The Fossils of Galactic Cannibalism
Galaxies eat each other like I eat post-Halloween sale candy. That is to say, voraciously. Astronomers have put together the evidence for such “galactic cannibalism” for years, and a neat spiral hidden inside an elliptical galaxy is the latest interesting evidence.
Meet Centaurus A, also known as NGC 5128, our nearest massive elliptical galaxy. It is well known for being a nearby active galaxy, meaning that there is material actively falling onto the central supermassive black hole, which causes a pretty display in all different wavelengths of light. Using the Sub-Millimeter Array (SMA) astronomers have discovered that inside that elliptical is a tiny spiral of rotating gas, with properties similar to the arms of a spiral galaxy.
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This is another complication in the canonical “Hubble tuning fork” classification of galaxies. When Edwin Hubble first started classifying galaxies by their shapes, they fell into two general types, elliptical and spirals, with the spiral being further divided into barred and un-barred. Classical thinking about galaxies was that spirals were full of gas, active star formation, and young blue stars, whereas those old elliptical galaxies are “red and dead,” not showing signs of recent star formation.
But the case is, as usual, never so simple. On closer inspection, many ellipticals have groupings of stars or gas and dust that rotate in a disk, hinting at a recent act of galactic cannibalism. That is, some smaller galaxy fell into the large elliptical and was gobbled up, but some of the new material can be distinguished by its motions, different from the rest of the big galaxy.
Centaurus A already has a prominent, visible dust lane and signs of recent activity. Also, because it is so close, a mere 12 million light years away, this is a perfect candidate for studying these hidden disks within ellipticals.
Using the SMA as an interferometer for measuring light at radio wavelengths, the group mapped carbon monoxide gas, or CO, thanks to the spectral line that it emits at submillimeter wavelengths. The resolution of their map is 45 times better than any previous map of the region using the CO molecular line. The two spiral arms are a few hundred light years across and rotate in a way similar to the arms of spiral galaxies.
It is not yet clear whether the spiral is a fleeting feature or if it is long-lasting, or how such a spiral can form and survive within the deep gravitational potential well of such a massive elliptical galaxy. Some infall of gas or an entire galaxy was probably the cause of the perturbation that created the spiral. After all, most spiral patterns that we see in astrophysics are the result of some sort of gravitational perturbation, or a “poke” if you will.
The odd shapes and behaviors of galaxies will continue to fascinate astronomers and defy traditional boundaries for some time to come. With the advent of bigger and better radio telescopes, this field is sure to promise more answers and questions in the future.
Images: Top: Left – Optical image of Centaurus A, showing its prominent dust lane (ESO/IDA/Danish 1.5m/R. Gendler, J.-E. Ovaldsen & S. Guisard, ESO). Right – The new CO map from SMA in green (Espada et al. 2012, infrared dust emission in red (Quillen et al. 2006), and x-rays in blue (NASA/CXC/M. Karovska et al.)