Hubble captures new star birth in an ancient galaxy

Elliptical galaxies were once thought to be aging star cities whose star-making heyday was billions of years ago.

But new observations with NASA's Hubble Space Telescope are helping to show that elliptical galaxies still have some youthful vigor left, thanks to encounters with smaller galaxies.

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 new study helps bolster the emerging view that most elliptical galaxies have young stars, bringing new life to old galaxies.

"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."

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."

The astronomers hope to study other elliptical galaxies in the SAURON survey to look for the signposts of new star birth. The team's results have been accepted for publication in The Astrophysical Journal.

Discovery could reveal secrets of ancient Martian and terrestrial atmospheres

Chemists at UC San Diego have uncovered a new chemical reaction on tiny particulates in the atmosphere that could allow scientists to gain a glimpse from ancient rocks of what the atmospheres of the Earth and Mars were like hundreds of millions years ago.

Their discovery also provides a simple chemical explanation for the unusual carbonate inclusions found in a meteorite from Mars that was once thought by some scientists to be evidence of ancient Martian life.

"We never knew before how the atmosphere could be trapped in carbonate," said Mark Thiemens, dean of UC San Diego's Division of Physical Sciences who headed the team of scientists that detailed its discovery in the early online edition of the Proceedings of the National Academy of Sciences. "This chemical reaction, which takes place on the surface of aerosols in the atmosphere, not only provides us with an understanding of how these carbonates can form on the Earth and Mars. It gives us a new tool to better understand climate change, as our planet warms and becomes more dusty."

Robina Shaheen, a postdoctoral researcher in Thiemens' laboratory, discovered the chemical reaction and detailed its importance in the Earth's atmosphere after four years of painstaking experiments in which she found a higher than expected proportion of oxygen 17 isotopes in the carbonates found on dust grains, aerosols and dirt from various parts of the world.

Martian meteorites, such as ALH84001, which was once thought to exhibit evidence of extraterrestrial life, have carbonates with similarly high oxygen 17 anomalies. Scientists have long attributed those anomalies to photochemical processes involving ozone and carbon dioxide in the thin atmosphere on Mars, which is bathed by intense ultraviolet radiation. But after finding similar anomalies on terrestrial carbonates formed in atmospheric aerosols, Shaheen surmised they might be the result of another chemical process more common to both planets.

She analyzed in painstaking detail in the laboratory and in the Earth's atmosphere how ozone molecules interacted with oxygen-bearing mineral aerosols from dust, sea spray and other sources to form hydrogen peroxide and carbonates containing this same oxygen-isotope anomaly.

"What she found is that the tiny little layer on the outside of the grain is where this chemistry all happens," said Thiemens. "It's the ozone in the atmosphere mixing with water and carbon dioxide that drives a completely different kind of chemistry, one that's not in any of the models."

While current models of atmospheric processes assume that the mixing of large volumes of gases drives the chemistry of the Earth's atmosphere, the UCSD chemists think their discovery may force a rethinking of this idea, particularly as the Earth's atmosphere becomes warmer and more dusty, providing more opportunities for this sort of chemistry to take place on aerosols.

"You can do chemistry on a grain that's a lot quicker and easier in many respects than is possible in other atmospheric processes," said Thiemens.

Shaheen, who analyzed the carbonates in the Martian meteorite ALH84001 and found that they could have been formed on aerosols in ancient Martian atmosphere, said that NASA's Phoenix lander recently detected carbonates associated with particulates in the dusty atmosphere of Mars. "We think it might be this same mechanism that is operating," she added.

Besides understanding current and future atmospheric processes on the Earth and Mars, the new discovery offers the possibility of mining information about the Earth's atmosphere, particularly its oxygen levels, from carbonates found in ancient rocks millions of years ago, far beyond the time period from which scientists can now obtain information about the ancient atmosphere from ice cores. The development of this new tool to probe ancient atmospheres could be the most significant aspect of the UCSD chemists' discovery.

"We've found a new way to measure the earth's atmosphere for time periods when we previously could not do it," said Thiemens. "What happened to ozone and oxygen levels 65 million years ago during the Cretaceous-Tertiary period when the dinosaurs and many other forms of life were killed in a mass extinction? Who died first? Did the food chain disappear before the dinosaurs? What happened 251 million years ago during the Permian-Triassic period, the most severe extinction of life on Earth, when 85 percent of life disappeared and no one knows why? There's no record of what happened in the atmosphere. But if you can find a record of what happened to oxygen levels, you can answer questions like that."

Other researchers at UCSD involved in the study in Thiemens' laboratory were undergraduates Anna Abramian and John Horn. The research was partially supported by grants from National Aeronautics and Space Administration, the National Science Foundation and the UC San Diego Chancellor's Associates.