The night sky is full of deceptive lights. Astronomers were searching for galaxies that ignited in the first few hundred million years after the Big Bang: their light has traveled to us for nearly 13 billion years. Six faint smudges were selected from images taken by the Hubble and Spitzer space telescopes. They pointed the giant array of radio dishes, ALMA, in Chile's Atacama Desert at them to catch the faint heat of distant dust.
One galaxy blazed brightly, like a campfire in the fog. But when the James Webb Space Telescope looked at it, it turned out the light had traveled not 13 billion years but only about 11. The galaxy was not an ancient giant but a dusty star factory, such objects are called ultra-luminous infrared galaxies. It resembles a fogged-up window hiding a bright lamp: dust absorbs starlight and re-emits it in the infrared, making the object look redder and farther away than it really is.
The mistake made astronomers wonder: how to tell a distant galaxy from a dusty impostor? The authors examined the color filters used to select ancient objects. It turns out the threshold for the color associated with the hydrogen break needs to be stricter. It's like raising the bar in a competition: weak candidates are eliminated, strong ones remain. This is especially important for future sky surveys that will collect millions of galaxies and cannot check each one by hand.
The galaxy UDS_18697 shines in the infrared a trillion times brighter than the Sun, even though its dust is colder than a warm oven — only about 42 kelvins (−231 °C).