Old galaxies get their second wind

NGC 4150

Images taken with the Hubble Space Telescope reveal fresh star birth in galaxy NGC 4150, 44 million light-years away. The inset shows the core of the galaxy in ultraviolet light, revealing the glow (blue) of young stars.

  • Elliptical galaxies were thought to comprise only very old stars
  • Evidence is now mounting that new stars being born within them
  • Rebirth is due to galactic cannibalism—big galaxies eating little ones

Evidence is mounting that some galaxies that were thought to be in their twilight years, are actually going through a rebirth process by forming lots of new stars.

Galaxies come in many different shapes and sizes, one of the most common being the elliptical type. Elliptical galaxies are generally very large and, unlike our Milky Way galaxy, they don’t have spiral arms.

Ellipticals 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 vigour left, thanks to close encounters with smaller galaxies.

Images of the core of the galaxy NGC 4150, taken in near-ultraviolet light with Hubble’s 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.

And evidence shows that the spurt of star birth was sparked when NGC 4150 merged 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.”

Hubble Space Telescope in orbit

Hubble's Wide Field Camera 3 can pick out the ultraviolet light of young stars in distant galaxies.

“Now we are finding evidence of star birth in many elliptical galaxies, fuelled mostly by cannibalising 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.”

Adding fuel to the cosmic fire

The Hubble images reveal turbulent activity deep inside the galaxy’s core. Clusters of young, blue stars form a ring around the centre 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 an analysis of the stars’ colours, 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 [rest of the] stars in the galaxy are around 10 billion years old.”

“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 elements heavier than hydrogen and helium in the young stars is very low, suggesting the galaxy that merged with NGC 4150 was also poor in heavy elements.

This points towards a small, dwarf galaxy, around one-twentieth the mass of NGC 4150.

Magnified view of NGC 4150's core

A magnified view of NGC 4150's core. The blue areas indicate a flurry of 'recent' (less than a billion years) star birth. Most of the rest of the stars in the galaxy are about 10 billion years old.

Hubble’s vision reveals new stars

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 picks out the glow of hot, young stars.

Crockett and his team selected NGC 4150 for their Hubble study because a ground-based telescope analysis gave tantalising hints that the galaxy’s core was not a quiet place. The 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.

Adapted from information issued by STScI / NASA / ESA / R.M. Crockett (University of Oxford, U.K.), S. Kaviraj (Imperial College London and University of Oxford, U.K.), J. Silk (University of Oxford), M. Mutchler (Space Telescope Science Institute, Baltimore), R. O’Connell (University of Virginia, Charlottesville), and the WFC3 Scientific Oversight Committee.

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