An international group of astronomers has discovered surprising gaseous spiral arms in the giant elliptical galaxy NGC 5128, also known as Centaurus A.
Most giant galaxies usually fit into two categories: spiral galaxies and elliptical galaxies. Spiral galaxies like our Milky Way Galaxy are hip and happening places, with plenty of gas and dust to birth new stars. Ellipticals are full of aging red giant stars. Now, the astronomers have suggested that one well-known elliptical – NGC 5128 – has a split personality.
“No other elliptical galaxy is known to have spiral arms. Centaurus A may be an old galaxy, but it’s still very young at heart,” said Dr Daniel Espada of the National Astronomical Observatory of Japan and Harvard-Smithsonian Center for Astrophysics, lead author of a paper describing the discovery in the Astrophysical Journal Letters (arXiv.org version).
“NGC 5128 isn’t your typical elliptical to begin with. Its most striking feature is a dark dust lane across its middle – a sign that it swallowed a spiral galaxy about 300 million years ago.”
NGC 5128 slurped that galaxy’s gases down, forming a disk that we see nearly edge on. From our point of view, any features in that disk have been hidden by the intervening dust. Its center hosts a supermassive black hole with a mass of about 100 million times that of the Sun.
The team used the sharp vision of the Smithsonian’s Submillimeter Array to tease out the disk’s structure. This radio telescope can see through dust to pick up signals from naturally occurring carbon monoxide gas. By mapping the gas, the astronomers unveiled two distinct spiral arms within the galaxy’s core.
These gaseous tendrils have sizes and shapes similar to spiral arms in galaxies like the Milky Way Galaxy. Also like the Milky Way’s spiral arms, they are forming new generations of stars.
“Centaurus A has been given a new lease on life by that past merger,” Dr Espada said.
Computer simulations suggest that the spiral features might endure for hundreds of millions of years to come.
Although NGC 5128 is the first elliptical galaxy found to have spiral arms, it may not be the last. Since it’s only 12 million light-years away, it’s relatively nearby and easy to study. The Atacama Large Millimeter/Submillimeter Array (ALMA) potentially can find more split-personality galaxies with its improved radio vision.
“We definitely will use ALMA to search for other objects that are similar to Centaurus A,” Dr Espada concluded.
Bibliographic information: D. Espada et al. 2012. Disentangling the Circumnuclear Environs of Centaurus A: Gaseous Spiral Arms in a Giant Elliptical Galaxy. ApJ 756, L10; doi: 10.1088/2041-8205/756/1/L10