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Description
Massive early-type galaxies are believed to be the end result of an extended mass accretion history. The stars formed in situ, very early during the initial phase of the assembly, might have originated from an extremely intense and rapid burst of star formation. These stars may still be found within the cores of such galaxies at z=0, depending on their accretion and merger histories. We wish to investigate the presence of a surviving high-z compact progenitor component in the brightest galaxy of the Hydra~I cluster, NGC~3311, by mapping its 2D kinematics and stellar population out to 2 effective radii. Our goal is to understand the formation of its several structural components and trace their mass assembly back in time. We combine MUSE observations, a customized and extended version of the state-of-the-art EMILES single stellar population models, and a newly developed parametric fully Bayesian framework to model the observed spectra using full-spectrum fitting. We present 2D maps, as well as radial profiles, of the stellar velocity dispersion, age, total metallicity, {alpha}-element, sodium abundance ([Na/Fe]), and the initial mass function slope. All properties have significant gradients, confirming the existence of multiple structural components, also including a "blue spot" characterized by younger and metal-richer stars. We find that the component dominating the light budget of NGC 3311 within R<2.0kpc, is the surviving z=0 analogue of a high-z compact core, according to the definition of Pulsoni et al. (2021). This concentrated structure has a relatively small velocity dispersion (sigma~180km/s), is very old (ages~11Gyr), metal-rich ([Z/H]~0.2 and [Na/Fe]~0.4), and has a bottom-heavy IMF (with slope Gamma_b_~2.4). In the outer region, instead, the line-of-sight velocity distribution becomes increasingly broader, and the stars have younger age. They are also metal and sodium poorer but {alpha}-element richer. The low-mass end of the IMF slope becomes Chabrier-like with increasing galactocentric distances. The existence of multiple structural components in NGC 3311 from photometry, kinematics, and stellar populations confirms the predictions from the two-phase formation scenario for NGC 3311, according to which a first very short, high-z star formation episode formed a compact stellar structure in its core, which then grew in size by the extended mass assembly of relatively massive satellites. Interestingly, the outer stellar population has an overabundant [alpha/Fe], most likely because NGC 3311, located at the center of the galaxy cluster, accreted stars from rapidly quenched satellites.
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