iFIT: A new fitting concept for the robust determination of Sérsic model parameters
The Sérsic 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 Sérsic exponent η effective radius Reff and surface brightness µeff, this is not necessarily the case for galaxies whose surface brightness profiles (SBPs) appreciably deviate from the SL (e.g., 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, corrections for point spread function (PSF) convolution effects and/or initial guesses. 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. The distinctive feature of the concept proposed here is that the fit is not constrained through standard χ2 minimization between an observed SBP and the SL model of it, but instead through the search for the best match between the observationally determined and theoretically expected radial variation of the mean surface brightness and light growth curve. This approach ensures quick convergence to a unique solution for both perfect and imperfect Sérsic profiles, even shallow and resolution-degraded SBPs. iFIT allows for correction of PSF convolution effects, offering the user the option of choosing between a Moffat, Gaussian, or user-supplied PSF. iFIT , which is a standalone FORTRAN code, can be applied to any SBP that is provided in ASCII format and it has the capability of convenient graphical storage of its output. The iFIT distribution package is supplemented with an auxiliary SBP derivation tool in python. 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.