Uncovering Ultra-Luminous Galaxies in the IRAS FSC Through Radio and Optical Cross-Identification

Extract

Ultraluminous infrared galaxies (ULIRGs) have luminosities (10¹² L⊙) once exclusive to QSOs. This suggests they might be the early, dust-enshrouded stages of QSOs. ULIRGs have ∼ 3.5× the space density of QSOs at the present epoch. Quasars reached their peak space density at z ∼ 2, so if ULIRGs are QSO precursors, there should be a dramatic increase in their space density up to z ∼ 2. The small number of known ULIRGs makes it difficult to explore links between ULIRGs and QSOs, much less measure their evolution. To do so, a large sample of ULIRG candidates must be identified. The IRAS FSC contains ∼ 60,000 probable galaxies, of which 1%–3% should have L_FIR > 10¹² L⊙. We discuss an efficient and reliable method which uses IRAS-VLA-APS cross-identification and flux ratios to mine the FSC for likely ULIRGs.