A Maximum Stellar Surface Density in Dense Stellar Systems

Understand

We compile observations of the surface mass density profiles of dense stellar systems, including globular clusters in the Milky Way and nearby galaxies, massive star clusters in nearby starbursts, nuclear star clusters in dwarf spheroidals and late-type disks, ultra-compact dwarfs, and galaxy spheroids spanning the range from low-mass cusp bulges and ellipticals to massive core ellipticals.

• We show that in all cases the maximum stellar surface density attained in the central regions of these systems is similar, Sigma_max ~ 10^11 M_sun/kpc^2 (~20 g/cm^2), despite the fact that the systems span 7 orders of magnitude in total stellar mass M_star, 5 in effective radius R_e, and have a wide range in effective surface density M_star/R_e^2.
• The surface density limit is reached on a wide variety of physical scales in different systems and is thus not a limit on three-dimensional stellar density.
• Given the very different formation mechanisms involved in these different classes of objects, we argue that a single piece of physics likely determines Sigma_max.