Galaxies Are Either Asleep or Awake

Galaxies Are Either Asleep or Awake

  8:26:00 PM    Science Lover

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Astronomers have probed into the distant universe and discovered that galaxies display one of two distinct behaviors: they are either awake or asleep, actively forming stars or are not forming any new stars at all.

Bluer galaxies are actively "awake" and forming stars, 
while redder galaxies have shut down and are "asleep." 
(Credit: NASA, ESA, S. Beckwith (STScI) and the 
HUDF team)

Scientists have known for several years that galaxies in the nearby universe seem to fall into one of these two states. But a new survey of the distant universe shows that even very young galaxies as far away as 12 billion light years are either awake or asleep as well, meaning galaxies have behaved this way for more than 85 percent of the history of the universe. (Looking at galaxies farther away is like looking back in time when they were much younger, because of how long it takes the light they emit to reach us here on Earth.)

"The fact that we see such young galaxies in the distant universe that have already shut off is remarkable," said Kate Whitaker, a Yale University graduate student and lead author of the paper, which is published in the June 20 online edition of the Astrophysical Journal.



In order to determine whether the galaxies were asleep or awake, Whitaker and her colleagues fabricated a new set of filters, each one sensitive to different wavelengths of light, which they used on a 4-meter Kitt Peak telescope in Arizona. They spent 75 nights peering into the distant universe and collecting light from 40,000 galaxies ranging in distance from the nearby universe out to 12 billion light years away. The resulting survey is the deepest and most complete ever made at those distances and wavelengths of light.

The team deciphered the galaxies' dual behavior based on the color of the light they emit. Because of the physics of star formation, active, wakeful galaxies appear bluer, while the light emitted by passive, sleepy galaxies tends toward the redder end of the spectrum.

The researchers found that there are many more active galaxies than passive ones, which agrees with the current thinking that galaxies start out actively forming stars before eventually shutting down.

"We don't see many galaxies in the in-between state," said Pieter van Dokkum, a Yale astronomer and another author of the paper. "This discovery shows how quickly galaxies go from one state to the other, from actively forming stars to shutting off."

Whether the sleeping galaxies have completely shut down remains an open question, Whitaker said. However, the new study suggests the active galaxies are forming stars at rates about 50 times greater than their sleepy counterparts.

"Next, we hope to determine whether galaxies go back and forth between waking and sleeping or whether they fall asleep and never wake up again," van Dokkum said. "We're also interested in how long it takes galaxies to fall asleep, and whether we can catch one in the act of dozing off."

Other authors of the study include Ivo Labbé (Leiden University and Carnegie Observatories); Gabriel Brammer (Yale University and European Southern Observatory); Mariska Kriek (Princeton University and Harvard-Smithsonian Center for Astrophysics); Danilo Marchesini (Tufts University); Ryan Quadri and Marijn Franx (Leiden University); Adam Muzzin, Rachel Bezanson, Kyoung-Soo Lee, Britt Lundgren, Erica Nelson, Tomer Tal and David Wake (Yale University); Rik Williams (Carnegie Observatories); Garth Illingworth (UCO/Lick Observatory); and Gregory Rudnick (University of Kansas).

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Black Hole Blows Big Bubble

Black Hole Blows Big Bubble

  9:14:00 PM    Science Lover

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Combining observations made with ESO's Very Large Telescope and NASA's Chandra X-ray telescope, astronomers have uncovered the most powerful pair of jets ever seen from a stellar black hole. This object, also known as a microquasar, blows a huge bubble of hot gas, 1000 light-years across, twice as large and tens of times more powerful than other known microquasars.

Combining observations done with ESO's Very Large 
Telescope and NASA's Chandra X-ray telescope, 
astronomers have uncovered the most powerful pair 
of jets ever seen from a stellar black hole. The black 
hole blows a huge bubble of hot gas, 1,000 light-years 
across or twice as large and tens of times more powerful 
than the other such microquasars. The stellar black 
hole belongs to a binary system as pictured in this 
artist's impression. (Credit: ESO/L. Calçada)

The discovery is reported this week in the journal Nature.

"We have been astonished by how much energy is injected into the gas by the black hole," says lead author Manfred Pakull. "This black hole is just a few solar masses, but is a real miniature version of the most powerful quasars and radio galaxies, which contain black holes with masses of a few million times that of the Sun."

Black holes are known to release a prodigious amount of energy when they swallow matter. It was thought that most of the energy came out in the form of radiation, predominantly X-rays. However, the new findings show that some black holes can release at least as much energy, and perhaps much more, in the form of collimated jets of fast moving particles. The fast jets slam into the surrounding interstellar gas, heating it and triggering an expansion. The inflating bubble contains a mixture of hot gas and ultra-fast particles at different temperatures. Observations in several energy bands (optical, radio, X-rays) help astronomers calculate the total rate at which the black hole is heating its surroundings.

The astronomers could observe the spots where the jets smash into the interstellar gas located around the black hole, and reveal that the bubble of hot gas is inflating at a speed of almost one million kilometres per hour.

"The length of the jets in NGC 7793 is amazing, compared to the size of the black hole from which they are launched," says co-author Robert Soria [1]. "If the black hole were shrunk to the size of a soccer ball, each jet would extend from the Earth to beyond the orbit of Pluto."

This research will help astronomers understand the similarity between small black holes formed from exploded stars and the supermassive black holes at the centres of galaxies. Very powerful jets have been seen from supermassive black holes, but are thought to be less frequent in the smaller microquasar variety. The new discovery suggests that many of them may simply have gone unnoticed so far.

The gas-blowing black hole is located 12 million light-years away, in the outskirts of the spiral galaxy NGC 7793 (eso0914b). From the size and expansion velocity of the bubble the astronomers have found that the jet activity must have been ongoing for at least 200 000 years.

Note

[1] Astronomers do not have yet any means of measuring the size of the black hole itself. The smallest stellar black hole discovered so far has a radius of about 15 km. An average stellar black hole of about 10 solar masses has a radius of about 30 km, while a "big" stellar black hole may have a radius of up to 300 km. This is still much smaller than the jets, which extend out to several hundreds light years on each side of the black hole, or about several thousand million million km!

More information

This result appears in a paper published in this week's issue of the journal Nature (A 300 parsec long jet-inflated bubble around a powerful microquasar in the galaxy NGC 7793, by Manfred W. Pakull, Roberto Soria and Christian Motch).

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Bizarre Galaxy Is Result Of Pair Of Spiral Galaxies Smashing Together

Bizarre Galaxy Is Result Of Pair Of Spiral Galaxies Smashing Together

  9:42:00 PM    Science Lover

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A recent NASA/ESA Hubble Space Telescope image captures what appears to be one very bright and bizarre galaxy, but is actually the result of a pair of spiral galaxies that resemble our own Milky Way smashing together at breakneck speeds. The product of this dramatic collision, called NGC 2623, or Arp 243, is about 250 million light-years away in the constellation of Cancer (the Crab).

Not surprisingly, interacting galaxies have a dramatic effect on each other. Studies have revealed that as galaxies approach one another massive amounts of gas are pulled from each galaxy towards the centre of the other, until ultimately, the two merge into one massive galaxy. NGC 2623 is in the late stages of the merging process, with the centres of the original galaxy pair now merged into one nucleus, but stretching out from the centre are two tidal tails of young stars, a strong indicator that a merger has taken place. During such a collision, the dramatic exchange of mass and gases initiates star formation, seen here in both the tails. (Credit: NASA, ESA and A. Evans (Stony Brook University, New York & National Radio Astronomy Observatory, Charlottesville, USA))

  
Not surprisingly, interacting galaxies have a dramatic effect on each other. Studies have revealed that as galaxies approach one another massive amounts of gas are pulled from each galaxy towards the centre of the other, until ultimately, the two merge into one massive galaxy. The object in the image, NGC 2623, is in the late stages of the merging process with the centres of the original galaxy pair now merged into one nucleus. However, stretching out from the centre are two tidal tails of young stars showing that a merger has taken place. During such a collision, the dramatic exchange of mass and gases initiates star formation, seen here in both the tails.

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Young Pulsar Shows Its Hand

Young Pulsar Shows Its Hand

  12:31:00 AM    Science Lover

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This graphic demonstrates the difference in physical size between the nebula around the pulsar B1509-58 and its more famous cousin, the Crab Nebula, seen on the left. The Crab was generated when a star collapsed, as seen in 1054 A.D. Since then, the nebula its pulsar created has increased in size to some 10 light years. In contrast, astronomers think the B1509-58 system is about 1,700 years old, yet its nebula now covers some 150 light years. The discrepancy in these sizes may be due to the different environment each pulsar was born into. (Credit: NASA/CXC/SAO/P. Slane et al.)

A small, dense object only twelve miles in diameter is responsible for a beautiful X-ray nebula that spans 150 light years.

At the center of a new image made by NASA's Chandra X-ray Observatory is a very young and powerful pulsar, known as PSR B1509-58, or B1509 for short. The pulsar is a rapidly spinning neutron star which is spewing energy out into the space around it to create complex and intriguing structures, including one that resembles a large cosmic hand.

In the new image, the lowest energy X-rays that Chandra detects are red, the medium range is green, and the most energetic ones are colored blue. Astronomers think that B1509 is about 1,700 years old and is located about 17,000 light years away.

Neutron stars are created when massive stars run out of fuel and collapse. B1509 is spinning completely around almost 7 times every second and is releasing energy into its environment at a prodigious rate -- presumably because it has an intense magnetic field at its surface, estimated to be 15 trillion times stronger than the Earth's magnetic field.

The combination of rapid rotation and ultra-strong magnetic field makes B1509 one of the most powerful electromagnetic generators in the Galaxy. This generator drives an energetic wind of electrons and ions away from the neutron star. As the electrons move through the magnetized nebula, they radiate away their energy and create the elaborate nebula seen by Chandra.

In the innermost regions, a faint circle surrounds the pulsar, and marks the spot where the wind is rapidly decelerated by the slowly expanding nebula. In this way, B1509 shares some striking similarities to the famous Crab Nebula. However B1509's nebula is 15 times wider than the Crab's diameter of 10 light years.

Finger-like structures extend to the north, apparently energizing knots of material in a neighboring gas cloud known as RCW 89. The transfer of energy from the wind to these knots makes them glow brightly in X-rays (orange and red features to the upper right of the image). The temperature in this region appears to vary in a circular pattern around this ring of emission, suggesting that the pulsar may be precessing like a spinning top and sweeping an energizing beam around the gas in RCW 89.

NASA's Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra's science and flight operations from Cambridge, Mass.

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