The velocity field of our Milky Way outer stellar halo based on DESI DR2

Abstract

Using 64,000 halo K giants from Dark Energy Spectroscopic Instrument (DESI) second Data Release (DR2), we decompose the Milky Way (MW) stellar halo between 3 and 160 kpc into metal-rich (MR) and metal-poor (MP) components via a Gaussian mixture model (GMM). The two populations are nearly equal in number but chemically and kinematically distinct: MR stars occupy highly radial orbits with velocity anisotropy of beta ~0.94 and metallicity dispersion sigma([Fe/H]) ~0.17 dex, without obvious dependence on distance, and are mainly contributed by Gaia-Sausage/Enceladus (GSE) debris. MR component dominates the inner 30 kpc and re-emerges beyond 50 kpc, implying GSE debris can extend to ~70-80 kpc. MP stars exhibit a weaker radial bias of beta ~0.46, decreasing to -0.5 beyond 80 kpc, and with a larger metallicity dispersion of sigma([Fe/H]) ~0.46 dex, showing signatures of multiple minor mergers. Both components exhibit net prograde rotation at ~10-30 kpc with a stronger azimuthal signal in the MP population. The non-equilibrium motions of the outer halo (>50 kpc) are quantified with a dipole-plus-contraction velocity field. We find that the outer halo is simultaneously contracting (~-19 km/s, distance-independent) and subject to reflex motions (increases from -19 to -44 km/s with radius), reflecting the perturbation from the Large Magellanic Cloud (LMC). We also confirm a linear dependence of mean polar velocity for the outer stellar halo on the dipole velocity field, a direct consequence of the LMC and MW interaction. Our results provide a quantitative distance-resolved description of the MW's last major accretion event and its ongoing response to the first infall of the LMC.

Figures (10)

• Figure 1: Error contours for different parameter combinations of the single-component model. Histograms show the marginalized posterior probability distributions for each parameter. $\langle V_r\rangle$, $\langle V_\phi\rangle$, and $\langle V_\theta\rangle$, which denote the three components of the mean velocity vector in Galactocentric spherical coordinates. $\sigma_r$, $\sigma_\phi$, and $\sigma_\theta$ are the corresponding velocity dispersions. $\mu_\mathrm{[Fe/H]}$ and $\sigma_\mathrm{[Fe/H]}$ characterize the mean and variance of the metallicity distribution, respectively. The yellow, light blue, and dark blue contours represent the $30\%$, $1\sigma$, and $2\sigma$ regions of the MCMC post-burn distributions, respectively.
• Figure 2: Left: Distance ($r_\mathrm{GC}$) dependence of the mean velocity (filled markers connected with solid lines) and velocity dispersion (open markers connected with dashed lines). The error bars represent the 1$\sigma$ uncertainties. Right: Velocity anisotropy ($\beta$) as a function of distance. The 1$\sigma$ uncertainties of velocity anisotropy are propagated from the uncertainties in velocity dispersion ($\sigma_r, \sigma_\phi, \sigma_\theta$). The black/blue, orange, and green dots denote the best-fit model in this work with/without masking Sgr stream, 2021ApJ...919...66B and yr1rrlyrae, respectively. Different line styles denote distinct tracer types: solid lines correspond to K giants, dashed line to RR Lyrae stars, and dotted line to BHB stars. The black horizontal dashed line denotes $\beta=0$, where $\beta>0$ corresponds to radial dominance and $\beta<0$ corresponds to tangentially dominance.
• Figure 3: Error contours for different parameter combinations of the double-component model (see Equation \ref{['equ:double_components_likelihood']}). Histograms show the marginalized posterior probability distributions for each parameter. $\langle V_x^\mathrm{MR}\rangle$ and $\langle V_x^\mathrm{MP}\rangle$ ($x=r,\phi,\theta$) are the means of the MR and MP halo populations in Galactocentric spherical coordinates. $\sigma_x^\mathrm{MR}$ and $\sigma_x^\mathrm{MP}$ ($x=r,\phi,\theta$) correspond to the velocity dispersions for two populations. $\mu_\mathrm{[Fe/H]}^\mathrm{MR/MP}$ and $\sigma_\mathrm{[Fe/H]}^\mathrm{MR/MP}$ are the mean and standard deviation of the metallicity distribution for two populations. $p_\mathrm{MR}$ is the fraction of the MR population in the stellar halo. The yellow, light blue, and dark blue contours represent the $30\%$, $1\sigma$, and $2\sigma$ regions of the MCMC post-burn distributions, respectively.
• Figure 4: Top: Fraction of the MR K giants as a function of Galactocentric distance $r_\mathrm{GC}$. Middle: Mean metallicity profiles of the MR population (magenta dots) and the MP population (cyan dots). Bottom: Metallicity dispersion profiles of the MR population (magenta dots) and the MP population (cyan dots). The error bars stand for 1$\sigma$ uncertainties.
• Figure 5: Distance dependence of velocity field for the MR population (first row) and the MP population (second row). In the left panel, the filled markers denote the mean velocity profile binned as a function of Galactocentric distance, and the open markers represent the velocity dispersion dependence on distance. The right panel shows the velocity anisotropy at different radii. The magenta/cyan dots stand for the MR/MP populations, and the black dots are from the best-fit single-component model in Section \ref{['sec:single_component_results']} (see the right panel of Figure \ref{['fig:single_component_result']}). The orange and green dots denote the best-fit model in 2021ApJ...919...66B and yr1rrlyrae, respectively. Different line styles denote distinct tracer types: solid lines correspond to K giants, dashed line to RR Lyrae stars, and dotted line to BHB stars. The error bars stand for 1$\sigma$ uncertainties. The black horizontal dashed line denotes $\beta=0$, where $\beta>0$ corresponds to radial dominance and $\beta<0$ corresponds to tangentially dominance.