oa MegaMorph: Multi-wavelength measurements of nearby galaxy structures

Fitting an analytic function to the two-dimensional surface brightness profile of a galaxy provides a powerful method of quantifying its internal structure. The resulting parameters reveal the size, shape and luminosity of the galaxy and its separate structural components (e.g., disk and bulge). Current galaxy fitting softwares packages consider only a single waveband image at a time. However, variations in stellar populations between and within galaxy structures mean that their observed properties depend on wavelength. Correctly studying the physical properties of galaxy components requires that these wavelength variations be accounted for. Multi-wavelength studies are presently possible, but require significant compromises: either the fits to each band must be done independently, or, one band must be favored for determining structural parameters, which are then imposed on fits to the other bands. Both of these approaches waste valuable information, and therefore result in suboptimal decompositions. Our project, 'MegaMorph', is developing a next-generation tool for decomposing galaxies, in terms of both their structures and stellar populations. We aim to present a modified version of the two-dimensional galaxy fitting software, GALFIT, developed as part of our MegaMorph project. These new additions enable a galaxy to be fit using images at many different wavelengths simultaneously. All the available data is therefore used to constrain a wavelength-dependent model of the galaxy, resulting in more robust, physically meaningful, component properties. We verify the advantages of our technique by applying it to a sample of 160 well-studied galaxies at redshifts smaller than 0.025, with ugriz imaging from the Sloan Digital Sky Survey to demonstrate how the resulting decompositions allow us to study the links between stellar population and galaxy structure in detail. Furthermore, we illustrate the advantages of our new method with regard to galaxy surveys, by fitting a sample of ~4000 artificially redshifted images of the galaxies described above. Our technique enables physical parameters of galaxy components to be robustly measured at lower signal-to-noise and resolution than would otherwise be possible. This paves the way for detailed statistical studies of the physical properties of galaxy disks and bulges.