Resolved optical–infrared spectral energy distributions of galaxies: universal relations and their break-down on local scales

Stefano Zibetti 0 1 Brent Groves 0 0 Max-Planck-Institut fur Astronomie , Konigstuhl 17, D-69117 Heidelberg, Germany 1 Dark Cosmology Centre, Niels Bohr Institute - University of Copenhagen Juliane Maries Vej 30 , DK-2100 Copenhagen, Denmark A B S T R A C T 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 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 earlyto 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 m). 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 H 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 wellknown 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 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. - One of the remarkable aspects of astronomy is the strong correlations that exist between the broad observable properties of galaxies. The morphological sequence of galaxies first noted by Hubble (1926) correlates well with surface brightnesses, sizes and optical colours. These in turn correlate with and arise from the intrinsic physical properties of these galaxies, such as the total stellar masses (M), star formation rates (SFR), and mean stellar ages and metallicities (for reviews on these correlations see Roberts & Haynes 1994; Kennicutt 1998). The rest-frame optical colours of galaxies broadly correlate with each other [i.e. (u g), with (g r)], as well as with surface brightness (SB). While forming continuous sequence in colours, galaxies can be broadly classified into blue and red galaxies, especially when compared with luminosity or absolute magnitude, with a strong correlation with late- and early-type, respectively (Blanton et al. 2003). The variation in optical colours arise from differences in the star formation histories of galaxies, with UV to blue light dominated by short-lived massive stars, while the red to near-infrared light is more sensitive to the total amount of stars. This is complicated, however, by the presence of interstellar dust, which attenuates and reddens the stellar light (see e.g. Kennicutt 1998; Bruzual & Charlot 2003). Thus, the colours of a galaxy broadly measure the ratio of young to old stars, which is generally parametrized by the ratio of current to past star formation (b = SFR/SFR), or the ratio of the current SFR to the total stellar mass, the specific star formation rate (sSFR = SFR/M). In the local Universe, these intrinsic physical parameters, and thus a galaxys colours, are related to the total stellar mass of a galaxy (e.g. Brinchmann et al. 2004), with more massive galaxies having lower sSFRs and thus redder colours. The stellar mass, and thus the sSFR and colours, is related to the size, concentration, SB and mean metallicity of a galaxy (e.g. Kauffmann et al. 2003; Tremonti et al. 2004; Gallazzi et al. 2005). In a similar manner, the IR colours of galaxies are also strongly associated with the intrinsic properties of the galaxies, with the IR fluxes increasing relative to the near-IR fluxes (i.e. becoming redder) from early- to late-type galaxies (see e.g. Kennicutt 1998, especially fig. 4), and with increasing sSFR (da Cunha, Charlot & Elbaz 2008). As with the optical colours, the infrared is sensitive to the younger stellar populations, as the IR emitting dust is heated predominantly by UVblue light. In fact, the fluxes in both of these wavelength regions can be used as measures of a galaxys SFR (Kennicutt 1998), and a clear correlation between the global optical and IR colours is observed, in the sense that optically blue galaxies are also found to have enhanced IR emission (e.g. Hogg et al. 2005). However on a physical basis, the processes that lead to the fluxes in these two wavelength regimes are contrary: the radiation from young stars that dominate the UVblue optical colours is also that preferentially absorbed by dust which then re-emits in the IR (see e.g. Draine 2003). This contrary nature is exacerbated by the association of the youngest stars with the clouds of gas and dust from which they form, leading to relatively higher attenuation observed in star-forming regions (see e.g. Calzetti 1997; Charlot & Fall 2000). This contradiction of the observed galaxy scale correlation of both blue colours and IR excess and the physical contrary nature of dust absorption and UVblue light emission indicates that on some spatial scale within galaxies this correlation must break down. In some respect this difference is expected as the blue light and infrared emission arise from physically distinct components (i.e. stars and dust), and this correlation will break down once these different components are resolved out, as seen within our own Galaxy. However, this distinction between IR and blue light exists on larger scales within galaxies, as we show here by examining seven nearby galaxies that span a range of galax (...truncated)