Light from Young Galaxies
The artist's illustration shows a typical massive galaxy as it would have appeared when the universe was only about a quarter of its current age. This young galaxy contains an active galactic nucleus (AGN), or quasar, in its center, a luminous object powered by the rapid growth of a supermassive black hole. Some of the light from the AGN is obscured by dense gas and dust near the center of the galaxy. The galaxy itself is undergoing a growth spurt, as shown by bright regions of star formation in the spiral arms.
Spitzer Space Telescope observations are extremely efficient at detecting distant AGN like this because dust and gas should absorb high-energy radiation from the AGN and re-emit it at longer wavelengths, generating copious amounts of infrared emission.
Large numbers of galaxies thought to contain such highly obscured AGN have been discovered in the Great Observatories Origins Deep Survey. The infrared emission for these galaxies exceeds the levels likely to be caused by star formation. However, X-ray observations were required to confirm the presence of obscured AGN, by looking for the high energy X-rays expected from such objects (less energetic X-rays are mostly absorbed).
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