Ghost Galaxies

Ghost Galaxies are thought to be some of the tiniest, oldest, and most pristine galaxies in the universe. They have been discovered over the past decade by astronomers using automated computer techniques to search through the images of the Sloan Digital Sky Survey.

NASA’s Hubble telescope recently has revealed that their stars share the same birth date. The galaxies all started forming stars more than 13 billion years ago — and then abruptly stopped — all in the first billion years after the universe was born in the big bang. These are claimed to be evidence for a transitional phase in the early universe that shut down star-making factories in tiny galaxies. During this time, the first stars burned off a fog of cold hydrogen in a process called reionization. The reionization of the universe began in the first billion years after the big bang. During this epoch, radiation from the first stars knocked electrons off primeval hydrogen atoms, ionizing the cool hydrogen gas. This process allowed the hydrogen gas to become transparent to ultraviolet light. The small irregular galaxies were born about 100 million years before reionization began and had just started to churn out stars. Roughly 2,000 light-years wide, the galaxies are the smaller cousins of the more luminous star-making dwarf galaxies near our Milky Way. The discovery could help explain the so-called “missing satellite problem,” where only a few dozen dwarf galaxies have been observed around the Milky Way while the simpler computer simulations predict that thousands should exist. One possible explanation is that there has been very little or even no star formation in the smallest of these dwarf galaxies, making them difficult to detect.