The Origin of Stars in the Inner 500 Parsecs in TNG50 Galaxies


 We investigate the origin of stars in the innermost 500 pc of galaxies spanning stellar masses of 5 × 108 − 12 M⊙ at z = 0 using the cosmological magnetohydrodynamical TNG50 simulation. Three different origins of stars comprise galactic centers: 1) in-situ (born in the center), 2) migrated (born elsewhere in the galaxy and ultimately moved to the center), 3) ex-situ (accreted from other galaxies). In-situ and migrated stars dominate the central stellar mass budget on average with 73% and 23% respectively. The ex-situ fraction rises above 1% for galaxies ≳ 1011 M⊙. Yet, only 9% of all galaxies exhibit no ex-situ stars in their centers and the scatter of ex-situ mass is significant (4 − 6 dex). Migrated stars predominantly originate closely from the center (1 − 2 kpc), but if they travelled together in clumps distances reach ∼10 kpc. Central and satellite galaxies possess similar amounts and origins of central stars. Star forming galaxies (≳ 1010 M⊙) have on average more ex-situ mass in their centers than quenched ones. We predict readily observable stellar population and dynamical properties: 1) migrated stars are distinctly young (∼2 Gyr) and rotationally supported, especially for Milky Way mass galaxies, 2) in-situ stars are most metal-rich and older than migrated stars, 3) ex-situ stars are on random motion dominated orbits and typically the oldest, most metal-poor and α-enhanced population. We demonstrate that the interaction history with other galaxies leads to diverse pathways of building up galaxy centers in a ΛCDM universe. Our work highlights the necessity for cosmological context in formation scenarios of central galactic components and the potential to use galaxy centers as tracers of overall galaxy assembly.