Lopsided galaxy as a result of a merger

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How the Webb Telescope will unveil the mysteries of cosmic star-making factories

Scientists know galaxy mergers play a big role in forming today’s universe. They just don’t know how they work.

NASA

The James Webb Space Telescope is less than a week away from delivering its first science images on July 12, 2022. The moment marks the beginning of a new era in extragalactic exploration — that is, the study of the billions of galaxies beyond the spiral arms of our own Milky Way.

Webb’s suite of high-tech cameras and its 6.5-meter mirror will enable scientists to peer farther and deeper than with any other telescope in history. What it reveals will likely alter our perception of how galaxies form and our fundamental understanding of the universe.

A major part of delivering this mind-bending science will be using Webb’s infrared and spectroscopic powers to peer into the churning cores of merging galaxies. What it finds will help scientists piece together the importance of mergers as star-marking factories, explain the obscure mechanics behind them, and fill in some of the missing puzzle pieces about how the universe formed.

“You’ll never be able to answer the question of ‘how do galaxies form’ if you don’t have a good understanding of mergers. It’s just impossible,” Christopher Conselice, an extragalactic astronomer at the University of Manchester, tells Inverse.

II Zw 096, a pair of galaxies well on their way to merging. NASA/JPL-Caltech/STScI/H. Inami (SSC/Caltech)

What makes a galaxy merge?

Galaxy mergers are what they say they are: A merger occurs when two (or sometimes three) galaxies crash into one another and become one. The process can take several hundred million to even a couple billion years to complete — a relatively short time compared to the lifespan of your typical galaxy.

These collisions happen more often than you might think. Anywhere from 5 percent to 25 percent of all existing galaxies are merging, and many others have likely experienced some kind of merger — whether a major or a minor one — in the past. Our own Milky Way is itself on a collision course: It will experience a major merger with the Andromeda galaxy in about 4.5 billion years.

When that happens, as is often the case with galaxy mergers, this new Milky Way-Andromeda hybrid will witness a rapid increase in star formation as gas clouds swirl, collapse, and form new stars — a lot of new stars.

Your typical solo galaxy will only produce one or two solar masses per year. Merging galaxies on the other hand can produce hundreds of solar masses per year and create massive starbursts as well as powerful active galactic nuclei.

Because these galaxies produce stars at such a rapid clip, they’re much brighter — especially in the infrared spectrum — than other, non-merging galaxies, so scientists designate them as Luminous Infrared Galaxies or LIRGs. Not all of these bright galaxies are necessarily merging galaxies, but most of them are.

This is where the Webb Telescope comes in: It will soon peer closely at four special Luminous Infrared Galaxies. All four have names only astronomers can love like NGC 7469, NGC 3256, II Zw 096, and VV114. But we can all gain from discovering how they work.

The program is designed to “be the first to explore the capabilities of James Webb and to see what we can see,” Vivian U, an extragalactic astronomer at the University of California, Irvine, and co-investigator on the Webb Telescope study, tells Inverse.

“The science is very broad…so we tried to pick objects that hit a variety of parameters.”

No two luminous infrared galaxies are the same. So the team selected various objects at different stages of merging as well as with differing amounts of dust, star clusters, and outflows. This range will offer a broad overview of galaxy mergers, how they form, and how they make stars so quickly.

NGC 3256, a galaxy mid-merger, which is a likely target for Webb.NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

Peering through the dust

Merging galaxies are some of the most powerful, star-making engines in the universe, but we don’t know much about them. We know they create far more solar mass than usual, but we don’t know how this happens and how resources in these gas-rich galaxies are shared between their merging black holes and the stars they go on to form.

That’s because luminous infrared galaxies kick up a lot of dust, especially at their cores — where most of the action is taking place. Older space telescopes like Hubble and Spitzer can’t penetrate the thick veil of dust to observe what lies within.

“Dust likes to absorb light in the UV,” Conselice says.

“So we see these patchy galaxies and we don’t really know what’s going on with them.”

Luckily, the Webb Telescope has a few tricks up its sleeve. With its Near Infrared Spectrograph (NIRSpec), Near-Infrared Camera (NIRCam), Mid-Infrared Instrument (MIRI), and 6.5-meter mirror, Webb will be able to peel back the curtain and get a good look at what’s going on inside. Not only will Webb provide high-res images of luminous infrared galaxies, but it will also take various spectra that can reveal their content and the movement inside them.

“An image tells you where things are or what things look like,” U says. “[With Webb] I get to see how things are moving. I get to see the kinematics and how gas is moving around a supermassive black hole.”

“With James Webb, we can see much deeper and much closer.”

Exactly how deep and how close? NASA thinks the Webb Telescope will provide data 50 to 100 times more sensitive than previous infrared surveys and will be able to zoom in on areas a mere 150 to 300 light-years across (galaxies can be hundreds of millions of light-years wide, so this is relatively small).

Not too shabby.

Big Telescope, Big Implications

While Webb will fill in a lot of gaps about what we know about merging galaxies and LIRGs, it’ll also glimpse back to some of the earliest galaxies in the universe

For his part, astronomer Conselice’s work probes along the very edge of the known universe. While the Hubble telescope could see “toddler galaxies” forming some 700 million years after the Big Bang, the Webb Telescope will see some of the universe’s very first stars — the “baby galaxies.”

Because the early universe was much hotter and denser than it is today, studies show that galaxy mergers were more common during the universe’s early years. This makes understanding the mechanics of galaxy mergers — whether in the local universe or on its very edges — all the more important.

“It’s a major process in the history of the formation of galaxies,” Conselice says.

Conselice says studying mergers could help answer lingering questions about specific parts of galaxies, like black holes and star formation, and also shine a light on broader mysteries of the cosmos like dark matter and dark energy. They can even help answer how the known universe came to be.

And the journey to answer that question has begun. The Webb Telescope has already captured infrared imaging of at least one of the luminous infrared galaxies it is geared to target, and U expects the data for all four luminous infrared galaxy targets in her program to be available by the end of the year.

A new age of Webb-powered astronomy has finally arrived.

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