In:
Astronomy & Astrophysics, EDP Sciences, Vol. 657 ( 2022-01), p. A15-
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)×10 13 and J z , thick = (1.20 ± 0.30)×10 13 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 10 15 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%).
Type of Medium:
Online Resource
ISSN:
0004-6361
,
1432-0746
DOI:
10.1051/0004-6361/202140983
Language:
English
Publisher:
EDP Sciences
Publication Date:
2022
detail.hit.zdb_id:
1458466-9
SSG:
16,12
Permalink