A New Galaxy Atlas Charts the Skies for Scientists and the Public Alike in Unprecedented Detail

Charting the galaxies in the sky has come a long way since the 18th century, when Charles Messier first included 40 nearby galaxies in his catalog of celestial objects. The New General Catalogue of Nebulae and Clusters of Stars of 1888 listed more than 6,000 such galaxies, while the Third Reference Catalog of Bright Galaxies, published in 1991, lists more than 23,000.

But the newly updated Siena Galaxy Atlas (SGA) has taken things to a whole new level, imaging and cataloging nearly 400,000 bright, nearby galaxies across half the sky, and making them available to the public through a Google Earth-style online viewing application. More than just a cool way to explore the night sky, the images and associated data will allow astronomers to pose new questions about galaxies and their formation, while simultaneously aiding the study of dark energy, the enigmatic force believed to be driving the accelerated expansion of the cosmos itself.

“I always had a fascination and a curiosity, scientific curiosity for what we could learn from galaxies,” John Moustakis, a New York Siena College astrophysicist and SGA project leader, told Parabolic Arc. “The driving question is, why do they look like they do? Why not something different? It gets at the heart of understanding where we are in the universe, how our universe came to be.”

Building the Galaxy Atlas

The atlas was built from surveys of the sky conducted between 2014 and 2017 using three observatories: the University of Arizona’s Steward Observatory, the Kitts Peak National Observatory, also in Arizona, and the Cerro Tololo Inter-American Observatory in Chile. None of these observations were conducted for the sake of building a galaxy atlas — they were preliminary work to aid the Dark Energy Spectroscopic Instrument (DESI) project.

“What [DESI] lets you do is get a spectrum of 5,000 galaxies or stars at once,” Dustin Lang, Perimeter Institute computer scientist and member of the SGA project, told Parabolic Arc. By breaking the light of distant galaxies up into their spectra, researchers can measure how much their light has been shifted further into the red portion of the electromagnetic spectrum as the light traversed the expanding cosmos, which can yield that object’s distance and position in three-dimensional space.

“It has these little robots that can position an optical fiber just so that the light from a single star or Galaxy hits that fiber, and then gets carried down to this instrument that can measure the spectrum,” Lang explained. “But one thing you need for a project like that is, you have to know where to steer each of those little fibers.”

The three preliminary surveys will provide the guidance needed for the DESI project to conduct surveys of very distant galaxies, not those included in the SGA. Ironically, the impetus for creating an atlas from those same datasets isn’t that different from what drove Charles Messier to begin cataloging galaxies more than 200 years ago.

Cataloging contaminated data

It turns out that Messier was a comet hunter, Moustakis explained, and therefore galaxies and anything else that might be confused with comets were “contaminants” in his data. “Messier cataloged 100 and some odd of these fuzzy objects,” he continued, in order to differentiate them from the comets he was truly looking for. The DESI survey is focused on faint, distant galaxies, which are useful for scientists trying to understand how the expansion rate of the universe has changed over time. That makes large, bright galaxies in the foreground contaminants once again, according to Moustakis. 

”It’s like having a little smudge on your glasses,” he said. “You want to figure out every little detail, but here’s this giant fingerprint in front of you.”

Moustakis and his colleagues could have used software to remove the large galaxies, contaminants as they are, from the survey data, and left it at that. “But [Moustakis] is somebody who is really good at finding and exposing the extra value in datasets,” Lang said. So, instead, the two worked together to write new image analysis software that could recognize and analyze the large, bright, nearby galaxies, while still recognizing the faint, distant galaxies needed for the DESI survey.

“Previous image analysis software tended to spread the big galaxies into a mess,” Lang said. “It would break this big, beautiful galaxy into 100 little fluffy bits of galaxy.”

The result is a visual atlas of bright, nearby galaxies spanning 20,000 degrees of the sky, and made available to the public through the online sky viewer application.

Making use of the SGA

 “The plan is to actually build a few future versions that eventually will be a comprehensive, large galaxy Atlas,” Moustakis told Parabolic Arc. He added that the atlas is actually a useful astronomy tool. “It’s not just pretty pictures,” he said.

With hundreds of thousands of galaxies, Lang says, the atlas will allow scientists to perform statistical studies of large, nearby galaxies either by analyzing the data that supports the atlas or by studying the images themselves.

“There is this technique called stacking where you average together a ton of images based on some property,” he said. “So you could take galaxies that share the same color range at some distance from their center and ask, what does the average galaxy with that property look like?”

Moustakis said that the team may invite the public to join in on research efforts after they release future versions of the atlas, possibly in the SGA 2024 release. That could involve connecting the atlas with Galaxy Zoo, a citizen science effort that trains non-scientists to examine and annotate images of galaxies. It’s a way to get human eyes on far more galaxies than any individual investigator can look at.

“That has actually led to dozens, if not over 100 scientific papers, trying to get at some of these questions of understanding the nature of galaxies,” he said. “That is something we would like to do is to have the broader public look through every image; because I’ve looked at thousands, but I haven’t looked at half a million [galaxies].”