In this paper, we demonstrate a new method for fitting galaxy profiles which makes use of the full multi-wavelength data provided by modern large optical-near-infrared imaging surveys. We present a new version of GALAPAGOS, which utilises a recently-developed multiwavelength version of GALFIT, and enables the automated measurement of wavelengthdependent Sérsic 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 profiles 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 structural parameters like half-light radius r e and Sérsic 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 sensible values can be derived to fainter limits. The technique utilises a smooth transition of galaxy parameters with wavelength, creating more physically meaningful transitions than single-band fitting and allows accurate interpolation between passbands, perfect for derivation of rest-frame values.

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. Dividing our sample by n and colour, the recovered wavelength variations in effective radius (R e ) and Sérsic index (n) reveal the internal structure, and hence formation history, of different types of galaxies. All these trends depend on n; some have an additional dependence on galaxy colour. Late-type galaxies (n r < 2.5) show a dramatic increase in Sérsic index with wavelength. This might be 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 R e with wavelength. This is strongest for early-types (n r > 2.5), even though they maintain constant n 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 R e 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.

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Supermassive black hole (SMBH) mass estimates are derived for 1743 galaxies from the Millennium Galaxy Catalogue (MGC) using the recently revised empirical relation between SMBH mass and the luminosity of the host spheroid. The MGC spheroid luminosities are based on R1/n bulge plus exponential‐disc decompositions. The majority of black hole masses reside between 106 M⊙ and an upper limit of 2 × 109 M⊙. Using previously determined space‐density weights, we derive the SMBH mass function which we fit with a Schechter‐like function. Integrating the black hole mass function over 106 < Mbh/M⊙ < 1010 gives an SMBH mass density of (3.8 ± 0.6) × 105 h370 M⊙ Mpc−3 for early‐type galaxies and (0.96 ± 0.2) × 105 h370 M⊙ Mpc−3 for late‐type galaxies. The errors are estimated from Monte Carlo simulations which include the uncertainties in the Mbh–L relation, the luminosity of the host spheroid and the intrinsic scatter of the Mbh–L relation. Assuming SMBHs form via baryonic accretion, we find that (0.008 ± 0.002) h370 per cent of the Universe's baryons are currently locked up in SMBHs. This result is consistent with our previous estimate based on the Mbh–n (Sérsic index) relation.

The European Space Agency has invested heavily in two cornerstones missions; Herschel and Planck. The legacy data from these missions provides us with an unprecedented opportunity to study cosmic dust in galaxies so that we can answer fundamental questions about, for example: the origin of the chemical elements, physical processes in the interstellar medium (ISM), its effect on stellar radiation, its relation to star formation and how this relates to the cosmic far infrared background. In this paper we describe the DustPedia project, which is enabling us to develop tools and computer models that will help us relate observed cosmic dust emission to its physical properties (chemical composition, size distribution, temperature), to its origins (evolved stars, super novae, growth in the ISM) and the processes that destroy it (high energy collisions and shock heated gas). To carry out this research we will combine the Herschel/Planck data with that from other sources of data, providing observations at numerous wavelengths (≤ 41) across the spectral energy distribution, thus creating the DustPedia database. To maximise our spatial resolution and sensitivity to cosmic dust we limit our analysis to 4231 local galaxies (v < 3000 km s In this the first of the DustPedia papers we describe the project objectives, data sets used and provide an insight into the new scientific methods we plan to implement.