A Cosmic Fog from Ancient Undetected Galaxies
Using The European Space Agency’s Herschel space telescope, astronomers in 2012 discovered that previously unseen distant galaxies are responsible for a cosmic fog of infrared radiation. The galaxies are some of the faintest and furthest objects seen by Herschel, and opened a new window on the birth of stars in the early Universe.
Astronomers estimate that there are billions and billions of galaxies in the observable universe –as well as some seven trillion dwarf galaxies. The image above shows A dwarf galaxy found by MIT’s Dr. Simona Vegetti and colleagues –a satellite of an elliptical galaxy almost 10 billion light-years away from Earth. The team detected it by studying how the massive elliptical galaxy, called JVAS B1938+666, serves as a gravitational lens for light from an even more distant galaxy directly behind it.
Here’s how astronomers breakout the visible universe within 14 billion light years:
Superclusters in the visible universe = 10 million
Galaxy groups in the visible universe = 25 billion
Large galaxies in the visible universe = 350 billion
Dwarf galaxies in the visible universe = 7 trillion
Stars in the visible universe = 30 billion trillion (3×10²²)
Astronomers recently realized that they may have underestimated the number of galaxies in some parts of the universe by as much as 90 percent, according to a study in 2011 by Matthew Hayes of the University of Geneva’s observatory, who led the investigation using the world’s most advanced optical instrument — Europe’s Very Large Telescope (VLT) in Chile, which has four 8.2-meter (26.65-feet) behemoths. They turned two of the giants towards a well-studied area of deep space called the GOODS-South field.
In the case of very distant, old galaxies, the telltale light may not reach Earth as it is blocked by interstellar clouds of dust and gas — and, as a result, these galaxies are missed by the map-makers.
“Astronomers always knew they were missing some fraction of the galaxies… but for the first time we now have a measurement. The number of missed galaxies is substantial,” said Matthew Hayes of the University of Geneva’s observatory, who led the investigation.
The team carried out two sets of observations in the same region, hunting for light emitted by galaxies born 10 billion years ago.The first looked for so-called Lyman-alpha light, the classic telltale used to compile cosmic maps, named after its U.S. discoverer, Theodore Lyman. Lyman-alpha is energy released by excited hydrogen atoms. The second observation used a special camera called HAWK-1 to look for a signature emitted at a different wavelength, also by glowing hydrogen, which is known as the hydrogen-alpha (or H-alpha) line.
The second sweep yielded a whole bagful of light sources that had not been spotted using the Lyman-alpha technique. They include some of the faintest galaxies ever found, forged at a time when the universe was just a child.
The astronomers conclude that Lyman-alpha surveys may only spot just a tiny number of the total light emitted from far galaxies. Astonishingly, as many as 90 percent of such distant galaxies may go unseen in these exercises.
“If there are 10 galaxies seen, there could be a hundred there, unseen” said Hayes. The discovery added powerfully to knowledge about the timeline by which stars and then galaxies formed.
Credits: ESA/PEP Consortium