Orphan stars could be window into visible universe's 'hidden half'

Astronomers have uncovered evidence for a vast population of previously undetected stars, stripped free of host galaxies that merged, sideswiped, or collided with other galaxies during the past 11 billion to 12 billion years.

The population is so large that it rivals the number of stars within known galaxies, according to the international research team that made the discovery. The results could help astrophysicists solve a lingering mystery. Though scientists have a good idea of the universe's makeup, their calculations suggest that there is more normal matter than known stars, planets, and their occupants account for.

The orphan stars could help fill the gap, researchers say.

“These ... results show us that there is still a large number of stars that we aren't accounting for,” says Karoline Gilbert, a researcher at the Space Telescope Science Institute in Baltimore. “If we want to study and account for all of the starlight in the universe, we can't ignore orphan stars.”

These stars can't be detected as individual points of light. Instead they collectively produce a near-infrared glow bathing the space between galaxies. They form extended halos around galaxies that reach much deeper into intergalactic space than do the typical galactic halos astronomers have studied.

The results answer a riddle first raised in 2005, when researchers using NASA's Spitzer Space Telescope reported detecting the glow, as well as variations in brightness from one patch of the sky to another, says James Bock, the lead scientist for the research project that uncovered the what some have called the hidden half of the visible universe.

These variations were more intense than they should have been if all of the known galaxies in the fields of view had been the only source of the light and scientists puzzled over the cause.

One possible explanation was that the variations represented a window on the universe's first galaxies, which brought the cosmos out of a prolonged dark age. During this period, a fog of neutral hydrogen permeated the universe, preventing light from traversing it. The first galaxies were producing the first generations of stars at a furious pace. The stars' intense ultraviolet radiation and stellar winds would have started to thin out the fog, eventually leaving the light free to roam.

Since the universe has been expanding since its birth 13.8 billion years ago, it in effect stretches this distant, early ultraviolet light to longer wavelengths. By the time it reaches the local universe, it appears as infrared light.

Researchers reasoned that subtle variations in the intensity of this glow across large patches of the sky could reveal information about the rate of star formation, the distribution of galaxy clusters and dark matter, and even supermassive black holes at the centers of the galaxies during the time the hydrogen fog was thinning – a period known as the epoch of re-ionization.

This possibility led the Cosmic Infrared Background Experiment team reporting the new results to make new measurements of the diffuse glow, says Dr. Bock an astrophysicist at the California Institute of Technology in Pasadena and at NASA's Jet Propulsion Laboratory.

The team used suborbital sounding rockets launched from White Sands, N.M., in July 2010 and March 2012. Each rocket carried two near-infrared cameras, each looking at a different wavelength of near-infrared light, and two types of spectrometers.

The team made several checks to ensure that their measurements were not the result of instrument defects or readings contaminated by diffuse near-infrared light from so-called zodiacal dust in the solar system, explains Michael Zemcov, a postdoctoral fellow at Caltech and the lead author of the paper describing the results. Then they compared their measurements against predictions from an early-galaxy model as well as with a second model in which orphan stars produce the glow.

The results showed that the excess glow the team recorded was coming from orphaned stars populating the space between galaxies that are older and are located no more than about 11.5 billion light-years from Earth.

In revealing this hidden half of the visible universe, the team's results also suggested that the extra-galactic background glow of near-infrared radiation may not open the best window on the epoch of re-ionization.

“There are many different ways for probing the epoch of re-ionization,” Dr. Gilbert says. “What these particular results are telling us is that this excess of background light is not one of them.”