Work

The properties of central stellar knots embedded in galactic bulges of CIELO simulations

Year: 2026

Type: article

Abstract: Context . Deeply bound stellar substructures of about 1 kpc at the center of bulges are found in a subset of galaxies from the cosmological chemo-hydrodynamical zoom-in CIELO simulation suite. They were identified as stellar overdensities at the lowest binding energies in each galaxy’s circularity-energy ( ϵ, E ) plane. We refer to these overdensities as stellar knots. Aims . We aim to characterize the nature and origin of the central stellar knots in the context of bulge assembly using CIELO-simulated galaxies spanning a wide range of stellar masses (10 8.0 –10 10.7 M ⊙ ) with diverse formation histories. Methods . We inspected 54 galactic bulges. Within them, we isolated stellar knot candidates in ( ϵ, E ) space and identified 28 robust knots that satisfy successive selection criteria based on kinematics, concentrations, and morphology. We characterized their chemical abundances, formation timescales, formation sites, progenitor gas origins, and spatial distributions. Results . We detect some systematic differences in the median statistics of the CIELO knots relative to their local and global surrounding bulge stellar populations, although substantial scatter exists within the sample. Across all galaxy masses, knots are systematically alpha-element enhanced, having assembled the bulk of their stellar mass at earlier epochs and on shorter timescales than other bulge populations, with a median of ∼2 Gyr versus ∼5 Gyr. Regarding their origins, knots were predominantly formed in situ (5% accreted mass fraction, roughly half that of the rest of the bulge), with a negligible disk-born star contribution and the largest gas fractions originating from a primordial central spheroid (21% versus 16–17% for the surrounding populations), which is consistent with being the primary gas fuel for the alpha-element enhancement. In terms of structure, knots exhibit a variety of morphologies, with spheroidal shapes predominating. Conclusions . Our dynamical selection in ( ϵ, E ) space of CIELO galaxies effectively recovers coeval stellar populations, pointing to stellar knots as plausible fossil signatures of early in situ bulge assembly.

Source: Astronomy and Astrophysics

DOI: 10.1051/0004-6361/202660969