A secret population of hidden galaxies suffusing the universe in a soft glow of far-infrared light have been strongly suggested to exist, based on careful detective work into some of the most unique data to come from Europe’s Herschel Space Observatory.
The galaxies, if they are real, are not necessarily a surprise. The cosmos is filled with light across all wavelengths — it’s just that the far-infrared component seems to be stronger than can be accounted for by all the galaxies we can see in visible light. In other words, there must be something else in the universe producing its glow.
Far-infrared light, associated with longer wavelengths than what even the James Webb Space Telescope can see, is emitted by cosmic dust that has absorbed starlight. Cosmic dust is produced by the cycle of star birth and death. Dust condenses around newly formed stars — it’s what planets like Earth are built out of, after all — and then is produced in huge quantities when stars die. The more intense the star formation, the more rapid the cycle of star birth and death. And the more rapid the cycle, the more dust is produced. Eventually, enough dust can be produced to literally hide the stars within a galaxy.
This has led astronomers to wonder whether there are countless galaxies out there shrouded in dust —- galaxies that are quietly contributing to the far-infrared background of the cosmos.
The trouble is, nobody had seen them —until possibly now.
“The cosmic infrared background is like a jigsaw puzzle, but there are some pieces missing,” Chris Pearson, an astronomer at the U.K.’s Rutherford Appleton Laboratory, told Space.com. “We’ve always known that we need something to complete the puzzle, but we haven’t really known what shape or form those missing pieces were going to be.”
Pearson led a team who used archival data from the Herschel Space Observatory to search for these missing pieces. Herschel, which was capable of viewing the universe in long wavelengths of far-infrared light, ended its mission all the way back in 2013. However, as a member of Herschel’s original instrumentation team, Pearson knew of some observations that hadn’t been available to regular astronomers.
One of Herschel’s primary instruments was SPIRE, the Spectral and Photometric Imaging Receiver. To ensure that SPIRE remained calibrated correctly, it was pointed towards a barren patch of sky just 3.5 degrees from the North Ecliptic Pole once or twice a month.
“By looking at the same area of sky we always expected to get roughly the same result, and if we didn’t, if we saw a systematic drift over time such as everything getting bright each month, then that would be indicative of some change in the characteristics of SPIRE, and we’d have to create a calibration correction,” Pearson said.
SPIRE imaged this “dark field” 141 times, but because it was only used by the instrument team to monitor the equipment itself, the dark field images were not released to the general astronomical community.
However, Pearson’s team realized that the images could be useful for more than just calibrating SPIRE. They stacked the 141 images — astro-imaging parlance for adding and merging the images on top of one another, which dramatically increases the signal-to-noise ratio — and threw in some data from NASA’s Spitzer Space Telescope to create the deepest far-infrared view of the cosmos ever made.
In this “dark field,” they identified 1,848 sources of far-infrared emission. Now, the problem with observations at long wavelengths is resolution: you just don’t get sharp images like you do with the Hubble or James Webb space telescopes.
Even though Herschel’s mirror, was larger than Hubble’s, at 3.5-meters (11.5 feet) in diameter, to Herschel, the 1,848 far-infrared sources all look like amorphous blobs.
Therefore, a careful statistical analysis had to be undertaken to figure out what these blobs actually are, and whether they match typical galaxy distributions. The conclusion is that they are dusty, star-forming galaxies at a range of distances from us; they are hard to find because they are faint, probably indicating that these are not large galaxies, but rather smaller dwarf galaxies undergoing their first intense bursts of star formation.
If one were to extrapolate the findings all across the sky, the result would be an awful lot of small, dusty, star-forming galaxies that collectively contribute a significant fraction of the far-infrared background, and of the overall energy budget of the universe.
Still, it’s not necessarily the first time that some of these galaxies have been seen; they may have turned up in deep images taken by Hubble or the JWST, for example. “But it’s making the link between what you see at one wavelength and what you see at another wavelength that’s the problem, and again it’s down to resolution,” Pearson said. For example, an optical image taken by Hubble might show a cluster of individual galaxies, but in the Herschel image they would appear as just one blob of infrared light. “You don’t know how many of those galaxies that you see at optical wavelengths are actually also contributing to the emission of this blob,” Pearson said.
What’s needed is more data to fill the gaps and confirm that this population of hidden galaxies is real. That data will not be forthcoming from Herschel, though: “We’ve pushed what Herschel could do right to the limit with this,” Pearson said.
On the bright side, there are two other possibilities.
One option is to conduct observations at submillimeter radio wavelengths, which is the next waveband up from far-infrared. Although the North Ecliptic Pole is not viewable from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, Pearson’s team does have some observing time coming up on the Submillimeter Array (SMA) in Hawaii, which can see the dark field. Beyond that, Pearson is a member of a consortium behind a proposed NASA mission called PRIMA, the Probe far-Infrared Mission for Astrophysics. PRIMA has made it to the final shortlist for NASA’s next billion-dollar Probe class mission, competing against one other mission, the Advanced X-ray Imaging Satellite (AXiS). Final selection takes place in 2026.
If PRIMA does go ahead, its telescope mirror will actually be quite a lot smaller than Herschel’s at just 1.8-meters (6 feet). “So in terms of taking pictures, it won’t help us because they’re still going to be blurry,” Pearson said.
What PRIMA will specialize in is spectroscopy, breaking down the infrared light into individual wavelengths to learn more about the constituents of these galaxies, how much star formation is taking place and how far away they are.
As Pearson said,, “If PRIMA goes ahead, it’s going to be absolutely instrumental in solving this.”
Two papers describing the results, one with Pearson as the lead author and another led by Thomas Varnish of the Massachusetts Institute of Technology, were published on April 9 in the journal Monthly Notices of the Royal Astronomical Society.
