domingo, 20 de mayo de 2012

The Ancient Massive Galaxy Mystery -- 80 percent Appear Extremely Active and Growing


NASA's Chandra X-ray Observatory image above of the distant galaxy 3C295 shows an explosive galaxy enveloped by a vast cloud of fifty million degree gas. The gas cloud, which is visible only with an X-ray telescope, contains more than a hundred galaxies and enough material to make a thousand more. The galaxies are too cool to be visible in X-rays. Roughly two million light years in diameter, the cloud and its cluster of galaxies are among the most massive objects in the universe. It is so distant that we see it as it was five billion years ago.
Important research at Tufts University in 2010 revealed that some of the most massive galaxies in the Universe may have formed billions of years earlier than the current models predicted. The identified galaxies were five to ten times more massive than our own Milky Way. 
"We found a relatively large number of very massive, highly luminous galaxies that existed almost 12 billion years ago when the universe was still very young, about 1.5 billion years old. These results appear to disagree with the latest predictions from models of galaxy formation and evolution," said Tufts astrophysicist Danilo Marchesini, lead author on the paper and assistant professor of physics and astronomy at the Tufts School of Arts and Sciences.
"Current understanding of the physical processes responsible in forming such massive galaxies has difficulty reproducing these observations."
Collaborating with Marchesini were researchers from Yale University, Carnegie Observatories, Leiden University, Princeton University, the University of Kansas and the University of California-Santa Cruz.

Redshift refers to the phenomenon of a light wave stretching and moving toward longer wavelengths (the red end of the spectrum) as the emitting object travels away from an observer (Doppler Effect). This is similar to the pitch of a siren getting lower as the siren moves away.

The redshift of distant galaxies is due to the expansion of the universe. The larger the redshift, the more distant the galaxy is, or the farther back in time we are observing. The larger the redshift, the younger the universe in which the galaxy is observed.
By complementing existing data with deep images obtained through a new system of five customized near-infrared filters, the researchers were able to get a more complete view of the galaxy population at this early stage and more accurately characterize the sampled galaxies.
The researchers made another surprising discovery: More than 80 percent of these massive galaxies show very high infrared luminosities, which indicate that these galaxies are extremely active and most likely in a phase of intense growth. Massive galaxies in the local universe are instead quiescent and do not form stars at all.  
The researchers noted that there are two likely causes of such luminosity: New stars may be forming in dust-enshrouded bursts at rates of a few thousand solar masses per year. This would be tens to several hundreds of times greater than the rates estimated by spectral energy distribution (SED) modeling. Alternatively, the high infrared luminosity could be due to highly-obscured active galactic nuclei (AGN) ferociously accreting matter onto rapidly growing super-massive black holes at the galaxies' centers.
There might be an explanation that would at least partially reconcile observations with model-predicted densities. The redshifts of these massive galaxies, and hence their distances, were determined from the SED modeling and have not yet been confirmed spectroscopically. Redshift measurements from SED modeling are inherently less accurate than spectroscopy. Such "systemic uncertainties" in the determination of the distances of these galaxies might still allow for approximate agreement between observations and model predictions. But this is pure speculation, and highly doubtful.
If half of the massive galaxies are assumed to be slightly closer, at redshift z=2.6, when the universe was a bit older (2.5 billion years old) and very dusty (with dust absorbing much of the light emitted at ultra-violet and optical wavelengths), then the disagreement between observations and model predictions becomes only marginally significant.
However, the discovery of the existence of such massive, old and very dusty galaxies at redshift z=2.6 would itself be a notable discovery.
"Either way, it is clear that our understanding of how massive galaxies form is still far from satisfactory," said Marchesini. "The existence of these galaxies so early in the history of the universe, as well as their properties, can provide very important clues on how galaxies formed and evolved shortly after the Big Bang."
In prior findings, it appears that the early universe was a place of puzzling extremes and seeming contradictions. That’s the conclusion scientists are drawing from new infrared observations of a very distant, unusually bright and massive elliptical galaxy. This galaxy was spotted 10 billion light years away, and gives us a glimpse of what the Universe looked like when it was only about one-quarter of its current age.
Measurements show that the galaxy is as large and equally dense as elliptical galaxies that can be found much closer to us. Coupled with recent observations by a different research team - which found a very compact and extremely dense elliptical galaxy in the early Universe - the findings deepen the puzzle over how ‘fully grown’ galaxies can exist alongside seemingly ‘immature’ compact galaxies in the young universe.
‘What our observations show is that alongside these compact galaxies were other ellipticals that were anything up to 100 times less dense and between two and five times larger - essentially ‘fully grown’ --- and much more like the ellipticals we see in the local universe around us,’ explains Michele Cappellari of Oxford University’s Department of Physics. ‘The mystery is how these two different extremes, ‘grown up’ and seemingly ‘immature’ ellipticals, co-existed so early on in the evolution of the Universe.’
Elliptical galaxies, which are regular in shape, can be over ten times as massive as spiral galaxies such as our own Milky Way and contain stars which formed over 10 billion years ago. One way of checking the density of such galaxies is to use the infrared spectrum they emit to measure the spread of the velocities of their stars, which has to balance the pull of gravity.
Measurements of a distant compact elliptical galaxy have shown that its stars were dispersing at a velocity of about 500 km per second, consistent with its size but unknown in local galaxies.
The new study, using the 8.3-m Japanese Subaru telescope in Hawaii, found a ‘fully grown’ elliptical with stars dispersing at a velocity of lower than 300 km per second, much more like similar galaxies close to us.
‘Our next step is to use the Subaru telescope to find the relative proportion of these two extremes, fully grown and compact ellipticals, and see how they fit in with the timeline of the evolution of the young Universe,’ Michele tells us. ‘Hopefully this will give us new insights into solving this cosmic puzzle.’
In earlier surveys, the Advanced camera for Survey (ACS) and the Infrared Camera for Multi-object Spectrometer (NICMOS), the Hubble Ultra Deep Field (HUDF) have revealed the presence of estimated 10,000 fully formed galaxies in a patch of sky in the constellation, Formax - a region just below the constellation, Orion. According to the NASA, these fully formed galaxies emerged just 700 million years after the Big Bang, when the universe was barely 5% of its current age.
Also, using ISAAC near- infrared instrument aboard ESO's Very Large Telescope(VLT), and the phenomenon of gravitational lensing, a team of French and Swiss astronomers using Very Large Telescope (VLT) of the European Southern Observatory, have identified an extremely faint galaxy, Abell 1835 (image left).
According to interpretations, Abell 1835 must have formed just 460 million years after the universe was born, during the "Dark Age" when the first stars and galaxies were supposedly being born More recently, fully formed galaxies were discovered which are at a greater distance, over 13.1 billion light years (American Astronomical Society 2010), and which may have already been billions of years in age, over 13 billion years ago .
There are fully formed distant galaxies that must have already been billions of years old over 13 billion years ago; which would make them older than the Big Bang. Then there is the problem of the oldest globular clusters so far discovered, whose ages are in excess of 16 billion years. The Milky Way and other galaxies are also so old that they must have formed before the so called "Dark Ages" and thus almost immediately after the Big Bang, which is not consistent with theory.
Using the Infrared Array Camera (IRAC) aboard NASA's Spitzer Space Telescope, astronomers have detected about a dozen very red galaxies at a distance of 10 to12 billion light years from Earth (cfa Harvard 2005). According to the Big Bang model, these galaxies existed when the universe was only about 1/5 of its present age of 13.75 billion years.
The unpredicted existence of "red and dead" galaxies so early in the universe challenges Big Bang theories relating to galaxy formation (cfa Harvard 2005). Analysis show that galaxies exhibit a large range of properties. Young galaxies with and without lots of dust, and old galaxies with and without dust.There is as much variety in the so called "early universe" as we see around "today" in galaxies closer to Earth.
Moreover, Spitzer Space Telescope, which is sensitive to the light from older and redder stars, has also revealed evidence for mature stars in less massive galaxies at similar distances (Spitzer 2005), when the Universe was supposedly less than one billion years old.
Image credit: Smithsonian Institute
Source: The Daily Galaxy - Tufts University and

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