Orphaned stars linger in dark matter haloes

Joanna Carver, reporter

(Image: NASA/JPL-Caltech)

Outcast stars stripped away by galaxy mergers may be trapped in the cocoons of dark matter that surround galaxies. If so, these stars would explain seemingly random smatterings of light in the infrared sky, which should illuminate studies of how the first galaxies formed and grew.

The universe is permeated by faint infrared light called the cosmic infrared background radiation, thought to be a collective glow emitted by all the stars and galaxies in the universe. This splotchy haze contains strange fluctuations that can't be explained by known sources.

Previous research hinted that the odd pattern might be caused by the very first stars and galaxies in the universe or the light of nearby dwarf galaxies that are too faint to resolve. But that would mean the universe spawned many more galaxies than we can detect, which would create turmoil for current models of how the cosmos has evolved.

Now, a new infrared map from NASA's Spitzer Space Telescope shows that unseen galaxies are not the culprits. When the intensity of light from the mysterious splotches is measured against that of known galaxies, it simply doesn't match, says Matthew Ashby, an astrophysicist at Harvard University who is on the study team.

(Image: NASA/JPL-Caltech/UC Irvine)

The team instead found that Spitzer's map fits nicely with theories that there should be stars embedded in dark matter haloes. Observations suggest that the visible matter in galaxies is wrapped in much bigger envelopes of mysterious mass, which seems to hold the galaxies together. As galaxies merge, so do their dark matter haloes.

During these violent collisions, most stars and gas sink toward the middle of the growing galaxy. But streams of stars also get ripped from their homes by gravitational forces, creating what are called tidal tails.

According to theory, some of these stars may settle in the newly swollen dark matter haloes, which are so large that the stars would seem to be adrift in intergalactic space. Fluctuations in the infrared background could be the faint light from these isolated stars, says Ashby.

Team member Edward L. Wright notes that more work is needed to confirm the idea. The new research looked at a larger area of sky than ever before, but Spitzer's sensitivity is limited. When the James Webb Space Telescope is completed sometime in 2018, its advanced infrared eye should be able to get a more detailed picture of the faint galaxies in the early universe, and perhaps expose the spattering of lights in the dark spaces between galaxies.

Journal reference: Nature, doi.org/jmd

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