How to read these gorgeous pictures of young galaxy clusters

This week, the European Space Agency (ESA) announced the discovery of what it believes to be proto-galaxy clusters, or young versions of the types of galaxy clusters that make up our universe. The new data could help scientists figure out how galaxy clusters were formed. “We still have a lot to learn about this new population, requiring further follow-up studies with other observatories. But we believe that they are a missing piece of cosmological structure formation,” said Hervé Dole, lead author of the study published on the findings this week, in a press release.

Images released by ESA in tandem with the announcement are pretty spectacular. But it’s not immediately clear how they relate to the news. Here’s how to decode them.

ESA and the Planck Collaboration/ H. Dole, D. Guéry & G. Hurier, IAS/University Paris-Sud/CNRS/CNES

First, let’s take a look at the central image, Planck’s “all-sky map at submillimetre wavelengths.” Herschel Project scientist Göran Pilbratt explains that what we’re looking at shouldn’t be taken too literally. “You need to understand that the colors are completely artificial.” At these wavelengths, he continues “we cannot see any lights at all with our eyes, so the concept of color is kind of arbitrary.” Here, light red represents the weakest light emission. The dark red portions show stronger emissions, and the white-gray, central band is the strongest. That part shows dust in the Milky Way.

ESA and the Planck Collaboration/ H. Dole, D. Guéry & G. Hurier, IAS/University Paris-Sud/CNRS/CNES

This is what it looks like with the colors inverted:

Institut national des sciences de l’univers

But, Pilbratt says, what we should really be looking at are all of those small, black dots at the periphery. These show where the potential proto-clusters announced by ESA are located. They correlate to the photos shown around the all-sky map.

Institut national des sciences de l’univers

The photographs, explains Pilbratt, are color-coded from the wavelength at which they are observed by Herschel’s Spectral and Photometric Imaging Receiver (SPIRE) instrument: Blue correlates to a wavelength of 250 microns, green correlates to 350, and red to 500. When the images are equally strong in each of the wavelengths we see a whiteish color. The yellow rings show the density of the galaxies shown.

The Herschel observatory captured more than 200 potential galaxy clusters. Here’s a collage showing what those images look like:

The strong emissions at these wavelengths, says Pilbratt, “tell us that star formation is ongoing in these galaxies in a big way.” France’s Institut d’Astrophysique Spatiale, whose scientist helped report the findings, explains it like this:

“These [light sources] appear bright either as a result of strong gravitational lensing, or because they represent the combined far-infrared (FIR)/sub−mm emission from multiple intense starbursts in a young galaxy cluster – the most massive and rapidly collapsing dark−matter environments in the early Universe.”

A note here — most of the light sources described above were, indeed, determined by scientists to be likely proto-galaxy clusters, rather than the result of gravitational lensing, which would signify individual galaxies. Back to Institut d’Astrophysique Spatiale:

“When Planck scientists started looking for these poorly understood sources, they were surprised to find so many candidates.”

Pilbratt adds, “We are [looking] back in time. When these clusters were forming, they look completely different from how they look today.”

Now, scientists face the task of taking a closer look at each of the galaxies: “The obvious next step is to establish accurate redshifts for these sources,” says Pilbratt. Redshift, per NASA, “refers to how much an object’s light has shifted into longer wavelengths as a result of the expansion of the universe.” So finding the redshift of the possible proto-galaxy clusters seen here will help us place them in the history of the cosmos, and learn more about how they shaped the universe as we see it today.

Danielle Wiener-Bronner is a news reporter.