On the estimation of galaxy structural parameters: the Sersic model

This paper addresses some questions which have arisen from the use of the Sersic r(1/n) law in modelling the luminosity profiles of early-type galaxies. The first issue deals with the trend between the half-light radius and the structural parameter n. We show that the correlation between these two parameters is not only real, but also a natural consequence from the previous relations found to exist between the model-independent parameters: total luminosity, effective radius and effective surface brightness. We also define a new galaxy concentration index which is largely independent of the image exposure depth, and is shown to be monotonically related with n. The second question concerns the curious coincidence between the form of the Fundamental Plane and the coupling between [I](e) and r(e) when modelling a light profile. We explain, through a mathematical analysis of the Sersic law, why the quantity r(e) [I](e)(0.7) appears almost constant for an individual galaxy, regardless of the value of n (over a large range) adopted in the fit to the light profile. Consequently, Fundamental Planes of the form r(e)[I](e)(0.7) proportional to sigma (x)(0)(for any x, and where sigma (0) is the central galaxy velocity dispersion) are insensitive to galaxy structure. Finally, we address the problematic issue of the use of model-dependent galaxy light-profile parameters versus model-independent quantities for the half-light radii, mean surface brightness and total galaxy magnitude. The former implicitly assume that the light-profile model can be extrapolated to infinity, while the latter quantities, in general, are derived from a signal-to-noise ratio truncated profile. We quantify (mathematically) how these parameters change as one reduces the outer radius of an r(1/n) profile, and reveal how these can vary substantially when n greater than or equal to 4.