Free Astronomy Magazine July-August 2015

SPACE CHRONICLES encountered in our everyday lives. Dark mat- ter is central to our understand- ing of how gal- axies form and evolve – and is ultimately one of the reasons for the exis- tence of life on Earth – yet we know almost nothing about it. “The surpris- ing finding of our study was that elliptical galaxies main- tain a remark- ably constant circular speed out to large distances from their centers, in the same way that spiral gal- axies are al- ready known to do,” said Cap- pellari. “This means that in these very dif- ferent types of galaxies, 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.” However, the conspiracy does not come out naturally from models of dark matter, and some disturbing fine-tuning is required to explain the observations. For this reason, the conspiracy even led some au- thors to suggest that, rather than being due to dark matter, it may be due to Newton's law of gravity becoming progressively less accu- rate at large distances. Remark- ably, decades after it was pro- posed, this alternative theory (with- out dark matter) still cannot be con- clusively ruled out. Spiral galaxies only constitute less than half of the stellar mass in the Universe, which is dominated by elliptical and len- ticular galaxies, and which have puffier configurations of stars and lack the flat disks of gas that spirals have. In these galaxies, it has been very difficult technically to measure their masses and to find out how much dark matter they have, and how this is distributed – until now. Because the elliptical galaxies have different shapes and formation his- tories than spiral galaxies, the new- ly discovered conspiracy is even more profound and will lead ex- perts in dark matter and galaxy for- mation to think carefully about what has happened in the "dark sector" of the universe. “This question is particularly timely in this period when physicists at CERN are about to restart the Large Hadron Collider to try to directly de- tect the same elusive dark matter particle, which makes galaxies ro- tate fast, if it really exists!,” said Professor Jean Brodie, principal in- vestigator of the SLUGGS survey. E xample of mapping out and analyzing the speeds of stars in an elliptical galaxy. Blue colors show regions where the stars are hurtling toward the observer on Earth, and red colors show regions that are moving away, in an overall pattern of coherent rotation. The top panel shows the original data, as collected using the DEIMOS spectrograph at the W.M. Keck Observatory. The bot- tom panel shows a numerical model that matches the data remarkably well, from using the com- bined gravitational influence of luminous and dark matter. [M. Cappellari and the SLUGGS Team] n