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This artist’s illustration shows NASA’s New Horizons spacecraft in the Outer Solar System. In the background are the sun and a luminous stripe representing the zodiacal light, caused by the reflection of sunlight on the dust. By traveling beyond the inner solar system and the light pollution that accompanies it, New Horizons was able to answer the question: how dark is space? At the bottom right are the background stars of the Milky Way. Credit: Joe Olmsted (STScI)
New measurements of the darkness of the sky show that galaxies number only in the hundreds of billions.
How dark is the sky and what does this tell us about the number of galaxies in the visible universe? Astronomers can estimate the total number of galaxies by counting everything visible in a deep Hubble field, then multiplying them by the total area of the sky. But other galaxies are too faint and too far away to be detected directly. Yet even though we cannot count them, their light permeates the space with a faint glow.
To measure this glow, astronomical satellites must escape the inner solar system and its light pollution, caused by sunlight reflecting off dust. A team of scientists used the observations of NASAThe New Horizons mission of Pluto and the Kuiper belt to determine the luminosity of this cosmic optical background. Their result sets an upper limit on the abundance of weak and unresolved galaxies, showing that they number only in the hundreds of billions, and not 2 trillion galaxies as previously believed.
This photo shows a phenomenon known as zodiacal light. At the bottom left, a glowing spot extends to the top right towards Jupiter, the luminous object to the left of the center. Zodiacal light is caused by sunlight reflecting off tiny particles of dust in the inner solar system – the disintegrated remnants of comets and asteroids. Attempts to measure how dark space uses telescopes like Hubble have been thwarted by this ambient glow. Credit: Z. Levay
How dark does the space get? If you walk away from the city lights and look up, the sky between the stars looks really, really dark. Above Earth’s atmosphere, outer space darkens even more, turning ink black. And yet even there the space is not absolutely black. The universe has a faint glow imbued with countless distant stars and galaxies.
New measurements of this weak background glow show that invisible galaxies are less abundant than some theoretical studies suggest, numbering only in the hundreds of billions rather than the previously reported two trillion galaxies.
“That’s an important number to know – how many galaxies are there?” said Marc Postman of the Space Telescope Science Institute in Baltimore, Maryland, lead author of the study. “We just don’t see the light of two trillion galaxies.”
The previous estimate was extrapolated from very deep sky observations by NASA The Hubble Space Telescope. He relied on mathematical models to estimate how many galaxies were too small and too weak for Hubble to see. This team concluded that 90% of the galaxies in the universe exceeded Hubble’s ability to detect in visible light. The new findings, which were based on measurements from NASA’s distant New Horizons mission, suggest a much smaller number.
“Take all the galaxies Hubble can see, double that number, and that’s what we see – but nothing more,” said Tod Lauer of NSF’s NOIRLab, lead author of the study.
These results will be presented on Wednesday, January 13 at a meeting of the American Astronomical Society, open to registered participants.
The cosmic optical background the team sought to measure is the visible light equivalent of the best-known cosmic microwave background – the faint afterglow of the big bang itself, before the stars existed.
“While the microwave cosmic background tells us about the first 450,000 years after the big bang, the cosmic optical background tells us a lot about the sum total of all the stars that have formed since,” Postman explained. “This puts a strain on the total number of galaxies that have been created, and where they might be in time.”
As powerful as Hubble was, the team couldn’t use it to make these observations. Despite being located in space, Hubble orbit the Earth and still suffers from light pollution. The inner solar system is filled with tiny dust particles from disintegrating asteroids and comets. Sunlight reflects off these particles, creating a glow called zodiacal light that can be observed even by sky-watchers on the ground.
To escape the zodiacal light, the team had to use an observatory that escaped the inner solar system. Fortunately, the New Horizons spacecraft, which delivered the closest ever-seen images of Pluto and the Kuiper Belt object Arrokoth, is far enough away to make these measurements. At its distance (more than 4 billion kilometers at the time of these observations), New Horizons experiences an ambient sky 10 times darker than the darkest sky accessible to Hubble.
“These types of measurements are extremely difficult. A lot of people have been trying to do this for a long time, ”Lauer said. “New Horizons has provided us with a vantage point to measure the cosmic optical background better than anyone has been able to.”
The team analyzed existing images from the New Horizons archives. To unravel the faint backlight, they had to correct a number of other factors. For example, they subtracted light from galaxies thought to exist that are too faint to be identifiable. The most difficult correction was to remove the light Milky Way stars reflecting off interstellar dust and into the camera.
The remaining signal, although extremely weak, was still measurable. Postman compared it to living in a remote area away from the city lights, lying in your bedroom at night with the curtains open. If a neighbor a mile down the road opened their fridge looking for a midnight snack and the light from their fridge reflected off the bedroom walls, it would be as bright as the background it detected. by New Horizons.
So what could be the source of this residual glow? It is possible that an abundance of dwarf galaxies in the relatively near universe lie just beyond detectability. Or the diffuse halos of stars that surround galaxies might be brighter than expected. There could be a population of rogue intergalactic stars spread across the cosmos. Perhaps most intriguing, there may be many more faint and distant galaxies than the theories suggest. This would mean that the even distribution of galaxy sizes measured to date increases sharply just beyond the weakest systems we can see – just as there are many more pebbles on a beach than there are rocks.
Upcoming from NASA James Webb Space Telescope may be able to help solve the mystery. If weak individual galaxies are the cause, then Webb ultra-deep field observations should be able to detect them.
This study is accepted for publication in the Astrophysical Journal.
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