New measurements of the blackness of the sky show that galaxies only reach hundreds of billions.
How dark is the sky and what does it 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 and multiplying them by the total area of the sky. But other galaxies are too faint and distant to detect them directly. Although we cannot count them, their light infuses 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 reflected off the dust. A team of scientists has used observations from NASAThe mission of New Horizons a Pluto and the Kuiper belt to determine the brightness of this cosmic optical background. Their result sets an upper limit on the abundance of weak unresolved galaxies, which show that they are only found in hundreds of billions, not 2 trillion galaxies as previously thought.
What night does it get dark? If you walk away from the city lights and look up, the sky between the stars looks very dark. Above the Earth’s atmosphere, outer space darkens even more, disappearing into an ink black. And yet there is no space there absolutely black. The universe has a faint and insufficient flash of countless distant stars and galaxies.
New measures of this faint background glow show that invisible galaxies are less abundant than some theoretical studies suggested, in the hundreds of billions alone instead of the two trillion galaxies reported above.
“It’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 above estimate was extrapolated from very deep observations of the sky by NASA Hubble Space Telescope. He relied on mathematical models to estimate how many galaxies were too small and weak for Hubble to see. That team concluded that 90% of the galaxies in the universe were beyond Hubble’s ability to detect with visible light. The new findings, which were based on measurements from NASA’s distant New Horizons mission, suggest a much more modest number.
“Take every galaxy that Hubble can see, twice that number, and that’s what we see, but nothing more,” said Tod Lauer of NOIRLab of NSF, lead author of the study.
These results will be presented Wednesday, Jan. 13, at a meeting of the American Astronomical Society, open to registered participants.
The cosmic optical background that the team tried to measure is the equivalent of visible light from the best-known cosmic microwave background: the faint glow of the Big Bang itself, before the stars existed.
“While the cosmic microwave background tells us about the first 450,000 years after the big bang, the cosmic optical background tells us something about the total sum of all the stars that have formed since then,” Postman explained. “Put a restriction on the total number of galaxies that have been created and where they might be in time.”
As powerful as Hubble is, the team was unable to do so to make these observations. Although in space, Hubble orbits the Earth and still suffers from light pollution. The inner solar system is full of small particles of asteroid dust and disintegrated comets. Sunlight is reflected in these particles, creating a glow called zodiacal light that can be observed even by observers from the sky to the ground.
To escape the zodiacal light, the team had to use an observatory that has escaped from the inner solar system. Fortunately, the New Horizons spacecraft, which has delivered the closest images of Pluto and the Arrokoth object from the Kuiper belt, is far enough away to make these measurements. At its distance (more than 4 billion miles away when these observations were made), New Horizons experiences an ambient sky 10 times darker than the darkest sky accessible to Hubble.
“Such measures are extremely difficult. A lot of people have been trying to do that for a long time, ”Lauer said. “New Horizons provided us with a point of view for measuring cosmic optical background better than anyone has been able to do.”
The team analyzed existing images from the New Horizons archives. To eliminate the faint background glow, several other factors had to be corrected. For example, they subtracted light from galaxies that were expected to exist, too faint to identify. The most difficult correction was to remove the light Milky Way stars reflected in the interstellar dust camera.
The remaining signal, though extremely weak, was still measurable. Postman compared it to living in a remote area away from city lights, lying in your room at night with the curtains open. If a neighbor a mile from the road opened the fridge looking for a snack at midnight and the light from the refrigerator reflected off the bedroom walls, it would be as bright as the background New Horizons detected.
So what could be the source of this excess of glow? It is possible that a large number of dwarf galaxies in the relatively close universe are beyond their detectability. Or the fuzzy halos of stars that surround galaxies may be brighter than expected. It is possible that there is a population of rogue intergalactic stars scattered throughout the cosmos. Perhaps most intriguing, there may be many galaxies farther and farther away than theories suggest. This would mean that the smooth distribution of galaxy sizes measured so far rises sharply beyond the weakest systems we can see, just as there are many more pebbles on the beach than rocks.
The next NASA James Webb Space Telescope can help solve the mystery. If they are weak, individual galaxies are the cause, Webb’s ultra-deep field observations should be able to detect them.
This study is accepted for publication in the Astrophysical Journal.