Lyman-alpha blob — Safekipedia Discoverer

In astronomy, a Lyman-alpha blob (LAB) is a huge concentration of gas that emits light in a special way called the Lyman-alpha emission line. These blobs are some of the largest single objects known in the Universe. Some of them stretch across more than 400,000 light years—much bigger than our own Milky Way galaxy, which is about 87,400 light years wide.

The giant Lyman-alpha blob LAB-1 (left) and an artist's impression of what it might look like if viewed from relatively close (right).

So far, scientists have found Lyman-alpha blobs only very far away in space, because they are very faint and their light is shifted to longer wavelengths. This shift, called redshift, allows their light to pass through Earth's atmosphere, which normally blocks this type of light. Because of this, these blobs can only be studied when their light has been stretched to longer wavelengths.

The most famous Lyman-alpha blobs were found in the year 2000 by a group of scientists. Later, more were discovered using the powerful Subaru Telescope in Japan. These blobs form a structure that stretches over 200 million light-years. Scientists are still trying to understand how these huge clouds of gas are connected to the galaxies around them and what makes them glow so brightly. Studying Lyman-alpha blobs may help us learn more about how galaxies formed in the early universe.

Examples

Composite of two different images taken with the FORS instrument on the Very Large Telescope of the Lyman-alpha blob LAB-1 in the constellation of Aquarius. Credit ESO/M. Hayes.

Some famous examples of Lyman-alpha blobs include Himiko, LAB-1, and EQ J221734.0+001701, known as the SSA22 Protocluster. Another example is TON 618, a very bright object in space that is powered by one of the most massive black holes known. These blobs are among the largest single objects observed in the universe.