Dust Masses, PAH Abundances, and Starlight Intensities in the SINGS Galaxy Sample

Physical dust models are presented for 65 galaxies in the SINGS survey thatare strongly detected in the four IRAC bands and three MIPS bands. For eachgalaxy we estimate (1) the total dust mass, (2) the fraction of the dust masscontributed by PAHs, and (3) the intensity of the starlight heating the dustgrains. We find that spiral galaxies have dust properties resembling the dustin the local region of the Milky Way, with similar dust-to-gas ratio, andsimilar PAH abundance. The observed SEDs, including galaxies with SCUBAphotometry, can be reproduced by dust models that do not require "cold" (T<10K)dust. The dust-to-gas ratio is observed to be dependent on metallicity. In theinterstellar media of galaxies with A_O=12+log(O/H)>8.1, grains contain asubstantial fraction of interstellar Mg, Si and Fe. Galaxies with A_O<8.1 andextended HI envelopes in some cases appear to have global dust-to-gas ratiosthat are low for their measured oxygen abundance, but in the regions whereinfrared emission is detected, the dust-to-gas ratio generally appears to beconsistent with a substantial fraction of interstellar Mg, Si, and Fe beingcontained in dust. The PAH index q_PAH -- the fraction of the dust mass in theform of PAHs -- correlates with metallicity. The nine galaxies in our samplewith A_O<8.1 have a median q_PAH=1.0%, whereas galaxies with A_O>8.1 have amedian q_PAH=3.55%. The derived dust masses favor a value X_CO approx 4e20cm^{-2}(K kms)^{-1} for the CO to H_2 conversion factor. Except for somestarbursting systems (Mrk33, Tolo89, NGC3049), dust in the diffuse ISMdominates the IR power.Comment: Accepted for publication in the Astrophysical Journa