19 October 2006
A team of international scientists headed by a South African astrophysicist have discovered that Andromeda, our nearest galactic neighbour, was involved in a violent head-on collision with another galaxy some 210-million years ago, in possibly the most important finding since Andromeda was first recorded in the year 964.
Professor David Block, director of Wits University’s Cosmic Dust Laboratory, announced on Wednesday that a study of images from Nasa’s Spitzer Space Telescope had revealed that the Andromeda galaxy was involved in a hit-and-run with the neighboring dwarf galaxy Messier 32 (M32) at around the time dinosaurs were roaming the earth.
Andromeda is the closest spiral galaxy to the Milky Way, separated by a distance of 2.5-million light years. Its disk spans about 260 000 light years, which means that a light beam would take 260 000 years to travel from one end of the galaxy to the other. Our home galaxy, the Milky Way, is about 100 000 light years across. When viewed from Earth, Andromeda occupies a portion of the sky equivalent to seven full moons.
The cloud of stars was first recorded by Persian stargazer Abd Al-Rahman Al Sufi in 964, and catalogued by astronomer Charles Messier – after whom Messier 32 is named – as a “diffuse object” in 1764.
The Spitzer space telescope image of Andromeda combined 3 000 individual images to obtain this final image showing the galaxy’s two rings. The inset is Andromeda’s inner ring (Image: Nasa/JPL/P Barmby (CfA). Sourced from the Harvard-Smithsonian Centre for Astrophysics)
Proof of Block’s discovery, reported in the 19 October issue of the journal Nature, was a never-before-seen dust ring revealed in the Spitzer images, deep within the Andromeda galaxy. When combined with a previously observed outer ring, the presence of both dust rings suggests a long-ago disturbance whose effects are still expanding outward through Andromeda.
“These dust rings are like ripples in a pond,” said Block. “Plop a stone into water and you get an expanding series of rings or waves. Let a small galaxy collide nearly head-on with a larger one, and you will see waves or rings of gas and dust that propagate outward as a result of the violent gravitational interaction.
“While our Atlantic Ocean was still forming, Messier 32 plowed head-long into Andromeda’s disk of gas and stars. Only roaming dinosaurs saw the crash and held the secret, until the Spitzer Space Telescope spilled the beans.”
The researchers conducted a series of computer simulations to model the collision between Andromeda and M32. They found that M32 plunged through the disk of Andromeda along the larger galaxy’s polar axis about 210 million years ago. As M32 is much less massive than Andromeda, the latter was not substantially disrupted, but the smaller galaxy lost more than half its initial mass in the course of the collision.
South African PhD student Robert Groess used sophisticated image-processing techniques to strip out the effects of foreground stars in our galaxy and extract the crucial information from Andromeda.
The galaxy Messier 2, highlighted in yellow near bottom right, smashed into Andromeda 210-million years ago, leaving a hole in the outer ring (right). The smaller galaxy lost much of its matter to Andromeda (Image: Nasa/JPL/P Barmby (CfA). Sourced from the Harvard-Smithsonian Centre for Astrophysics)
“Like a CSI team, we gathered clues and reconstructed the scene of the crime,” said Pauline Barmby of the Harvard-Smithsonian Centre for Astrophysics, a member of the research team. “The evidence clearly shows that M32 is guilty of committing a hit-and-run.
“You could compare M32 to a compact car while Andromeda would be an 18-wheeler,” she explained. “In a collision between the two, the truck would be almost unharmed while the car would be wrecked. Similarly, M32 was much more damaged than Andromeda.”
Astronomers have predicted that Andromeda and the Milky Way will collide in five- to 10-billion years. That collision will erase the separate identities of each galaxy, leaving a single elliptical galaxy in their place.
Galaxy expert Professor Ken Freeman, a fellow of the Royal Society, told Business Day that the new discovery provides the greatest leap in our understanding of Andromeda in the 300 years astronomers have been training their telescopes on the night sky.
“The finding . ranks as one of the most important discoveries yet made regarding that galaxy’s history,” he said.
According to Block, a dramatic cosmic crash of this kind had previously thought to only occur in far-distant galaxies, usually more than 10-billion light years away. He told Business Day that finding one so close to home, in a galaxy once thought fairly ordinary, fundamentally challenges scientists’ views of Andromeda.
“Up to now, astronomers believed the structure of Andromeda was billions of years old. We believe now that what we see in Andromeda today is primarily the result of a head-on collision only 210-million years ago.
“It proves galaxies are continually refashioning themselves, and the idea of galaxies being isolated island universes is out.”