Morphological Dependence of MIR Properties of SDSS Galaxies in the Spitzer SWIRE Survey

We explore the correlation between morphological types and mid-infrared (MIR) properties of an optically flux-limited sample of 154 galaxies from the Forth Data Release (DR4) of Sloan Digital Sky Survey (SDSS), cross-correlated with Spitzer SWIRE (Spitzer Wide-Area InfraRed Extragalactic Survey) fields of ELAIS-N1, ELAIS-N2 and 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 bulgedisk decomposition on SDSS g− and r−band images. The average bulge-tototal ratio (B/T ) is a smooth function over different morphological types. Both the 8μm(dust) and 24μm(dust) luminosities and their relative luminosity ratios to 3.6μm (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 latetypes 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 [8μm(dust)]/νLν [3.6μm] or νLν [24μm(dust)]/νLν [3.6μm] 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 3.6μm 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. Subject headings: galaxies: morphology — galaxies: formation— infrared: galaxies — galaxies: statistics National Astronomical Observatories, Chinese Academy of Sciences, A20 Datun Road, Beijing 100012, P.R.China; [email protected]; [email protected] Graduate School of Chinese of Academy of Sciences, Beijing 100039, P.R.China Visiting Scholar, Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822