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 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^(-1), consistent with results from optical studies