Preserved red nugget in the heart of the BCG of the Hydra I cluster revealed with MUSE 2D stellar population analysis

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C.E. Barbosa, C. Spiniello, M. Arnaboldi, L. Coccato, M. Hilker, T. Richtler

The population of passive, compact, high-redshift galaxies known as red nuggets is believed to end up in the core of current giant early-type galaxies. We investigate this scenario for NGC 3311, the brightest galaxy of the Hydra I cluster, a massive early-type galaxy with multiple structural components, including a low-velocity dispersion core and an extended stellar halo. We use MUSE observations to map the stellar populations of NGC 3311 out to 2 effective radii to trace its mass assembly back in time. Using a customized and extended version of the state-of-the-art single stellar population models EMILES, and a newly developed parametric fully Bayesian framework, we model the observed spectra using full-spectrum fitting and produce 2D maps of the stellar velocity dispersion, age, total metallicity, α-element, sodium abundance, and the initial mass function (IMF) slope. The core, which we identify as a red nugget, dominates the light budget within R≲1.5 kpc, has a relatively small velocity dispersion (σ∗≈180 km s−1), is old (ages≳11 Gyr), metal-rich ([Z/H]∼0.2 and [Na/Fe]∼0.4), and has a bottom-heavy IMF (with slope Γb∼2.4). In the outer region, stars become increasingly hotter, younger, metal and sodium poorer, α-element richer, and the IMF slope becomes Chabrier-like with increasing galactocentric distances. The multiple structural components in NGC 3311 confirm the expectations of the two-phase formation scenario for NGC 3311, where red nuggets are formed first in a very short, high-z star formation episode, and later grow in size by the extended mass assembly of satellites. Interestingly, the outer stellar population has an overabundant [α/Fe], most likely because NGC 3311, located at the center of the galaxy cluster, accreted stars from rapidly quenched satellites. [Abridged]

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