The early Universe was a busy place some 13 billion years ago. That’s when countless young galaxies began to evolve and birthed stars at a prodigious rate. The hearts of those very distant galaxies show turbulent, lumpy disks studded with even thicker clumps of dust and gas that spawned their huge batches of stars. Astronomers want to understand what’s driving the clumping, so they’ve turned to recent surveys of closer galaxies in the “local Universe” that contain similar lumpy regions.
The big challenge to understanding the clumps in the very early Universe is actually being able to “see into” their clumps. The thick dust clouds obscure the view. Even JWST hasn’t been able to lift the veil on the clumps seen in the earliest galaxies. So, what can astronomers do to learn more about them? According to Sean Linden, a researcher at the University of Arizona, clues lie in galaxy surveys focused on luminous and ultra-luminous infrared galaxies (LIRGs and ULIRGs). They’re relatively rare in the local Universe. Around 200 are known to lie within ~400 megaparsecs (1.3 billion light-years). That’s close enough to study in detail, which makes them valuable observational targets to understand their counterparts in the early Universe.
What LIRGs and ULIRGs Reveal
LIRGs and ULIRGs are very luminous, massive, dense star factories. LIRGs output more than a billion times the output of the Sun. Their brighter siblings, the ULIRGs, are the brightest at up to a trillion times as bright as the Sun and emit more energy in the infrared part of the spectrum. That’s because their visible light emissions are absorbed by large regions of gas and dust. “These galaxies are very clumpy, very different from the beautiful spiral galaxies that we see now, such as the Milky Way,” Linden said. “And we know from cosmological simulations that these clumps were the building blocks of galaxies in the early Universe.”
IRAS 14348-1447 is a good example of a ULIRG. It lies more than a billion light-years away and shows all the hallmarks of a galaxy collision, including the distorted shape, starburst areas, and clumpy clouds. Courtesy: ESA/Hubble
In the early epochs of cosmic history, infant galaxies crashed together much more often and lit up with bursts of star formation. Those early mergers twisted and deformed them. So, their clumps could very well be quite similar to the ones seen in LIRGs and ULIRGs. Curiously, the closer-by LIRGs and ULIRGs often show similar strange shapes, tidal tails, and other deformities. Those all occur thanks to the intense gravitational interactions they experienced as they merged. The action also forms the dense clumps that caught astronomers’ attention. That confirms that LIRGs and ULIRGs are good analogs for similar-type galaxies in the early Universe.
“What we would like to have is a laboratory in the nearby Universe where we can actually study these kinds of extreme stellar systems with great detail,” Linden said. “We know, from detailed hydrodynamics simulations of galaxy mergers, that these massive clumps form in the densest and dustiest regions of galaxy mergers. So, we need a sample of very dusty extreme mergers where we can study them in their centers, where the gas densities and pressures are so high that they can produce these extreme clumps that we are now seeing at a high redshift. That brings me to the Great Observatories All-sky LIRG survey (GOALS). This is a survey that was started in 2009, originally with the great observatories Hubble (HST), Spitzer, Chandra, and GALEX. We have imaged 200 of these galaxies across those telescopes.”
A selection of luminous infrared galaxies from the GOALS survey. These objects are very common in the early but rarely found in the “local” Universe. Courtesy: Great Observatories All-sky LIRG Survey
Surveying LIRGs and ULIRGs
All the galaxies in the data set were at a distance of less than a billion light-years. They’re star-forming galaxies, according to Linden, which makes them a great laboratory to study clump formation as a result of mergers. Even though they’re relatively close, he pointed out they’re still problematic. “Even with HST, we were not able to resolve the clumps in their center, and in many cases, the dust actually obscures HST’s ability to detect clumps.”
This is where JWST comes in, with its enhanced infrared capabilities. Linden and his team did imaging and spectra on these galaxies using the telescope’s Near-Infrared Camera (NIRCam) and Near-Infrared Spectrograph (NIRSpec). “We zeroed in on the central regions of a handful of these ultraluminous infrared galaxies,” Linden explained. “And what you notice immediately is that we’re able to detect hundreds—actually thousands of clumps in these systems, down to the very center, where JWST can now look through the dust. These clumps are the sort of youngest and most dusty clumps that we’re looking for, because we think that these are the sites of the most extreme star formation that we can potentially compare to the high-redshift Universe. And we know that from initial studies, at least 20 percent of these sources are undetected at optical wavelengths. So they truly are being revealed now for the first time with JWST.”
The comparison between the earliest galaxies and their star-forming clumps and the more recent, similar-type galaxies tells a story of cosmic evolution, as well,” explained Linden. “The Universe used to be much more violent and extreme in the past, and it’s now settling down,” Linden said. “That’s why these rare examples of extreme galaxies no longer exist in the local Universe, because, by and large, most galaxies have settled down as well.”
Hints of the Future
Two scenes from the possible merger between Andromeda Galaxy and the Milky Way Galaxy. Notice the starburst regions and gas clumps being driven by the merger. Courtesy: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger
Linden and his team’s JWST studies give astronomers a way to verify that the properties of these extreme clumps actually match their galaxy evolution simulations. In addition, the data match what they’re seeing in galaxies at very high redshift – out to a time when the Universe was less than a billion years old. It’s also a preview of galaxy mergers to come. For example, at some point, the Milky Way and Andromeda galaxies may interact or collide, over the course of several billions of years. When that happens, the merger could ignite another round of massive star formation in both galaxies. “As Andromeda gets closer,” he said, “and the pressure in the interstellar medium goes up, all of a sudden, the clumps that you will find that the Milky Way is forming will be more and more massive.”
Eventually, if that merger comes to pass, the resulting “Milkdromeda Galaxy” (as some astronomers have named it) will also show the same types of starburst regions that characterize the galaxies in the study.
For More Information
In the Belly of the Beast: Massive Clumps Reveal Star Factories From a Bygone Era of the Cosmos
GOODS Survey