A Sky Too Empty
When this century dawned, astronomers knew our galaxy, the Milky Way, had 11 small satellite galaxies orbiting around it, just the way moons orbit their planets. The largest of these are the Greater and Lesser Magellanic Clouds, within just 200,000 lys of us. The Greater Magellanic Cloud is 14,000 lys across The other satellites are smaller, some as small as 2300 lys across.
But, calculations showed that our galaxy should have as many as 100 satellite galaxies. Where were the other Milky Way satellite galaxies? No matter where or how they looked, astronomers could not find anymore.
A young doctoral candidate, Josh Simon, at the University of California and his fellow researchers observing the dwarf galaxy NGC 4605, a satellite galaxy of M81, a big spirial in the M81/M82 group, found something very peculiar that may shed some light on this dilemma. Some 15 million lys away, a medium sized dwarf satellite, NGC 4605 exhibited a distinct halo around its group of stars. Measurements made of the star’s velocities as they orbited the galactic center and of radiation signatures from the halo seemed to suggest that most of the galaxy’s mass resided not in the central core—as do most galaxies—but in the halo. Yet the halo contained few stars, consisting mainly of hydrogen and dust glowing from radiation.
What was the mass coming from?
Sherlock Holmes said: “When you have eliminated the impossible, whatever is left, no matter how improbable, is the truth."
That’s what Simon and his fellow researchers were left with. The invisible mass had to be cold dark matter (CDM).
Were the Milky Way’s dim dwarf galaxies also hiding CDM? That was the question Simon asked for his doctoral thesis. He looked at three other dwarf galaxies. Like NGC 4605, they were far more massive than their collection of stars could account for. It seemed the Milky Way’s dwarf satellite galaxies were hiding places for cold dark matter. The image below is an artist's impression of what the CDM halo might look like if we could see it.