Morphological Dependence of Mid-Infrared Properties of SDSS Galaxies in the Spitzer SWIRE Survey

Abstract

We explore the correlation between morphological types and mid-infrared(MIR) properties of an optically flux-limited sample of 154 galaxiesfrom 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 theSDSS and Spitzer images to obtain optical and MIR properties. Themorphological classifications are given based on both visual inspectionand bulge-disk decomposition in SDSS g- and r-band images. The averagebulge-to-total ratio (B/T) is a smooth function over differentmorphological types. Both the 8 μm (dust) and 24 μm (dust)luminosities and their relative luminosity ratios to 3.6 μm (MIRdust-to-star ratios) present obvious correlations with both the HubbleT-type and B/T. The early-type galaxies notably differ from the latetypes in the MIR properties, especially in the MIR dust-to-star ratios.It is suggested that the MIR dust-to-star ratio of eitherνLν[8 μm (dust)]/νLν[3.6 μm] orνLν[24 μm (dust)]/νLν[3.6 μm] isan effective way to separate the early-type galaxies from the late-typeones. Based on the tight correlation between the stellar mass and the3.6 μm luminosity, we have derived a formula to calculate the stellarmass from the latter. We have also investigated the MIR properties ofboth edge-on galaxies and barred galaxies in our sample. Since theypresent similar MIR properties to the other sample galaxies, they do notinfluence the MIR properties obtained for the entire sample.