Buried Active Galactic Nucleus

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Science  31 Aug 2001:
Vol. 293, Issue 5535, pp. 1559
DOI: 10.1126/science.293.5535.1559a

Ultraluminous infrared galaxies (ULIRGs) are some of the brightest objects that have formed in the past few billion years, and older objects observed in the submillimeter background emission appear to be similar to ULIRGs. The high luminosity and other characteristics suggest that ULIRGs are starburst galaxies with high rates of star formation, but recent work has indicated that some ULIRGs may get their energy from a central source engine, namely an active galactic nucleus (AGN) that is buried within a shroud of dust.

Imanishi et al. obtained infrared spectra of the galaxy UGC 5101 and found a weak polycyclic aromatic hydrocarbon emission coupled with a strong 3.4-micrometer carbonaceous dust absorption. The weak emission cannot account for the object's luminosity, suggesting that an AGN is needed and that the strong absorption is due to the AGN being buried in a sphere of dust. Thus, not all ULIRGs may be starburst galaxies, and this may alter estimates of star formation rates in the early universe and the present. — LR

Astrophys. J.558, astro-ph0108156v2.

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