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

TL;DR

The paper introduces iFIT, a robust fitting method for accurately determining Sersic model parameters in galaxy surface brightness profiles, even when profiles deviate from ideal models or are affected by PSF effects.

Contribution

The novel iFIT method provides a reliable way to fit Sersic profiles to imperfect galaxy data, improving structural analysis accuracy for large surveys.

Findings

iFIT accurately recovers Sersic parameters with errors <0.2 for synthetic data.

iFIT maintains high accuracy with PSF-degraded data when FWHM< R_eff.

Subtraction of fitted models matches observed color profiles well.

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.