Scientists Discover Huge ‘Extragalactic Structure’ in Hidden Region of Space

Scientists have discovered a huge “extragalactic structure” hidden behind the Milky Way in a mysterious area of the sky known as the “zone of avoidance” because it is obscured by our own galaxy’s opaque bulge, according to a new preprint study.

The discovery of the structure, which appears to be a large galaxy cluster, helps to fill in this shadowy part of our cosmic map, which may as well be labeled “here be space dragons” because it is so unclear what exists there.

The star stuff that makes up our galaxy, the Milky Way, is distributed inside a thin plane that orbits around a central bulge that contains a supermassive black hole. The galactic plane and bulge are packed with stars, dust, and gas that block our view of whatever is on the other side. Though scientists have been able to use different wavelengths to peer through the zone of avoidance (ZoA), a region that obscures 10 to 20 percent of the sky, most of this region still remains out of view.

Now, a team led by Daniela Galdeano, an astronomer at the National University of San Juan in Argentina, report the discovery of “ a new galaxy cluster, VVVGCl-B J181435-381432, behind the Milky Way bulge,” which helps to complete “the picture of the large scale structure in this still little explored area of the sky,” according to a study posted this week on the preprint server arxiv. (The study has been submitted to the journal Astronomy & Astrophysics but has not yet been peer-reviewed.)

“This result is very satisfying for us,” said Galdeano in an email to Motherboard. “For many years, the ZoA had a lack of information, but now with new studies we could cover a little region of the sky, and in the near future, a bigger region with data.”

“It is incredibly difficult to find galaxies behind the galactic plane, because of the high density of stars and also the obscuration by dust along the line of sight, and this looked like one of the most prominent candidates,” noted Dante Minniti, director of the Institute of Astrophysics at Andrés Bello National University in Chile and a co-author of the study, in another email to Motherboard. “We suspected the presence of structure,” he added, “but since this was a ‘blind region’ before, this discovery of a new galaxy cluster was a nice confirmation.”

Galdeano and her colleagues were able to spot this cluster within the ZoA using the VVV Survey, a project that scans the Milky Way bulge at infrared wavelengths using the European Southern Observatory’s Visible and Infrared Survey Telescope for Astronomy (VISTA) in Paranal, Chile. Whereas the galactic plane blocks out almost all visible light in the zone, longer wavelengths of light, including in the infrared band, are able to travel through the Milky Way’s haze to reach telescopes on Earth. 

The team previously used the infrared glow captured in the VVV Survey to probe an “overdensity region” in the ZOA that suggested the presence of “new extragalactic sources that have not been identified by other catalogs,” according to a 2021 study also led by Galdeano. 

To zoom in on the tantalizing region, the researchers used a near-infrared instrument called FLAMINGOS-2, which is on the Gemini South telescope in Chile, to identify measurements called “redshifts” that can be used to estimate the distance and velocities of its objects in space. The results exposed new details about five galaxies some three billion light years away, which the researchers think are part of a much bigger cluster. 

“I started working with VVV data in 2017, and from the beginning we noticed an excess of galaxies in a small region of the sky,” Galdeano said. “During all these years we had the suspicion that these galaxies belonged to the same structure. These suspicions were based on photometric techniques, so we could not confirm these conclusions.” 

“It is for this reason that we requested follow-up telescope time to obtain the spectra of the brightest galaxies in these overdensity regions with the aim to confirm or discard our suspicions,” she added. “Fortunately we could confirm our conclusions, so we are very happy and proud with these results.”

The team estimated that the cluster contains about 58 galaxies, but it will take more observations to be sure of its mass and contents. 

“It looks quite big, but it is difficult to tell yet how massive,” Minniti said. “We need more spectroscopic redshifts to estimate the mass of this cluster.”

The discovery of this cluster offers an exciting glimpse behind the Milky Way, and scientists will need to continue pulling back this curtain in order to understand our place in space. For instance, in addition to hidden galaxy clusters, the ZoA contains the so-called Great Attractor, an unidentified gravitational anomaly that is tugging galaxies and clusters toward it. The nature of this huge attractor is a mystery that can only be solved by more observations and research.

To that end, it’s unclear whether we will ever be able to figure out what lies in this eclipsed zone, but regardless, Minniti noted that he and his colleagues “are prepared to be surprised.”

“There are some areas that have a lot of dust and stars, so the absorption [of light] is very high, and this is an obstacle that is very difficult,” Galdeano concluded. “Nevertheless, we work hard to explore these mysterious regions, so we hope to have an approach and find out interesting results in the near future.”