Spatial and Kinematic Clustering of Stars in the Galactic Disk

Abstract

The Galactic disk is expected to be spatially and kinematically clustered on many scales due to both star formation and the Galactic potential. In this work we calculate the spatial and kinematic two-point correlation functions (TPCF) using a sample of 1.7 × 106 stars with radial velocities from Gaia DR2. Clustering is detected on spatial scales of 1–300 pc and a velocity scale of 15 km s−1. After removing bound structures, the data have a power-law index of γ ≈ −1 for 1 pc < Δr < 100 pc and γ ≲ −1.5 for Δr > 100 pc. We interpret these results with the aid of a star-by-star simulation of the Galaxy, in which stars are born in clusters orbiting in a realistic potential that includes spiral arms, a bar, and giant molecular clouds. We find that the simulation largely agrees with the observations at most spatial and kinematic scales. In detail, the TPCF in the simulation is shallower than the data at ≲20 pc scales, and steeper than the data at ≳30 pc. We also find a persistent clustering signal in the kinematic TPCF for the data at large Δv (>5 km s−1) that is not present in the simulations. We speculate that this mismatch between observations and simulations may be due to two processes: hierarchical star formation and transient spiral arms. We also predict that the addition of ages and metallicities measured with a precision of 50% and 0.05 dex, respectively, will enhance the clustering signal beyond current measurements.

Two-point Correlation Function Studies for the Milky Way: Discovery of Spatial Clustering from Disk Excitations and Substructure

“…The appearance of substructure is inferred through comparison of the data to a reference theoretical background distribution. Extensive study has shown the LS method to be a superior choice (Keihanen et al 2019;Lancaster et al 2019), which we and Kamdar et al (2021) also employ, though we note Wall & Jenkins (2012) for discussion of the broader possibilities. In Kamdar et al (2021) spatial and kinematic clustering of the stars in the Galactic disk is studied using a sample of 1.7 × 10 6 stars with 6D phasespace information within 1 kpc of the Sun from Gaia DR2 data.…”

“…Extensive study has shown the LS method to be a superior choice (Keihanen et al 2019;Lancaster et al 2019), which we and Kamdar et al (2021) also employ, though we note Wall & Jenkins (2012) for discussion of the broader possibilities. In Kamdar et al (2021) spatial and kinematic clustering of the stars in the Galactic disk is studied using a sample of 1.7 × 10 6 stars with 6D phasespace information within 1 kpc of the Sun from Gaia DR2 data. The construction of a suitable reference distribution is essential to the determination of the 2PCF, and in the Galaxy its construction is challenging (Kamdar et al 2021;Mao et al 2015).…”

“…We develop this line of thinking in this paper, starting in Section 2. We note that studies of the 2PCF in the Galaxy do exist (Cooper et al 2011; Kamdar et al 2021;Lancaster et al 2019;Mao et al 2015), though they are focused on somewhat different questions and are also limited in different ways. An assumption of spherical symmetry has carried over to those studies.…”

“…In Kamdar et al (2021) spatial and kinematic clustering of the stars in the Galactic disk is studied using a sample of 1.7 × 10 6 stars with 6D phasespace information within 1 kpc of the Sun from Gaia DR2 data. The construction of a suitable reference distribution is essential to the determination of the 2PCF, and in the Galaxy its construction is challenging (Kamdar et al 2021;Mao et al 2015). Kamdar et al (2021) employ Dirichlet process Gaussian mixture models to that end.…”

The non-monotonic, strong metallicity dependence of the wide-binary fraction

“…Here we assume that both stars have the same metallicity and adopt the metallicity from the stellar component with LAMOST measurements. We argue that this assumption is justified because previous studies have shown that wide binaries with separations 10 4 AU have nearly identical elemental abundances (Andrews et al 2018(Andrews et al , 2019Hawkins et al 2020), and simulations have suggested that most pairs with separations 10 6 AU and small relative velocities ( 2 km/s) are conatal (Kamdar et al 2020). Due to turbulent mixing, conatal stars from the same gas cloud are expected to be homogeneous in metallicity (Feng & Krumholz 2014).…”