SIMBAD references

A large body of evidence have demonstrated that the global rest-frame optical and infrared colours of galaxies correlate well with each other [i.e. (u-g), with (r- 8 µ m)], as well as with other galaxy properties such as surface brightness and morphology. However, the processes that lead to the observed correlations are contrary; the stellar light that contributes to the optical is readily absorbed by dust which emits in the IR. Thus, on small scales we expect these correlations to break down. We examine here seven nearby galaxies ranging from early- to late-types, which have all been smoothed to the same physical scale and signal-to-noise ratio, using data from the optical to the mid-IR (u band to 8 µm). By examining these galaxies on a pixel-by-pixel basis we demonstrate that there is disconnect between the optical and IR when normalized to the near-IR (H band). For five of the seven galaxies we can decompose this disconnect into two distinct components through a principal component analysis of the H-band normalized spectral energy distribution (SED) of the pixels: one mainly correlated with variations in the IR, and the other correlated with variations in the optical. The two exceptions are the elliptical galaxy NGC 4552, whose SED can be well reproduced using a single principal component, and the highly inclined spiral NGC 3521, due to its complex dust geometry. By mapping these two components in the five `regular' galaxies, it is clear they arise from distinct spatial regions. By comparing these components with the surface brightnesses in Hα and H band we demonstrate that the IR-dominated component is strongly associated with the specific star formation rate, while the optical-dominated component is broadly associated with the stellar mass density. However, when the pixels of all galaxies are compared, the well-known optical–IR colour correlations return, demonstrating that the variance observed within galaxies is around a mean which follows the well-known trend. As a final step, we extend this work by examining the extremely tight correlations observed between the Infrared Array Camera (IRAC)–near-IR colours, and demonstrate that these correlations are tight enough to use a single IRAC–near-IR colour (i.e. 8 µ m -H) to determine the fluxes in the other IRAC bands. These correlations arise from the differing contribution of stellar light and dust to the IRAC bands, enabling us to determine pure `stellar' colours for these bands, but still demonstrating the need for dust (or stellar) corrections in these bands when being used as stellar (dust) tracers.