A preserved high-z compact progenitor in the heart of NGC 3311 revealed with MUSE 2D stellar population analysis

Abstract

Context. Massive early-type galaxies are believed to be the end result of an extended mass accretion history. The stars formed in situ very early on in the initial phase of the mass 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. Aims. 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. Methods. We combined 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. Results. We present 2D maps and radial profiles of the stellar velocity dispersion, age, total metallicity, α-element, sodium abundance ([Na/Fe]), and the initial mass function (IMF) slope. All properties have significant gradients, confirming the existence of multiple structural components, also including a “blue spot” characterized by younger and more metal-rich stars. We find that the component dominating the light budget of NGC 3311 within R ≲ 2.0 kpc is the surviving z = 0 analog of a high-z compact core. This concentrated structure has a relatively small velocity dispersion (σ* ≈ 180 km s−1), is very 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, instead, the line-of-sight velocity distribution becomes increasingly broad, and the stars are younger. They are also more metal and sodium poor but are richer in α-elements. The low-mass end of the IMF slope becomes Chabrier-like with increasing galactocentric distance. Conclusions. 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 [α/Fe], most likely because NGC 3311, located at the center of the galaxy cluster, accreted stars from rapidly quenched satellites.

Fornax 3D project: Assessing the diversity of IMF and stellar population maps within the Fornax Cluster

“…In addition to the internal dynamics of galaxies within Fornax, the gas in the cluster is found to be shock heated at a temperature of κT = 1.48 keV (Frank et al 2013), which would also prevent fueling of cool gas and thus star formation. This interpretation is in agreement with the recent measurements of similarly positive [Mg/Fe] gradients in some nearby cluster galaxies (Ferreras et al 2019;Barbosa et al 2021a).…”

Modelling simple stellar populations in the near-ultraviolet to near-infrared with the X-shooter Spectral Library (XSL)

“…The comparison with the E-MILES is relevant because these models are widely used in the study of intermediate and old stellar populations (Neumann et al 2021;Rodriguez Beltran et al 2021;Barbosa et al 2021;Lonoce et al 2021, to name a few recent works). Here, we use the E-MILES models calculated using the Salpeter IMF and both the Padova00 and BaSTI (Pietrinferni et al 2004(Pietrinferni et al , 2006Cordier et al 2007;Percival et al 2009) isochrones.…”

Galaxy populations in the Hydra I cluster from the VEGAS survey I. Optical properties of a large sample of dwarf galaxies

“…Several other works, studying the light distribution and the kinematics of the Hydra I cluster core, demonstrated that the mass assembly is still active around NGC 3311, and showed the presence of ongoing interactions between galaxies (Arnaboldi et al 2012;Ventimiglia et al 2011;Hilker et al 2018;Barbosa et al 2018Barbosa et al , 2021Koch et al 2012). Hereafter, we assume that the cluster center is represented by NGC 3311: R. A.…”