44) Evidence from APOGEE for the presence of a major building block of the halo buried in the inner Galaxy

Abstract: It is known that the Galactic stellar halo holds crucial information for disentangling the mass assembly history of the Galaxy. Of particular importance is the spatial region enclosed within the inner few kpc of the Galactic centre, which contains a large fraction of the total mass of the halo. However, observational difficulties associated with crowding and extinction have hampered our understanding of the stellar population content of the inner halo. In this talk, I will provide evidence based on data from APOGEE, Gaia, and the EAGLE simulations, for the presence of a metal-poor structure located within ~4 kpc from the Galactic centre. Characterised by a chemical composition resembling those of low mass satellite galaxies of the Milky Way, this structure appears to be chemically and dynamically distinct from its more metal-rich counterparts in the inner Galaxy. We speculate this newly identified inner Galaxy structure (Heracles) is associated with an accretion event that occurred in the early life of the Milky Way, likely constituting a major building block of the Milky Way halo. Furthermore, I will show that such accretion scenario is not atypical for Milky Way mass galaxies in the EAGLE numerical simulations, which reveal similar chemical compositions for merger events resembling the substructure presented. Contrasting the APOGEE data with chemical compositions of simulated EAGLE dwarf satellite galaxies, we estimate the mass of the progenitor to be roughly twice that of the Gaia-Enceladus system. I will discuss alternative interpretations of our results. If confirmed, the discovery of such an early accretion event will help shed light into the early mass assembly history of the Galaxy and the origin of the metal poor stars in the Galactic centre.

Bio: My name is Danny Horta, and I am a PhD student at Liverpool John Moores University in the United Kingdom. I am a member of the APOGEE survey, and work with my supervisor Ricardo Schiavon on Galactic archaeology. Specifically, I focus on studying the chemo-dynamical properties of Galactic stellar halo populations, as well their globular cluster counterparts, in order to decipher the mass assembly history of the Galaxy. Aside from astronomy, I play a lot of basketball, like to travel and love nature/outdoor expeditions. Check out more stuff about me and my research at my personal website: https://dhortad.github.io.