartikel
• Fakultät: Physik
• Themengebiete: 500 Naturwissenschaften und Mathematik > 530 Physik
• ISSN: 0004-6361
• Sprache: Englisch