An infrared study of a sample of optically selected galaxies

Abstract

Infrared and optical photometry of a complete sub-sample of 194 galaxies in the Anglo-Australian Redshift Survey is presented. The Colour-Redshift diagrams are constructed and used, in a self-consistent manner, to reduce the data to zero-redshift colours at a fixed aperture size. Infrared and optical-to-ir Colour-Luminosity relations are analysed both as a function of morphological type and, in conjunction with published data, as a function of environment. An infrared Colour-Luminosity relation is found for all galaxy types. The slope seen for E-SO galaxies strengthen the hypothesis that metallicity is the driving factor in the relation at infrared and optical wavelengths. The Colour-Luminosity relations for spirals have slopes that are substantially steeper. The infrared Colour-Luminosity relations of spirals do not vary significantly with Hubble type and the type-dependence of their optical-ir Colour-Luminosity relation disappears when corrections for bulge contamination are made. The results imply that both bulge and disk material have distinct enrichment histories that depend on luminosity in a simple way irrespective of the host galaxy type. Comparison with stellar synthesis models has revealed that the optical-to-ir colours of spirals are also sensitive to metallicity variations. Moreover, a change in the slope of the initial mass function of star formation (from X=1.35 to X=3) is needed to produce their optical-ir colours over the observed range. The infrared Colour-Luminosity relation is fairly tight; the intrinsic scatter is small and does not appear to be environmental dependent. The relation for the field E-SOs, analysed in this study, agrees closely with that seen in both the Virgo and Coma clusters and is consistent with recent estimates of the Local Group motion; there is no evidence for any anomalous population of cool stars in the Virgo cluster galaxies. Although promising as a distance indicator in cluster sample, infrared photometry of high precision is required. The optical-ir relation for spirals, however, has an intrinsic scatter which is most likely due to varying amounts of star formation at a fixed luminosity and bulge-to-disk ratio. An application of the bulge-corrected optical-ir Colour-Luminosity relation to Bothun et al.'s sample of cluster spirals, reduces the scatter in the mean optical-ir colours amongst the clusters. However, the scatter is still larger than that observed in the infrared Tully-Fisher relation. A detailed analysis of the infrared luminosity function of 'field' galaxies is carried out in conjunction with its optical counterpart. The infrared luminosity function is found to be type-dependent. This implies that the E-SOs and the bulges of spirals have different evolutionary or formation histories. Moreover, the infrared luminosity function for a given morphological type of galaxies is found to be the same as its optical counterpart shifted by the respective mean optical- ir colours. Regarding this result, it is proposed that galaxies with strong/weak near-ir emission are not very common. The sensitivity of the luminosity function to inhomogeneities in the galaxian distribution is investigated. The best estimates for the luminosity function parameters are then used to predict deep number-magnitude counts at 2.2 µm (K ~ 18 - 20 mag).