Gaia Reveals a Metal-rich, in situ Component of the Local Stellar Halo

We use the first Gaia data release, combined with the RAVE and APOGEE spectroscopic surveys, to investigate the origin of halo stars within $\lesssim 3$ kpc from the Sun. We identify halo stars kinematically as moving at a relative speed of at least 220 km s⁻¹ with respect to the local standard of rest. These stars are generally less metal-rich than the disk, but surprisingly, half of our halo sample is comprised of stars with $\left[Fe/H\right]>-1$. The orbital directions of these metal-rich halo stars are preferentially aligned with the disk rotation, in sharp contrast with the intrinsically isotropic orbital distribution of the 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, whereas 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 actually 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.