Resolved optical-infrared SEDs of galaxies: universal relations and their break-down on local scales

Abstract

A large body of evidence has demonstrated that the global rest-frame optical and IR colours of galaxies correlate well with each other, which can be readily interpreted as a sign of typically smooth star formation histories. 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. In this contribution we present our recent results (Zibetti & Groves 2011) from a pixel-by-pixel multi-wavelength (u-band to 8μm) analysis of seven nearby galaxies ranging from early- to late-types. We show that such a break-down occurs already on scales on few 100 pc, as a result of the different physical conditions in spatially distinct regions inside the galaxy, as we demonstrate by means of a Principal Component Analysis. Despite the lack of internal correlation between optical and IR within individual galaxies, when the pixels of all galaxies are compared the well known optical-IR colour correlations return, demonstrating that the variance observed within galaxies is limited around a mean which follows the well-known trends. We also examine the extremely strong correlations between the mid IR (Spitzer-IRAC)-NIR colours which extend continuously across all galaxies. 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.