2007ApJ...663..908R

We present an analysis of Spitzer Space Telescope observations of the three low surface brightness (LSB) optical giant galaxies Malin 1, UGC 6614, and UGC 9024. Mid- and far-infrared morphology, spectral energy distributions, and integrated colors are used to derive the dust mass, dust-to-gas mass ratio, total infrared luminosity, and star formation rate (SFR). We also investigate UGC 6879, which is intermediate between high surface brightness (HSB) and LSB galaxies. The 8 µm images indicate that polycyclic aromatic hydrocarbon (PAH) molecules are present in the central regions of all three metal-poor LSB galaxies. The diffuse optical disks of Malin 1 and UGC 9024 remain undetected at mid- and far-infrared wavelengths. The dustiest of the three LSB galaxies, UGC 6614, has infrared morphology that varies significantly with wavelength; 160 µm (cool) dust emission is concentrated in two clumps on the northeast and northwest sides of a distinct ring seen in the 24 and 8 µm images (and a broken ring at 70 µm) at a radius of ∼40" (18 kpc) from the galaxy center. The 8 and 24 µm emission is cospatial with Hα emission previously observed in the outer ring of UGC 6614. The estimated dust-to-gas ratios, from less than 10^–3 to 10^–2, support previous indications that the LSB galaxies are relatively dust-poor compared to the HSB galaxies. The total infrared luminosities are approximately 1/3 to 1/2 the blue-band luminosities, suggesting that old stellar populations are the primary source of dust heating in these LSB objects. The SFR estimated from the infrared data ranges ∼0.01-0.88 M_☉/yr, consistent with results from optical studies.