Ancient Elliptical Galaxies Reveal Secrets of the Early Universe:
"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."
Could the Universe be much older than we think? Early on its life it appears that our 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," Cappellari 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.
"Elliptical galaxies were thought to have made all of their stars billions of years ago," says astronomer Mark Crockett of the University of Oxford, leader of the Hubble observations. "They had consumed all their gas to make new stars. Now we are finding evidence of star birth in many elliptical galaxies, fueled mostly by cannibalizing smaller galaxies.
"These observations support the theory that galaxies built themselves up over billions of years by collisions with dwarf galaxies," Crockett continues. "NGC 4150 is a dramatic example in our galactic back yard of a common occurrence in the early Universe."
Images of the core of NGC 4150, taken in near-ultraviolet light with the sharp-eyed Wide Field Camera 3 (WFC3), reveal streamers of dust and gas and clumps of young, blue stars that are significantly less than a billion years old. Evidence shows that the star birth was sparked by a merger with a dwarf galaxy.
The Hubble images reveal turbulent activity deep inside the galaxy's core. Clusters of young, blue stars trace a ring around the center that is rotating with the galaxy. The stellar breeding ground is about 1,300 light-years across. Long strands of dust are silhouetted against the yellowish core, which is composed of populations of older stars.
From a Hubble analysis of the stars' colors, Crockett and his team calculated that the star-formation boom started about a billion years ago, a comparatively recent event in cosmological history. The galaxy's star-making factory has slowed down since then.
"We are seeing this galaxy after the major starburst has occurred," explains team member Joseph Silk of the University of Oxford. "The most massive stars are already gone. The youngest stars are between 50 million and 300 to 400 million years old. By comparison, most of the stars in the galaxy are around 10 billion years old."
The encounter that triggered the star birth would have been similar to our Milky Way swallowing the nearby Large Magellanic Cloud.
"We believe that a merger with a small, gas-rich galaxy around one billion years ago supplied NGC 4150 with the fuel necessary to form new stars," says team member Sugata Kaviraj of the Imperial College London and the University of Oxford. "The abundance of 'metals'—elements heavier than hydrogen and helium—in the young stars is very low, suggesting the galaxy that merged with NGC 4150 was also metal-poor. This points towards a small, dwarf galaxy, around one-twentieth the mass of NGC 4150."
Minor mergers such as this one are more ubiquitous than interactions between hefty galaxies, the astronomers say. For every major encounter, there are probably up to 10 times more frequent clashes between a large and a small galaxy.
Major collisions are easier to see because they create incredible fireworks: distorted galaxies, long streamers of gas, and dozens of young star clusters. Smaller interactions are harder to detect because they leave relatively little trace.
Over the past five years, however, ground- and space-based telescopes have offered hints of fresh star formation in elliptical galaxies. Ground-based observatories captured the blue glow of stars in elliptical galaxies, and satellites such as the Galaxy Evolution Explorer (GALEX), which looks in far- and near-ultraviolet light, confirmed that the blue glow came from fledgling stars much less than a billion years old. Ultraviolet light traces the glow of hot, young stars.
Crockett and his team selected NGC 4150 for their Hubble study because a ground-based spectroscopic analysis gave tantalizing hints that the galaxy's core was not a quiet place. The ground-based survey, called the Spectrographic Areal Unit for Research on Optical Nebulae (SAURON), revealed the presence of young stars and dynamic activity that was out of sync with the galaxy.
"In visible light, elliptical galaxies such as NGC 4150 look like normal elliptical galaxies," Silk says. "But the picture changes when we look in ultraviolet light. At least a third of all elliptical galaxies glow with the blue light of young stars."
Adds Crockett: "Ellipticals are the perfect laboratory for studying minor mergers in ultraviolet light because they are dominated by old red stars, allowing astronomers to see the faint blue glow of young stars."
Provided by The Daily Galaxy - Casey Kazan / journalofcosmology.com and ox.ac.uk
