Friday, May 30, 2008

Cosmic Web

Credit: NASA, ESA, and A. Feild (STScI)

This illustration shows how the Hubble Space Telescope searches for missing ordinary matter, called baryons, by looking at the light from quasars several billion light-years away. Imprinted on that light are the spectral fingerprints of the missing ordinary matter that absorbs the light at specific frequencies (shown in the colourful spectra at right). The missing baryonic matter helps trace out the structure of intergalactic space, called the "cosmic web."

Credit: NASA, ESA, and E. Hallman (University of Colorado, Boulder)

This graphic represents a slice of the spider-web-like structure of the universe, called the "cosmic web." These great filaments are made largely of dark matter located in the space between galaxies. The Hubble Space Telescope probed the structure of intergalactic space to look for missing ordinary matter, called baryons, that is gravitationally attracted to the cosmic web.

Thursday, May 22, 2008

Galaxies Collide

Galaxies Collide in NGC 3256 - Credit: NASA, ESA, Hubble Heritage (STScI / AURA) - ESA/Hubble Collaboration, & A. Evans (UVa, NRAO, SUNYSB)

Galaxies don't normally look like this. NGC 3256 actually shows a current picture of two galaxies that are slowly colliding. Quite possibly, in hundreds of millions of years, only one galaxy will remain.

Today, however, NGC 3256 shows intricate filaments of dark dust, unusual tidal tails of stars, and a peculiar center that contains two distinct nuclei.

Although it is likely that no stars in the two galaxies will directly collide, the gas, dust, and ambient magnetic fields do interact directly. NGC 3256, part of the vast Hydra-Centaurus supercluster of galaxies, spans over 100 thousand light-years across and is located about 100 million light-years away.

Thursday, May 01, 2008

Black Holes get a kick

A colossal black hole has been spotted exiting its home galaxy, kicked out after a huge cosmic merger took place. The event, seen for the first time, was announced on Tuesday April 29.

When two colliding galaxies finally merge, it is thought that the black holes at their cores may fuse together too. Astronomers theorize that the resulting energy release could propel the new black hole from its parent galaxy out into space, but no one has found such an event.

"We have observed the pre-merger stages of black holes," said Stefanie Komossa of the Max Planck Institute for extraterrestrial Physics, part of the team that made the new discovery. "But we haven't seen the actual merger event."

Komossa and her team have now detected the consequences of such a merger: a black hole in the process of leaving its home galaxy. "The consequence was that the merged black hole, the final product, the new black hole was expelled from the galaxy," Komossa said. The team's results are detailed in the May 10 issue of the journal Astrophysical Journal Letters.

Komossa explained that the theory behind these mergers follows from the observations that many galaxies have very massive black holes at their cores. If two galaxies with these black holes collide, "then it's sort of inevitable that these two black holes will come very close to each other."

The black holes may not merge right away though. "One possibility is that for a long time they just orbit each other," like binary stars, said Komossa.

The orbiting black holes might interact with a star or surrounding gas which could cause them to lose angular momentum. That would be a way to push them ever-closer towards each other.

Eventually, the black holes would fuse, and in the final coalescence, or merger, of these two black holes, a giant burst of gravitational waves is emitted. Since these waves are generally emitted in one preferred direction, the black hole is then kicked in the other direction.

The "kick" the black hole receives is akin to the recoil of a rifle. It can propel the black hole to speeds of up to several thousand kilometers per second, according theoretical simulations. The escaping black hole Komossa and her team observed was racing along at 5,900,000 mph (2,650 kilometers per second).

The pull of the galaxy's gravity is no match for these incredible speeds, and the black hole, "will inevitably go to intergalactic space," Komossa said.

In theory, these mergers and escapes would leave several black holes without galaxies and galaxies without black holes out in space.

Detecting black holes at the center of galaxies is a difficult process. Because their gravity is so powerful, light is trapped, which is why they're black. Only by looking at their effects on surrounding material are they presumed to exist, and this is typically done only with relatively nearby galaxies, so looking for a missing black hole in the center of a distant galaxy is a tricky prospect.

The evolution of black holes and galaxies is very closely linked, so what exactly the effect would be on the separated partners is uncertain and the subject of further research.

In simulations where a black hole receives a slightly weaker kick, it can't escape the galaxy's gravity, so it falls back and oscillates until it comes to rest again at the galaxy's core. Recent simulations of this situation showed that stellar orbits adjust to the yo-yoing black hole, "so it clearly has an effect on the core of the galaxy," Komossa said.