High-accuracy analytical solutions for the projected mass (counts) and surface density (brightness) of Einasto profiles

Abstract: The Einasto profile has been successful in describing the density profiles of dark matter haloes in $\Lambda$CDM N-body simulations. It has also been able to describe multiple components in the surface brightness profiles of galaxies. However, analytically projecting it to calculate quantities under projection is challenging. In this paper, we will see the development of a highly accurate analytical approximation for the mass (or counts) enclosed in an infinitely long cylindrical column for Einasto profiles--also known as the projected mass (or counts)--using a novel methodology. We will then develop a self-consistent high-accuracy model for the surface density from the expression for the projected mass. Both models are quite accurate for a broad family of functions, with a shape parameter $\alpha$ varying by a factor of 100 in the range $0.05 \lesssim \alpha \lesssim 5.0$, with fractional errors $\sim 10^{-6}$ for $\alpha \lesssim 0.4$. Profiles with $\alpha \lesssim 0.4$ have been shown to fit the density profiles of dark matter haloes in N-body simulations as well as the luminosity profiles of the outer components of massive galaxies. Since the projected mass and the surface density are used in gravitational lensing, I will illustrate how these models facilitate (for the first time) analytical computation of several quantities of interest in lensing due to Einasto profiles. The models, however, are not limited to lensing and apply to similar quantities under projection, such as the projected luminosity, the projected (columnar) number counts and the projected density or the surface brightness.