Monday, May 17, 2010

Hubbles 20 Greatest Discoveries: 20 - 17

17. Hubble Maps the Cosmic Web of "Clumpy" Dark Matter in 3-D

This three-dimensional map offers a first look at the web-like large-scale distribution of dark matter, an invisible form of matter that accounts for most of the universe's mass. This milestone takes astronomers from inference to direct observation of dark matter's influence in the universe. Because of the finite speed of light, regions furthest away are also seen as they existed a long time ago. The map stretches halfway back in time to the beginning of the universe.

The map reveals a loose network of dark matter filaments, gradually collapsing under the relentless pull of gravity, and growing clumpier over time. This confirms theories of how structure formed in our evolving universe, which has transitioned from a comparatively smooth distribution of matter at the time of the big bang. The dark matter filaments began to form first and provided an underlying scaffolding for the subsequent construction of stars and galaxies from ordinary matter. Without dark matter, there would have been insufficient mass in the universe for structures to collapse and galaxies to form.

[Top] - Three slices through the evolving distribution of dark matter. The dataset is created by splitting the background source galaxy population into discrete epochs of time (like cutting through geologic strata), looking back into the past. This is calibrated by measuring the cosmological redshift of the lensing galaxies used to map the dark matter distribution, and binning them into different time/distance "slices". Each panel represents an area of sky nine times the angular diameter of the full Moon. Note that this fixed angle means that the survey volume is a really a cone, and that the physical area of the slices increases (from 19 Mpc on a side to 31 Mpc on a side) from left to right.

[Bottom] - When the slices across the universe and back into time are combined, they make a three-dimensional map of dark matter in the universe. The three axes of the box correspond to sky position (in right ascension and declination), and distance from the Earth increasing from left to right (as measured by cosmological redshift). Note how the clumping of the dark matter becomes more pronounced, moving right to left across the volume map, from the early universe to the more recent universe.

The dark matter distribution was mapped with Hubble Space Telescope's largest ever survey of the universe, the Cosmic Evolution Survey ("COSMOS"). To compile the COSMOS survey, Hubble photographed 575 adjacent and slightly overlapping views of the universe using the Advanced Camera for Surveys' (ACS) Wide Field Camera onboard Hubble. It took nearly 1,000 hours of observations. The distances to the galaxies were determined from their spectral redshifts, using the Subaru telescope in Hawaii.

18. Hubble Provide Complete View of Jupiter's Auroras

19. Pluto's Two Small Moons Officially Named Nix and Hydra

A pair of small moons that NASA's Hubble Space Telescope discovered orbiting Pluto now have official names: Nix and Hydra. Photographed by Hubble in 2005, Nix and Hydra are roughly 5,000 times fainter than Pluto and are about two to three times farther from Pluto than its large moon, Charon, which was discovered in 1978.

The names were approved this week by the International Astronomical Union (IAU), the recognized authority for assigning designations to celestial bodies.

In Greek mythology, Nyx is the goddess of the night. Among her many offspring was Charon, the boatman who ferried the dead across the river Styx into the Underworld. (Because asteroid 3908 already bears the Greek name Nyx, the IAU decided to use the Egyptian equivalent, Nix, for the name of Pluto's moon.) The mythological Hydra was a nine-headed serpent with poisonous blood. The Hydra had its den at the entrance to Hades, where Pluto and his wife Persephone entered the Underworld.

The team of researchers who selected the names, out of a list of more than two- dozen candidate names, used Hubble images to make the discovery in support of NASA's New Horizons mission to Pluto and the Kuiper Belt beyond. Team members are based at Southwest Research Institute (SwRI) in Boulder, Colo., the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., the Space Telescope Science Institute in Baltimore and Lowell Observatory in Flagstaff, Ariz.

"You're going to be hearing a lot more about Nix and Hydra in coming years, says co-leader of the discovery team, Alan Stern of SwRI. "Astronomers are already applying for telescope time to study their orbits and physical properties. And when New Horizons flies by Pluto in the summer of 2015, each moon will be mapped in detail."

In making the selection, team members explained that Nix and Hydra honor the search for new satellites and the New Horizons mission to Pluto by starting with the letters "N" and "H." The first letter of Hydra also honors the Hubble Space Telescope that was used to detect the satellites. This has historical tradition. Pluto's name begins with the letters "P" and "L" to honor Boston astronomer Percival Lowell, who inaugurated the search that led to Pluto's discovery. (Lowell did not live to see Pluto's discovery, which was made by Clyde Tombaugh at Lowell Observatory in 1930.)

20. Quaoar

OCTOBER 7, 2002: NASA's Hubble Space Telescope has measured the largest object discovered in the solar system since the discovery of Pluto 72 years ago. Approximately half the size of Pluto, the icy world is called "Quaoar" (pronounced kwa-whar). Quaoar is about 4 billion miles away, more than a billion miles farther than Pluto. Like Pluto, Quaoar dwells in the Kuiper belt, an icy belt of comet-like bodies extending 7 billion miles beyond Neptune's orbit.

1. How large is "Quaoar"?

Using Hubble's Advanced Camera for Surveys, astronomers determined that "Quaoar" has a diameter of about 800 miles (1300 kilometers). Hubble's observation is the first direct measurement of a Kuiper belt object's true size. Although astronomers have discovered 500 Kuiper belt objects over the past decade, they have estimated the sizes of only the largest of them. But astronomers could not measure the true sizes of the objects because they are small, dim, and far away.

2. What is the significance of this finding?

This new object is the "biggest fish" astronomers have snagged in surveys of Kuiper belt objects. Astronomers theorize that even larger icy worlds reside in the Kuiper belt, a reservoir of material from the birth of our solar system. The Kuiper belt is a vast frontier where astronomers are just beginning to hunt for clues about the early solar system. Objects like "Quaoar" could help enlighten astronomers about the birth of our solar system planets.

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