Spitzer Spectroscopy of IRAS Sources Having Extremely High IR/Opt Flux Ratios

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One of the main achievements of IRAS and the current Spitzer Space Telescope (SST), was the discovery and study of a large population of high-luminosity IR galaxies, ULIRGs and HLIRGs. Sources with extremely high IR/opt flux ratio (>100, sometimes reaching a few thousands) are of special interest. However, it is not easy to locate such sources among hundreds of thousands of infrared sources without having optical identifications and photometry. An attempt was made by one of the authors (AMM) to select such candidates among the 173,044 sources within the IRAS Faint Source Catalog (FSC). Sources having quality flags 2 or 3 both at 25 and 60 microns were taken. Given that the IRAS positions were not sufficiently accurate to select the correct optical counterpart a cross-correlation was made with the radio FIRST catalog (811,117 sources). Although stellar objects were lost in this way, the remaining sources made up a reliable sample of extragalactic IR sources. After discarding “false” associations and combining the double data, we were left with 2,310 IRAS-FSC sources. Accurate FIRST positions allowed optical identifications giving the optical magnitudes, thus making possible the estimation of IR/opt flux ratio of the objects. The DSS2 images were examined as well, and a number of erroneous sources were discarded. 237 objects compiled the final sample of high IR/opt flux ratio sources. The best 28 sources have been observed to obtain low resolution infrared spectra with the Infrared Spectrograph on Spitzer. Optical magnitudes in B are in the range 19m-23m, R magnitudes, 18m-21m, and the Spitzer flux at 25 mum is 50-1700 mJy. Infrared spectra show silicate absorption features or PAH emission features and are similar to those for classical ULIRGs. We make detailed comparisons with IRS spectra of ULIRGS to determine how the infrared characteristics depend on the infrared to optical ratio.

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