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

Physical dust models are presented for 65 galaxies in SINGS that are strongly detected in the four IRAC bands and three MIPS bands. For each galaxy we estimate (1) the total dust mass, (2) the fraction of the dust mass contributed by PAHs, and (3) the intensity of the starlight heating the dust grains. We find that spiral galaxies have dust properties resembling the dust in the local region of the Milky Way, with similar dust-to-gas ratio and similar PAH abundance. The observed SEDs, including galaxies with SCUBA photometry, can be reproduced by dust models that do not require "cold" (T ≲ 10 K) dust. The dust-to-gas ratio is observed to be dependent on metallicity. In the interstellar media of galaxies with A_O ≡ 12 +(O/H) > 8.1, grains contain a substantial fraction of interstellar Mg, Si, and Fe. Galaxies with A_O < 8.1 and extended H I envelopes in some cases appear to have global dust-to-gas ratios that are low for their measured oxygen abundance, but the dust-to-gas ratio in the regions where infrared emission is detected generally appears to be consistent with a substantial fraction of interstellar Mg, Si, and Fe being contained in dust. The PAH index q_PAH, the fraction of the dust mass in the form of PAHs, correlates with metallicity. The nine galaxies in our sample with A_O < 8.1 have a median q_PAH = 1.0%, whereas galaxies with A_O > 8.1 have a median q_PAH = 3.55%. The derived dust masses favor a value X_CO ≈ 4 × 10²⁰ cm^-2 (K km s^-1)^-1 for the CO-to-H₂ conversion factor. Except for some starbursting systems (Mrk 33, Tol 89, NGC 3049), dust in the diffuse ISM dominates the IR power.