Morphological Dependence of Mid-Infrared Properties of SDSS Galaxies in theSpitzerSWIRE Survey

Abstract

We explore the correlation between morphological types and mid-infrared (MIR) properties of an optically flux-limited sample of 154 galaxies from the Fourth Data Release (DR4) of the Sloan Digital Sky Survey (SDSS), cross-correlated with Spitzer SWIRE fields of ELAIS-N1, ELAIS-N2, and the Lockman Hole. Aperture photometry is performed on the SDSS and Spitzer images to obtain optical and MIR properties. The morphological classifications are given based on both visual inspection and bulge-disk decomposition in SDSS g- and r-band images. The average bulge-to-total ratio (B/T) is a smooth function over different morphological types. Both the 8 μm (dust) and 24 μm (dust) luminosities and their relative luminosity ratios to 3.6 μm (MIR dust-to-star ratios) present obvious correlations with both the Hubble T-type and B/T. The early-type galaxies notably differ from the late types in the MIR properties, especially in the MIR dust-to-star ratios. It is suggested that the MIR dust-to-star ratio of either νLν[8 μm (dust)]/νLν[3.6 μm] or νLν[24 μm (dust)]/νLν[3.6 μm] is an effective way to separate the early-type galaxies from the late-type ones. Based on the tight correlation between the stellar mass and the 3.6 μm luminosity, we have derived a formula to calculate the stellar mass from the latter. We have also investigated the MIR properties of both edge-on galaxies and barred galaxies in our sample. Since they present similar MIR properties to the other sample galaxies, they do not influence the MIR properties obtained for the entire sample.