A new fitting concept for the robust determination of S\'ersic model parameters

Abstract

The S\'ersic law (SL) offers a versatile functional form for the structural characterization of galaxies near and far. Whereas applying it to galaxies with a genuine SL luminosity distribution yields a robust determination of the S\'ersic exponent eta and effective surface brightness $\mu_{\rm eff}$, this is not necessarily the case for galaxies whose surface brightness profiles (SBPs) appreciably deviate from the SL (eg, early-type galaxies with a depleted core and nucleated dwarf ellipticals, or most late-type galaxies-LTGs). In this general case of "imperfect" SL profiles, the best-fitting solution may significantly depend on the radius (or surface brightness) interval fit and corrections for point spread function (PSF) convolution effects. Such uncertainties may then affect, in a non-easily predictable manner, automated structural studies of galaxies. We present a fitting concept (iFIT) that permits a robust determination of the equivalent SL model for the general case of galaxies with imperfect SL profiles. iFIT has been extensively tested on synthetic data with a S\'ersic index 0.3<${\eta}$<4.2 and an effective radius 1<$\rm{R}_{eff}$ (arcs)<20. Applied to non PSF-convolved data, iFIT can infer the S\'ersic exponent eta with an absolute error of <0.2 even for shallow SBPs. As for PSF-degraded data, iFIT can recover the input SL model parameters with a satisfactorily accuracy almost over the entire considered parameter space as long as FWHM(PSF)<$\rm{R}_{eff}$. Tests indicate that iFIT shows little sensitivity on PSF corrections and the SBP limiting surface brightness, and that subtraction of the best-fitting SL model in two different bands yields a good match to the observed radial color profile. The publicly available iFIT offers an efficient tool for the non-supervised structural characterization of large galaxy samples, as those expected to become available with Euclid and LSST.