NGC 4449B, illustrated here to scale as a ghostly companion to our Milky Way, would stretch from the center of the Milky Way to the Sun's position, if placed in our galaxy. This is an artist's conception; NGC 4449B lies some 13 million light years distant from our galaxy.
Credit: Milky Way image from Robert Hurt, JPL; image produced by Claire Welch, Nature.
ADELAIDE: A unique dwarf galaxy has been caught interacting with a larger, companion galaxy first the first time, 12.4 million light years away from Earth.
Reporting in Nature today, scientists from Germany and the U.S. describe the newly discovered dwarf, named NGC 4449B due to its proximity to an irregular galaxy called NGC 4449, located in the constellation Canes Venatici.
The team says NGC 4449B has several characteristics that suggest the two galaxies have interacted. Such close encounters in space can provide important clues for unravelling the puzzle of how galaxies formed during the evolution of the universe.
Michael Rich from the University of California and Polaris Observatory Association in the U.S. and lead author on the paper said that the captured telescopic images of NGC 4449 and NGC 4449B may represent the first time the two galaxies have interacted. "[the results are] telling us something interesting about the formation conditions of these galaxies," he said.
Approaching galaxy formation
How large galaxies like NGC 4449 and indeed our own Milky Way develop is not fully understood. Astronomers use photographic data from our skies in combination with mathematical modelling to gain an insight into events that happened millions and even billions of years ago.
In this instance, the close proximity of NGC 4449B to NGC 4449 as well as its morphological characteristics add to the growing body of evidence that large galaxies may form by a process of amalgamating smaller galaxies. This theory is referred to as the 'bottom-up' approach to galaxy generation.
Dwarf galaxy NGC 4449B was detected using a Centurion 28-inch telescope, a relatively small piece of equipment and the first example of its type to be 'on the sky'. The telescope has been specifically designed to study low-surface-brightness structures (such as dwarf galaxies) in the vicinity of other galaxies.
Whereas the Milky Way and other large galaxies are composed of hundreds of billions of stars, dwarf galaxies typically consist of only several billion stars. If overlaid for comparative purposes, NGC 4449B would stretch from the centre of the Milky Way to the Sun's position if placed in our galaxy.
Detecting galaxy interaction
Rich and his colleagues used several key features of NGC 4449B to deduct that a galactic interaction had occurred. Firstly, NGC 4449B is in a transient stage of tidal disruption, similar to that of the Sagittarius dwarf near the Milky Way. This means that the structure of the dwarf galaxy has been altered by the impact of the gravitational forces of the larger galaxy.
NGC 4449B also exhibits a striking S-shaped morphology that has been predicted for disrupting galaxies but previously only seen in a dissolving globular cluster. And lastly, the dwarf has a 2-component stellar halo, suggesting an unusual degree of heterogeneity in its stars.
Compared to the 12 billion year-old Milky Way, the interaction between NGC 4449 and NGC 4449B is relatively young, having commenced approximately 200 million years ago. "The two-component structure to the old stellar halo is to my knowledge, unprecedented," said Rich. "I would like to understand the origin of this."
"NGC 4449B is a beautiful example of [..] a tidal galaxy," commented Helmut Jerjen from the Mt Stromlo Observatory in Canberra. "Finding satellite dwarf galaxies around large galaxies, like NGC 4449 or the Milky Way, is very exciting."
