MegaMorph - multiwavelength measurement of galaxy structure: complete Sersic profile information from modern surveys

In this paper, we demonstrate a new method for fitting galaxy p rofiles which makes use of the full multi-wavelength data provided by modern large optical-near-infrared imaging sur- veys. We present a new version of GALAPAGOS, which utilises a recently-developed multi- wavelength version of GALFIT, and enables the automated measurement of wavelength- dependent Sersic profile parameters for very large samples of galaxies. Our new technique is extensively tested to assess the reliability of both pieces of software, GALFIT and GALAPAGOS on both real ugrizY JHK imaging data from the GAMA survey and simulated data made to the same specifications. We find that fitting galaxy light profi les with multi-wavelength data increases the stability and accuracy of the measured parameters, and hence produces more complete and meaningful multi-wavelength photometry than has been available previously. The improvement is particularly significant for magnitudes in low S/N bands and for struc- tural parameters like half-light radius re and Sersic index n for which a prior is used by constraining these parameters to a polynomial as a function of wavelength. This allows the fitting routines to push the magnitude of galaxies for which s ensible values can be derived to fainter limits. The technique utilises a smooth transiti on of galaxy parameters with wave- length, creating more physically meaningful transitions t han single-band fitting and allows accurate interpolation between passbands, perfect for der ivation of rest-frame values.