Ghost Galaxies Firmly Set In Hubble’s Sites
Credit: NASA, ESA, and T. Brown (STScI)
They are extremely faint and tiny. Not only are they barely detectable, but thanks to Hubble they are thought to be the oldest and most pure formation in the Universe.
They have only come to light within the last ten years and even then they had to be detected through a computer search of the Sloan Digital Sky Survey. What are they? They are “ghost galaxies” and scientists are puzzled as to why they harbor so few stars.
Thanks to the Hubble Space telescope, astronomers are now able to peer into the mystery of ghost galaxies. By examining a trio of these ethereal island universes, Hubble was able to uncover their star populations all seem to share a common birth date. They began star formation approximately 13 billion years ago and then unexpectedly stopped… all within the first billion years following the Big Bang. These antique galaxies are proof a transitional phase occurred in the early Universe which put a halt to strong star-formation in small galaxies. Just how do we know what switched them off? It may be a process called “reionization” – where stars consume their envelope of cold hydrogen.
“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 Tom Brown of the Space Telescope Science Institute in Baltimore, Md., the study’s leader. “The most likely explanation is reionization.”
Sloan Digital Sky Survey telescope (Photo credit: Wikipedia)
Some time during the first billion years after the Big Bang, the Universe began the process of reionization and the very first stars scattered the electrons of these primeval hydrogen atoms. This caused the cool hydrogen gas to ionize and become transparent to ultraviolet light. Oddly enough, the radiation which caused universal reionization apparently also shut-down star formation in dwarf galaxies.
These dwarfs were young at the time – only about 100 million years – and had just began to form stars in earnest. The “ghost galaxies” are only about 2,000 light years wide… much smaller than their stellar producing peers which reside near the Milky Way. Being of small mass, they weren’t able to shield themselves from the intense ultraviolet light and their small gas supply was stripped away, leaving their stellar wombs barren.
Through this new revelation, astronomers may be able to explain the “missing satellite problem” – where only a few dozen dwarf galaxies have been found around the Milky Way when computer simulations predict there should be thousands. It offers a plausible explanation – one where little or no star formation makes them almost impossible to detect.
The Sloan Survey has been responsible for bringing more than a dozen of these ghost galaxies to light… tiny galaxies which are at home in the Milky Way area and only account for about a quarter of the sky surveyed. Now researchers predict there may be dozens more waiting to be discovered. These ultra-faint dwarf galaxies are proof that there may be even more where no stars formed at all.
“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 Jason Tumlinson of the Space Telescope Science Institute, a member of the research team.
“These are the fossils of the earliest galaxies in the universe,” adds Dr, Brown. “They haven’t changed in billions of years. These galaxies are unlike most nearby galaxies, which have long star-formation histories.”
Just what is inside a ghost galaxy? It would appear there are anywhere from a few hundred to a few thousand stars about the same mass as the Sun or smaller. While these galaxies might be starved for stars, they are certainly glutted on dark matter. Ordinary dwarf galaxies contain 10 times more dark matter than the ordinary matter that makes up gas and stars. In ultra-faint dwarf galaxies, dark matter outweighs ordinary matter by at least a factor of 100. “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,” Brown explained.
Past History Examined
On a curious note, these “dark matter islands” have been here all along – coexisting with the Milky Way, but unseen. Thanks to the Sloan Sky Survey, they came to light and astronomers began to be curious as to why they contained very few stars. There have been many theories, such as internal dynamics where a supernova event cleared the gas. Others contended the galaxies just depleted their gas content naturally. Even reionization was considered, where the formation of the galaxy coincided with the epoch during the early Universe. However, no perfect explanation is forthcoming. Ground-based observations reveal the stars are incredibly old.
“Astronomers have said before that certain galaxies should be ancient, and then someone studies them hard enough and finds younger stars,” 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.”
Through the use of spectroscopy, Brown was able to determine the stars’ ages. For reference, Brown compared the galaxies’ stars with the stars in the ancient globular cluster M92, located 26,000 light-years away. M92 is more than 13 billion years old… one of the oldest objects in the Universe. The analysis revealed that the galaxies’ stars are as old as those in M92.
“The stars in the ultra-faint dwarf galaxies are very sparse,” 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.”
Original Story Source: Hubble News Release.
