One of the most surprising scientific discoveries of the 20th century was that spiral galaxies, such as our own Milky Way, rotate much faster than expected, powered by an extra gravitational force of invisible ‘dark matter’. Since this discovery 40 years ago, we have learned this mysterious substance, which is probably an exotic elementary particle, makes up about 85 per cent of the mass in the known Universe, leaving only 15 per cent to be the ordinary stuff encountered in our everyday lives.
Surprising gravitational similarities between spiral and elliptical galaxies were discovered this past May by an international team, implying the influence of hidden forces. In the first such survey to capture large numbers of these galaxies, researchers have mapped out the motions of stars in the outer parts of elliptical galaxies using the world’s largest optical telescope at W M Keck Observatory in Hawaii.
“By combining Keck telescope time from Swinburne and the University of California, we were able to investigate a larger number of galaxies which allowed us to make this important discovery,” Swinburne’s Professor Duncan Forbes said.
The team, led by Michele Cappellari from the University of Oxford, used the powerful DEIMOS (DEep Imaging and Multi-Object Spectrograph) to conduct a major survey of nearby galaxies called SLUGGS, which mapped out the speeds of their stars. The scientists used Newton's law of gravity to translate these speed measurements into the amounts of matter distributed within the galaxies.
“One of the surprising findings of this study was that spiral galaxies maintain a remarkably constant rotation speed throughout their disks,” Dr Cappellari said. “This means stars and dark matter conspire to redistribute themselves to produce this effect, with stars dominating in the inner regions of the galaxies, and a gradual shift in the outer regions to dark matter dominance.”
The image below illustrates speeds of stars on circular orbits have been measured around both spiral and elliptical galaxies. Without dark matter, the speeds should decrease with distance from the galaxy, at different rates for the two galaxy types. Instead, the dark matter appears to conspire to keep the speeds steady. (M. Cappellari and the Sloan Digital Sky Survey.
But the ‘conspiracy’ does not come out naturally from the models, and some fine-tuning is required to explain the observations. For this reason, some astronomers have suggested that, rather than being due to dark matter, it may be due to Newton's law of gravity becoming progressively less accurate at large distances.
Remarkably, decades after it was proposed, this alternative theory (without dark matter) still could not be conclusively ruled out.
Spiral galaxies constitute less than half of the stellar mass in the Universe, which is dominated by elliptical and lenticular galaxies. These have puffier configurations of stars and lack the flat disks of gas that spiral galaxies have. Until now it has been technically difficult to measure the masses of elliptical galaxies and to find out how much dark matter they have, and how this is distributed.
Because elliptical galaxies have different shapes and formation histories than spiral galaxies, the newly discovered conspiracy is even more profound and will lead experts in dark matter and galaxy formation to think carefully about what has happened in the ‘dark sector’ of the Universe.
The galaxy, NGC 7049 shown at the top of the page, is a giant galaxy on the border between spiral and elliptical galaxies that spans about 150,000 light-years. It is located about 100 million light-years away from Earth in the southern constellation of Indus. NGC 7049 is the “brightest” galaxy of the Indus Triplet of galaxies (NGC 7029, NGC 7041, NGC 7049), and its structure might have arisen from several recent galaxy collisions.
Bright Cluster Galaxies are among the most massive galaxies in the universe and are also the oldest. They provide astronomers the opportunity of studying the many globular clusters contained within them. NGC 7049 has far fewer such clusters than other similar giant galaxies in very big, rich groups. This indicates to astronomers how the surrounding environment influenced the formation of galaxy halos in the early Universe.
The globular clusters in NGC 7049 are seen as the sprinkling of small faint points of light in the galaxy’s halo. The halo – the ghostly region of diffuse light surrounding the galaxy – is composed of myriads of individual stars and provides a luminous background to the remarkable swirling ring of dust lanes surrounding NGC 7049′s core.
NGC 7049′s striking appearance is primarily due to this unusually prominent dust ring, seen mostly in silhouette. The opaque ring is much darker than the millions of bright stars glowing behind it. Generally these dust lanes are seen in much younger galaxies with active star forming regions. Not visible in this image is an unusual central polar ring of gas circling out of the plane near the galaxy’s center.
The image was taken by the Advanced Camera for Surveys on the Hubble Space Telescope, which is optimized to hunt for galaxies and galaxy clusters in the remote and ancient Universe, at a time when our cosmos was very young.
The study was published in The Astrophysical Journal Letters.
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