Bastionstelsels

Bastionstelsels is a term used in some astronomical contexts to describe a class of galactic systems characterized by a compact, dense core that remains bound in the face of tidal forces in dense environments, such as galaxy clusters. The term evokes a fortress-like central region that acts as a bastion against disruption, preserving much of the system’s stellar content.

Formation and evolution: Bastionstelsels are thought to originate through early, rapid star formation in deep potential wells, followed by tidal stripping and mergers that remove outer layers while the inner core remains intact. In simulations, repeated interactions can gradually grow a central density peak, strengthening the core potential.

Physical properties: They typically exhibit high central velocity dispersion relative to their luminosity, small effective radii, and high central surface brightness. Mass-to-light ratios can be elevated, reflecting either dark matter concentration or an old, metal-rich stellar population. Some models allow for a central massive black hole that influences core dynamics.

Observational status: The term is not universally applied as a formal class, but several compact ellipticals and ultra-compact dwarfs in nearby clusters share core properties reminiscent of bastionstelsels. Observational evidence relies on spectroscopy for velocity dispersion and high-resolution imaging for core structure.

Significance: If realized in nature, bastionstelsels would provide constraints on tidal evolution, subhalo survival, and central black hole growth. They highlight the role of core stability in galaxy evolution and may help distinguish between formation pathways for compact systems.