Astronomers estimate that there are between 100 billion and 200 billion galaxies in the known universe. A single galaxy such as the Milky Way contain upwards of 200 billion normal stars. About 75 percent of of all stars in the Milky Way are less than half as massive as our Sun. In the universe at large, the majority of galaxies are classified as dwarfs, each with less than a few hundred million stars. The image above is a computer simulation of a colliding dwarf galaxy triggering the formation of the Milky Ways spiral arms.
This reveals the complex web of the large-scale structure of the Universe in great detail. The light of each galaxy is spread out into its component colours within VIMOS. Follow up analysis then allows astronomers to work out how fast the galaxy appears to move away from us — its redshift. This in turn reveals its distance and, when combined with its position on the sky, its location in the Universe.
By studying the cosmic web astronomers can test theories of how the Universe formed and evolved and help to track down the properties of the mysterious dark energy that is making the expansion of the Universe speed up. By mapping how large-scale structure grows with time they can also check whether Einstein’s theory of general relativity holds precisely, or whether it may need to be revised.
VIPERS is the most detailed survey so far of galaxies that are seen from the time when astronomers think that the Universe became dominated by dark energy, as it is today. This happened when the Universe was between about five and nine billion years old — about half its current age of 13.7 billion years. Although it is not yet complete, VIPERS is already delivering exciting science results, including both a first estimate of the growth rate of large-scale structure at this time and the best census ever of the average number of massive galaxies during this period of the Universe’s history.
ESA astronomers say that for every ten far galaxies observed, a hundred go undetected. The European Space Agency’s Herschel space telescope has 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 open 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).
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 (3x10²²)
Astronomers realize that they may have underestimated the number of galaxies in some parts of the universe by as much as 90 percent, according to 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 an infant.
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," said Hayes. The discovery adds powerfully to knowledge about the timeline by which stars and then galaxies formed.
The image below shows the discovery of a previously unresolved population of galaxies in the GOODS fields and the first measurements of properties of galaxies in the almost unexplored far-infrared domain are among the first exciting scientific results achieved by Herschel's PACS and SPIRE instruments. These findings confirm the extraordinary capabilities of ESA's new infrared space observatory to investigate the formation and evolution of galaxies.
Image credit: GOODS-South field, ESA/PACS Consortium/PEP Key Programme Consortium and http://hipacc.ucsc.edu/images/sgr_galaxyfield_web_highres.jpg