A new analysis shows a surprisingly simple relationship between the way galaxies move, and the distribution of ordinary matter within them. Unexpectedly this seems to hold however much mysterious dark matter they contain. That’s funny.
John Butterworth in The Guardian:
On 25 August 2003, a Delta II rocket launched the Spitzer Space Telescope into a orbit from Cape Canaveral, Florida. It went into orbit trailing the Earth around the Sun, and began making precise observations of hundreds of galaxies. More than 13 years later, on 19 September 2016, an intriguing analysis of some of these observations was posted by three astrophysicists, Stacy McGaugh and Federico Lelli from Case Western reserve University, and Jim Schombery from the University of Oregon. The analysis seems to be telling us something surprising.
Galaxies are made up of three components. Stars, which we can see. Gas, which we can also see, although much of what we ‘see’ is infrared light with a wavelength too long for our eyes but which we can nevertheless measure. And most elusive of all, ‘Dark Matter’, which we can’t see at all. We deduce its presence from its gravitational influences – on the way galaxies move and the way light bends as it passes by them. We don’t know what Dark Matter is made of, a situation which especially annoys and intrigues particle physicists like me, who want to know what everything is made of.
Key to the analysis is the measurement of rotation curves of galaxies. This is the way the average speed of the stars orbiting in galaxies changes as they get further from the centre. To measure this you need a good spatial resolution (to distinguish the distance from the centre) and a measurement of the wavelength of the light, because the wavelength tells us the speed – from the ‘Doppler Shift’, similar to the way the pitch of a horn is higher for an approaching train and lower as it recedes. McGaugh, Lelli and Schmobery have analysed 2693 measurements in 153 galaxies studied by Spitzer.