# Gaia reveals a metal-rich in-situ component of the local stellar halo

## TL;DR

Using Gaia, RAVE, and APOGEE data, the study reveals a significant in-situ, metal-rich component of the local stellar halo, challenging the traditional view of the halo as primarily accreted material.

## Contribution

The paper demonstrates that a substantial fraction of local halo stars are metal-rich and formed in-situ within the Galactic disk, supported by observational data and cosmological simulations.

## Key findings

- Half of local halo stars have [Fe/H]>-1 and disk-like orbital directions.
- Metal-rich halo stars likely formed inside the solar circle in the Galactic disk.
- Simulations show in-situ formation of metal-rich halo stars within the Milky Way.

## Abstract

We use the first Gaia data release, combined with RAVE and APOGEE spectroscopic surveys, to investigate the origin of halo stars within <~3 kpc from the Sun. We identify halo stars kinematically, as moving with a relative speed of at least 220 km/s with respect to the local standard of rest. These stars are in general more metal-poor than the disk, but surprisingly, half of our halo sample is comprised of stars with [Fe/H]>-1. The orbital directions of these metal-rich halo stars are preferentially aligned with the disk rotation, in sharp contrast with the isotropic orbital distribution of the more metal-poor halo stars. We find similar properties in the Latte cosmological zoom-in simulation of a Milky Way-like galaxy from the FIRE project. In Latte, metal-rich halo stars formed primarily inside of the solar circle, while lower-metallicity halo stars preferentially formed at larger distances (extending beyond the virial radius). This suggests that metal-rich halo stars in the Solar neighborhood in fact formed in-situ within the Galactic disk rather than having been accreted from satellite systems. These stars, currently on halo-like orbits, therefore have likely undergone substantial radial migration/heating.