Shiva and Shakti: Presumed Proto-Galactic Fragments in the Inner Milky Way

Abstract

Using $\textit{Gaia}$ DR3 astrometry and spectroscopy, we study two new substructures in the orbit-metallicity space of the inner Milky Way: $\textit{Shakti}$ and $\textit{Shiva}$. They were identified as two confined, high-contrast overdensities in the $(L_z, E)$ distribution of bright ($G<16$) and metal-poor ($-2.5<\rm{[M/H]}<-1.0$) stars. Both have stellar masses of $M_\star \gtrsim 10^7M_\odot$, and are distributed on prograde orbits inside the Solar circle in the Galaxy. Both structures have an orbit-space distribution that points towards an $\textit{accreted}$ origin, however, their abundance patterns -- from APOGEE -- are such that are conventionally attributed to an $\textit{in situ}$ population. These seemingly contradictory diagnostics could be reconciled if we interpret the abundances [Mg/Fe], [Al/Fe], [Mg/Mn] $\textit{vs.}$ [Fe/H] distribution of their member stars merely as a sign of rapid enrichment. This would then suggest one of two scenarios. Either these prograde substructures were created by some form of resonant orbit trapping of the field stars by the rotating bar; a plausible scenario proposed by Dillamore et al. (2023). Or, $\textit{Shakti}$ and $\textit{Shiva}$ were proto-galactic fragments that formed stars rapidly and coalesced early, akin to the constituents of the $\textit{Poor Old Heart}$ of the Milky Way; just less deep in the Galactic potential and still discernible in orbit space.