Co-formation of the Galactic disc and the stellar halo (1802.03414v1)

Abstract: Using a large sample of Main Sequence stars with 7-D measurements supplied by Gaia and SDSS, we study the kinematic properties of the local (within ~10 kpc from the Sun) stellar halo. We demonstrate that the halo's velocity ellipsoid evolves strongly with metallicity. At the low [Fe/H] end, the orbital anisotropy (the amount of motion in the radial direction compared to the tangential one) is mildly radial with 0.2<beta\<0.4. However, for stars with [Fe/H]>-1.7 we measure extreme values of beta~0.9. Across the metallicity range considered, i.e. -3<[Fe/H]<-1, the stellar halo's spin is minimal, at the level of 20<v_theta (km/s) \<30. Using a suite of cosmological zoom-in simulations of halo formation, we deduce that the observed acute anisotropy is inconsistent with the continuous accretion of dwarf satellites. Instead, we argue, the stellar debris in the inner halo were deposited in a major accretion event by a satellite with Mvir\>10^10 Msun around the epoch of the Galactic disc formation, i.e. between 8 and 11 Gyr ago. The radical halo anisotropy is the result of the dramatic radialisation of the massive progenitor's orbit, amplified by the action of the growing disc.

• The paper uses SDSS data to show that velocity anisotropy in the Galactic stellar halo strongly depends on metallicity, suggesting different formation histories for metal-rich and metal-poor stars.
• Analysis reveals metal-rich halo stars exhibit extreme radial anisotropy while metal-poor stars show significantly lower anisotropy, indicating varying origins.
• Cosmological simulations suggest massive accretion events during Galactic disc formation contributed significantly to the stellar halo, explaining its observed high radial anisotropy.

Analysis of the Co-formation of the Galactic Disc and Stellar Halo

Introduction

The paper by Belokurov et al. explores the intricate dynamics of the local stellar halo, focusing on its kinematic and chemical properties. By leveraging a large sample of Main Sequence stars with 7-D measurements provided by SDSS, the research investigates the velocity anisotropy and orbital properties of the stellar halo as a function of metallicity. The paper utilizes cosmological zoom-in simulations to interpret the observational data and hypothesizes significant insights into the formation mechanisms of the Milky Way's stellar halo and disc.

Methodological Overview

The paper harnesses a robust sample of Main Sequence stars situated within approximately 10 kpc from the Sun. These stars are analyzed to assess the evolution of the halo's velocity ellipsoid. Importantly, the research distinguishes the behavior of the halo across a broad metallicity range, revealing a noticeable transition in orbital anisotropy. The paper employs the Extreme Deconvolution algorithm to model velocity distributions with multivariate Gaussians, allowing for an expert dissection of the ellipsoidal shapes and rotational properties of the stellar halo.

Key Findings

• Anisotropy and Metallicity: The paper finds a pronounced dependence of velocity anisotropy on metallicity. Specifically, halo stars with [Fe/H] > -1.7 exhibit extreme radial anisotropy with $\beta$ nearing values of 0.9. Conversely, metal-poor stars (with [Fe/H] < -1.7) display significantly lower anisotropy ($0.2<\beta<0.4$), hinting at varying formation histories within the stellar halo.
• Rotational Properties: The research reports minimal halo spin across all metallicity ranges. However, subtle prograde rotation is observed for more metal-rich stars, remaining relatively invariant with the distance from the Galactic plane, suggesting influences from precursor dynamical conditions.
• Simulation Insights: The cosmological simulations present a scenario where massive early accretion, coinciding with the epoch of disc formation, plays a pivotal role. The most massive progenitors ($M_{\rm vir}>10^{10} M_{\odot}$) contribute stars to the current day halo, with simulations indicating that this results in heightened radial anisotropy, especially when compounded by the influence of an evolving Galactic disc.

Implications

The implications of this research touch upon several fundamental aspects of galaxy formation and evolution. The results imply a correlation between the kinematic properties of stellar debris and the mass of its progenitor systems, suggesting robust evidence for a major accretion event shaping the inner Galactic halo. The extreme radial anisotropy could be evidence of the significant role that such accretion events play in the stellar makeup of the galaxy, providing a lens to view past dynamical interactions.

Speculations on Future Research

In future developments, extending the analysis to a broader spatial distribution alongside more refined simulations could enhance our understanding of the Galactic halo's relic dynamics. Investigations using upcoming datasets, such as those expected from next-generation telescopic surveys, could illuminate the evolutionary narratives underlying the chemical and kinematic bifurcation observed in this paper.

Conclusion

This paper enriches our understanding of the Galactic halo's formation by dissecting its anisotropic nature and associating its properties with past large-scale accretion events. Combining observational datasets with robust simulations offers a compelling narrative of the co-evolution of the Galactic disc and halo, hinting at an intertwined cosmological history pivotal to constructing our contemporary understanding of galaxy dynamics.