Authors: Benedetta Vulcani, Steven P. Bamford, Boris Haussler, Marina Vika, Alex Rojas, Nicola K. Agius, Ivan Baldry, Amanda E. Bauer, Michael J.I. Brown, Simon Driver, Alister Graham, Lee S. Kelvin, Jochen Liske, Jon Loveday, Cristina C. Popescu, Aaron S.G. Robotham, Richard J. Tuffs
We investigate the relationship between colour and structure within galaxies using a large, volume-limited sample of bright, low-redshift galaxies with optical–near-infrared imaging from the GAMA survey. We fit single-component, wavelength-dependent, elliptical Sérsic models to all passbands simultaneously, using software developed by the MegaMorph project. The recovered wavelength variations in eective radius (Re) and Sérsic index (n) are robust and meaningful on an individual galaxy basis. These variations are closely related to radial colour gradients, but provide complementary physical insights. Dividing by Sérsic index and colour, we confirm that averaged trends with wavelength reflect the typical behaviour of galaxies in each sample. The wavelength variations of Re and n are useful for selecting objects that are traditionally difficult to identify, such as red spirals and blue ellipticals.
The trends in Re and n with wavelength reveal the internal structure, and hence formation history, of different types of galaxies. All these trends depend on Sérsic index; some have an additional dependence on galaxy colour. Late-type galaxies show a dramatic increase in Sérsic index with wavelength. We argue that this is a result of their two-component (bulge-disk) nature, though stellar population gradients within each component and dust attenuation are likely to play a role. All galaxies show a substantial decrease in eective radius with wavelength. This is strongest for early-types, even though they maintain constant Sérsic indices with wavelength, revealing that ellipticals are a superimposition of different stellar populations associated with multiple collapse and merging events. Processes leading to structures with larger Re must be associated with lower metallicity or younger stellar populations. This appears to rule out the formation of young cores through dissipative gas accretion as an important mechanism in the recent lives of luminous elliptical galaxies.