Great Glowing Galaxies

Way back in the 1980’s, astronomers uncovered anomalous galaxies that shined far brighter in the infrared than expected when compared to galaxies of similar size and age. The ultra-luminous infrared (ULI) galaxies were illuminated by starlight (much like the majority of the galactic population) but their visible light was diminished and their infrared dramatically increased. They have remained a subject of speculation for years, but now the Hubble Space Telescope may have cracked the case.

Utilizing the light focusing power of gravitational lensing around clusters of galaxies, Hubble has observed some ULI galaxies at extreme distances and found the source of their unexpected infrared. They are so far away that the light we are receiving dates back to a time around the peak age of star formation in our universe. At that point in time, these massive and dust-rich galaxies were churning out new stars around ten thousand times faster than our galaxy, the Milky Way, is today. While the exact mechanism of this increased rate is still murky, some ideas are that material is being “piped in” along dark matter filaments or that maybe primordial hydrogen, the raw stuff of stars, was more abundant in the Universe back then? As James Lowenthal of Smith College is quoted, “There are so many unknowns about star and galaxy formation, we need to understand the extreme cases, such as these galaxies, as well as the average cases, like our Milky Way, in order to have a complete story about how galaxy and star formation happen.”

If there are so many stars forming, why don’t they shine brightly in visible light as well? The very dust and gas that allows for increased star formation absorbs and blocks visible light but glows in the infrared. This enables the galaxies to shine in infrared as brightly as 10–100 trillion suns! Since this is an ancient and not widely explored age of the Universe, the astronomers studying the ULI galaxies are hoping that it might “shed some light” on the formation process and first age of galaxies that began in our Universe over 10 billion years ago.

The challenge is that since most of this information is obtained via gravitational lensing, and the lenses made by galaxy clusters are far from optically perfect, so it is like looking at the Universe through a water droplet—or trying to look at something submerged in water. The distortion can make details hard to discern with any degree of fidelity, but the more we study this family of objects, the more details will come to light (infrared and otherwise).