Astronomers have solved the mystery of ghost galaxies, according to a NASA press release. Ghost galaxies, which are galaxies with few stars, have stumped astronomers over the past decade. These ghost galaxies have been located by scientists using automated computer technology to look through the images of the Sloan Digital Sky Survey. It took NASA’s Hubble Space Telescope to help solve the mystery of the star-starved galaxies.
Astronomers say that views of three of the ghost galaxies suggest that their stars have the same birth date. The star-starved galaxies all began forming stars more than 13 billion years ago and then immediately stopped creating stars in the first billion years after the big bang.
These ancient galaxies suggest that a transitional phase occurred in the early universe that put an end to star formation in small galaxies. During the transitional phase, scientists say, the early stars burned off a fog of cold hydrogen in a process called reionization.
“These galaxies are all ancient and they’re all the same age, so you know something came down like a guillotine and turned off the star formation at the same time in these galaxies,” said study leader Tom Brown of the Space Telescope Science Institute in Baltimore. “The most likely explanation is reionization.”
Scientists say that the reionization of the universe took place in the first billion years after the big bang. During this period of time, radiation from the first stars smacked electrons off primeval hydrogen atoms, ionizing the cool hydrogen gas. This action allowed the hydrogen gas to become transparent to ultraviolet light. MIT’s Haystack Observatory provides some additional explanation of the Epoch of Reionization.
Astronomers believe that the same radiation that started universal reionization appears to have stopped star-making activities in small galaxies. The ghost galaxies were born about 100 million years before reionization began and had just started to form stars. Scientists say these galaxies, at approximately 2,000 light-years wide, are smaller than the more luminous star-creating dwarf galaxies near our Milky Way.
Unlike their larger relatives, the tiny galaxies were not big enough to prevent the harsh ultraviolet light from stripping away the gas they had. Stripped of gas, the puny galaxies couldn’t make new stars.
Scientists believe that Hubble’s discovery could help solve the so-called “missing satellite problem,” where only a few dwarf galaxies have been seen around the Milky Way while computer simulations calculate that thousands should exist. Without any star formation in the ghost galaxies, these dwarf galaxies would be exceedingly difficult to detect, scientists posit.
“By measuring the star formation histories of the observed dwarfs, Hubble has confirmed earlier theoretical predictions that star formation in the smallest clumps would be shut down by reionization,” said research team member Jason Tumlinson of the Space Telescope Science Institute.
“These are the fossils of the earliest galaxies in the universe,” Mr. Brown said. “They haven’t changed in billions of years. These galaxies are unlike most nearby galaxies, which have long star-formation histories.”
Scientists say there are anywhere from a few hundred to a few thousand stars in the ghost galaxies. While the galaxies may be star-deprived, the galaxies have a lot of dark matter, the building block of galaxies.
These ghost galaxies contain 10 times more dark matter than the ordinary matter that makes up gas and stars. Evidence reveals that dark matter outweighs ordinary matter by at least a factor of 100 in ghost galaxies. “The small galaxies in our study are made up mostly of dark matter because their hydrogen gas was ionized and the stars got turned off,” Mr. Brown said.
When astronomers first began uncovering the ghost galaxies, theories as to their shortage of stars were plentiful. Some scientists argued that internal dynamics, such as a supernova blast, blew out the gas needed to create additional stars. Other scientists said that the galaxies used up the gas they had. Only a few correctly guessed that reionization prevented the galaxies from forming stars.
Mr. Brown wanted to use Hubble’s Advanced Camera for Surveys to carefully examine six of the galaxies to study the population of stars and figure out when they were born. As of this press release, Mr. Brown and his research team have completed analyzing the Hubble data of the Hercules, Leo IV, and Ursa Major galaxies.
“Astronomers have said before that certain galaxies should be ancient, and then someone studies them hard enough and finds younger stars,” Mr. Brown said. “Some of us expected to uncover younger stars and prove that the galaxies are not relics from the early universe. We were surprised to find that all the stars were ancient.”
Mr. Brown calculated the stars’ ages by looking at their brightness and colors. He used the stars in the ancient globular cluster M92 as a reference. The examination revealed that the galaxies’ stars are as old as those in M92.
“The stars in the ultra-faint dwarf galaxies are very sparse,” Mr. Brown said. “This is one reason why no one went after them with Hubble. However, we thought they were good targets for Hubble, given Hubble’s ability to measure precise ages. You look at the Hubble images and there are almost no stars, but the ones we have are enough to give us the ages of these galaxies.”
The results of the study appeared in the July 1 issue of The Astrophysical Journal Letters.