On the origin of S\'ersic profiles of galaxies and Einasto profiles of dark-matter halos

TL;DR

This paper explores the common analytic functions describing galaxy brightness and dark-matter halo density profiles, linking their shapes to formation histories influenced by initial density fluctuations and merger activity.

Contribution

It demonstrates that the Sersic and Einasto profiles originate from the initial density fluctuation field and merger history, providing a unified understanding of their shapes.

Findings

High-alpha (low-m) systems form in smooth regions with few mergers.

Low-alpha (high-m) systems form in clumpy regions with multiple mergers.

Profiles are linked to initial conditions and merger history.

Abstract

The surface-brightness profiles of galaxies I(R) and the density profiles of dark-matter halos rho(r) are well represented by the same analytic function, named after either S\'ersic, I~exp[-(R/R*)^(1/m)], or Einasto, rho~[exp[-(r/r*)^alpha], where R* and r* are characteristic radii. Systems with high S\'ersic index m (or low Einasto index alpha) have steep central profiles and shallow outer profiles, while systems with low m (or high alpha) have shallow central profiles and steep profiles in the outskirts. We present the results of idealized numerical experiments which suggest that the origin of these profiles can be traced back to the initial density fluctuation field: high-alpha (low-m) systems form in smooth regions via few mergers, while low-alpha (high-m) systems form in clumpy regions via several mergers.