Unveiling the Lens Galaxy of FLS 1718+59: A Galaxy-Galaxy Gravitational Lens System

We analyze a newly discovered galaxy-galaxy scale gravitational lens system, FLS 1718+59 in the Spitzer First Look Survey (FLS) field. A background galaxy (z = 0.245) is severely distorted by a foreground galaxy (z = 0.08), via gravitational lensing. We analyze this system by several methods, including surface brightness fitting (Galfit and Ellipse), gravitational lens modeling (gravlens), and spectral energy distribution fitting (Magphys). From Galfit and Ellipse we measure properties of the lens galaxy, such as the effective radius and the average surface brightness inside it, the ellipticity, and the position angle. gravlens gives us the total mass inside the Einstein radius ($R_{Ein}$), and Magphys provides us an estimate of the stellar mass inside $R_{Ein}$. By comparing these obtained parameters, we confirm that the lens galaxy is an elliptical galaxy on the Fundamental Plane, and calculate the stellar mass function inside $R_{Ein}$, and discuss the implications of the results regarding the initial mass function.