Exploring infrared properties of giant low surface brightness galaxies

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 mu m 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 mu m ( cool) dust emission is concentrated in two clumps on the northeast and northwest sides of a distinct ring seen in the 24 and 8 mu m images ( and a broken ring at 70 mu m) at a radius of similar to 40 '' ( 18 kpc) from the galaxy center. The 8 and 24 mu m emission is cospatial with H alpha 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 similar to 0.01-0.88 M-circle dot yr(-1), consistent with results from optical studies.