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Starts With A Bang

Can This Newfound Dark, Massive Galaxy Be Astronomy’s ‘Missing Link’ In The Universe?

If this newfound galaxy is just the tip of the iceberg, the entire Universe may fall into place.


One of the greatest challenges for a scientist is that every time you make a new advance, it only raises more questions. When we look out at our Universe today, we see galaxies with all sorts of different properties. We see giant ellipticals that haven’t formed stars in billions of years; we see Milky Way-like spirals that are rich in heavy elements; we see irregular galaxies; we see dwarf galaxies; we see ultra-distant galaxies that appear to be forming stars for just the first or second time.

But when you put this all together, there are some puzzles. Some galaxies have grown to be so large so early that they’ve defied a coherent explanation. With only small, low-mass galaxies found at great distances by Hubble, the active formation of a large galaxy has long been astronomy’s missing link. With a new discovery of a dark, massive galaxy, astronomers may have just cracked the mystery, and solved a longstanding cosmic puzzle.

Galaxies comparable to the present-day Milky Way are numerous, but younger galaxies that are Milky Way-like are inherently smaller, bluer, more chaotic, and richer in gas in general than the galaxies we see today. For the first galaxies of all, this ought to be taken to the extreme, and remains valid as far back as we’ve ever seen. There is an unexplained gap between the earliest proto-galaxies and the first large galaxies that astronomers have struggled to explain. (NASA AND ESA)

To understand how galaxies form and grow up in our Universe, it’s always best to start at the very beginning. Cosmologists have assembled a comprehensive and coherent picture of the Universe, and if we trace out how that Universe evolves and grows from its humble beginnings to the cosmos we inhabit today, we should be able to come up with a story that tells us what we ought to see.

The Universe, in the aftermath of the Big Bang (post-inflation), arrives on the scene with the seeds for our modern-day galaxies already planted. Our Universe is hot, dense, expanding, and filled with matter, antimatter, dark matter and radiation. It’s also born almost perfectly uniform, but with tiny density imperfections in it. On all scales, the densest regions are just a few parts-in-100,000 denser than average, but that’s all the Universe needs.

The largest-scale observations in the Universe, from the cosmic microwave background to the cosmic web to galaxy clusters to individual galaxies, all require dark matter to explain what we observe. The large scale structure requires it, but the seeds of that structure, from the Cosmic Microwave Background, require it too. (CHRIS BLAKE AND SAM MOORFIELD)

As the Universe expands and cools, the regions that have slightly more matter (normal and dark combined) than others will begin to preferentially attract more and more of the matter from surrounding regions towards it. As time goes on, radiation becomes less important, and these matter imperfections can grow at a faster rate as they continue to grow in density.

Although it takes somewhere between 50 and 100 million years for the very first region in the Universe to become dense enough to form stars, that’s just the start of the story. These first stars, once they start turning on, herald the arrival of energetic, ultraviolet photons that start streaming through the Universe. Over time, as stars form in more and more locations, the neutral atoms throughout space begin to be reionized, as the Universe slowly becomes transparent to visible light.

The most distant galaxy ever discovered in the known Universe, GN-z11, has its light come to us from 13.4 billion years ago: when the Universe was only 3% its current age: 407 million years old. But there are even more distant galaxies out there, and we all hope that the James Webb Space Telescope will discover them. (NASA, ESA, AND G. BACON (STSCI))

At around 200–250 million years after the Big Bang, the first galaxies begin to form, increasing the rate of reionization as star-forming regions cluster and merge together. The earliest galaxy we’ve ever identified (with today’s instrumentation limits) appears about 400 million years after the Big Bang, with all the earliest galaxies actively forming stars at an alarming rate, but no more massive than 1% the mass of our modern Milky Way.

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After a total of 550 million years, the Universe finally becomes fully reionized, and light can freely travel without being absorbed. Yet we continue to see only these bright but low-mass galaxies for some time, until about a billion years after the Big Bang, when enormous galaxies even more massive than our Milky Way appear in our telescopes. The big puzzle here is the missing link between these two populations.

In theory, the way these cosmic structures should form is through gravitational growth and mergers. Individual proto-galaxies should attract the matter from surrounding regions of space, while different proto-galaxies should attract one another. As time goes on, the gravitational influence of the various galaxies starts to affect larger and larger scales, leading to galaxies growing by eating one another and merging together.

But if that were the case, we wouldn’t expect to see only the small, early proto-galaxies and the large, mature, post-merger galaxies. We would expect to see that intermediate stage, where the proto-galaxies are merging together, during the growth phase where star-formation is actively occurring. But all of the early galaxies we’ve seen aren’t forming stars at a fast enough rate to explain these mature galaxies.

The distant galaxy MACS1149-JD1 is gravitationally lensed by a foreground cluster, allowing it to be imaged at high resolution and in multiple instruments, even without next-generation technology. This galaxy’s light comes to us from 530 million years after the Big Bang, but the stars within it are at least 280 million years old. How we go from tiny galaxies like this to the massive ones we see a few hundred million years later is a mystery in galaxy evolution. (ALMA (ESO/NAOJ/NRAO), NASA/ESA HUBBLE SPACE TELESCOPE, W. ZHENG (JHU), M. POSTMAN (STSCI), THE CLASH TEAM, HASHIMOTO ET AL.)

The standard expectation is that there’s got to be some undiscovered type of galaxy in between these low-mass, early-type proto-galaxies and the heavy, massive, mature galaxies that we see. For those elusive galaxies to not appear in the same surveys that find both of the other types of galaxies means there must be something that’s obscuring the light we’re expecting to arrive.

For the most distant galaxies that are actively forming new stars at the greatest rates, we expect the light they’ll emit will peak in ultraviolet wavelengths, just like they do for all massive star-forming regions where the light is dominated by stars significantly more massive than the Sun. After traveling through the expanding Universe, that light should redshift from ultraviolet through the visible part of the spectrum and all the way into the infrared. Yet our deepest infrared observations reveal only the early and late-type galaxies, not the intermediate type.

A young, star-forming region found within our own Milky Way. Note how the material around the stars gets ionized, and over time becomes transparent to all forms of light. Until that happens, however, the surrounding gas absorbs the radiation, emitting light of its own of a variety of wavelengths. In the early Universe, it takes hundreds of millions of years for the Universe to fully become transparent to light, and newly merged galaxies might require very long timescales to ionize all the obscuring gas-and-dust while the galaxy grows and forms stars. (NASA, ESA, AND THE HUBBLE HERITAGE (STSCI/AURA)-ESA/HUBBLE COLLABORATION; ACKNOWLEDGMENT: R. O’CONNELL (UNIVERSITY OF VIRGINIA) AND THE WFC3 SCIENTIFIC OVERSIGHT COMMITTEE)

Why could this be? The simplest explanation would be if something were blocking that light somehow. By the time the Universe is in the process of forming these very massive galaxies, it’s already reionized, so we cannot blame the intergalactic medium for absorbing the light. But what might be a reasonable culprit is the gas and dust that belongs to the proto-galaxies which merge to form the late-type galaxies we eventually see.

Whenever you have a star-forming region, even if that region encompasses the entire galaxy, those stars are only able to form where you have neutral gas clouds collapsing. But neutral gas is exactly what we expect to block ultraviolet and visible light by absorbing it, and then re-radiating it at much longer wavelengths, dependent on the gas temperature. That light should be radiated in the infrared, and ought to be redshifted far into the microwave or even radio bands.

Light may be emitted at a particular wavelength, but the expansion of the Universe will stretch it as it travels. Light emitted in the ultraviolet will be shifted all the way into the infrared when considering a galaxy whose light arrives from 13.4 billion years ago; the Lyman-alpha transition at 121.5 nanometers becomes infrared radiation at the instrumental limits of Hubble. But warm gas, emitting in the infrared normally, will be redshifted all the way into the radio portion of the spectrum by the time it arrives at our eyes. (LARRY MCNISH OF RASC CALGARY CENTER)

So instead of looking for redshifted starlight, you’d want to look for the signatures of warm dust that gets redshifted by the expansion of the Universe. You wouldn’t use an optical/near-infrared observatory like Hubble, but rather a millimeter/submillimeter array of radio telescopes.

Well, the most powerful such array is ALMA, the Atacama Large Millimeter/submillimeter Array, which contains a collection of 66 radio telescopes designed for achieving high angular resolution and unprecedented sensitivity to detail in exactly that critical set of wavelengths. If you can find a faint, distant source of light that appears in these wavelengths and no others, you’ll have discovered a candidate for exactly this type of “missing link” in galaxy formation. For the first time, a team of astronomers appears to have struck gold with exactly this discovery, by pure luck, in their observing field.

The Atacama Large Millimeter/submillimeter Array (ALMA) are some of the most powerful radio telescopes on Earth. These telescopes can measure long-wavelength signatures of atoms, molecules, and ions that are inaccessible to shorter-wavelength telescopes like Hubble, but can also measure details of protoplanetary systems and faint, early galaxies that may be obscured to more familiar wavelengths of light. (ESO/C. MALIN)

They made this discovery by looking at galaxies in the COSMOS field, a deep-field set of observations where many different observatories, including both Hubble and ALMA, have taken copious amounts of data. The team found two signals that corresponded to galaxies filled with warm dust and, therefore, rapid amounts of star formation. One of these corresponded to a run-of-the-mill late-type galaxy, but the other corresponded to no known galaxy at all.

When all the observations of this new galaxy candidate were combined, the astronomers studying it determined that it was:

  • very massive, with nearly 100 billion solar masses worth of stars and even more in neutral gas,
  • a star formation rate of 300 new solar masses’ worth of stars every year (hundreds of times what we find in the Milky Way),
  • extremely highly obscured, as though it were shrouded in light-blocking dust,
  • and incredibly distant, with its light coming to us just 1.3 billion years after the Big Bang.
Looking back through cosmic time in the Hubble Ultra Deep Field, ALMA traced the presence of carbon monoxide gas. This enabled astronomers to create a 3-D image of the star-forming potential of the cosmos. Gas-rich galaxies are shown in orange. You can clearly see, based on this image, how ALMA can spot features in galaxies that Hubble cannot, and how galaxies that may be entirely invisible to Hubble could be seen by ALMA. (R. DECARLI (MPIA); ALMA (ESO/NAOJ/NRAO))

The study’s authors have expressed extreme excitement that this galaxy ⁠ — which appears in a survey area of just 8 square arcminutes (it would take 18 million such regions to cover the sky) ⁠ — might be a prototype for the “missing link” galaxies required to explain how the Universe grew up. According to study author Kate Whitaker,

“These otherwise hidden galaxies are truly intriguing; it makes you wonder if this is just the tip of the iceberg, with a whole new type of galaxy population just waiting to be discovered.”

While other large galaxies, including star-forming galaxies, had been spotted before, none of them had large enough star-formation rates to possibly explain how the Universe’s galaxies grew up so fast. But this galaxy changes all of that, according to first author Christina Williams, who noted,

“Our hidden monster galaxy has precisely the right ingredients to be that missing link, because they are probably a lot more common.”

Optical telescopes like Hubble are extraordinary at revealing optical light, but the expansion of the Universe redshifts much of the light from distant galaxies out of Hubble’s view. Infrared and longer wavelength observatories, like ALMA, can pick up the distant objects that are too redshifted for Hubble to see. In the future James Webb and ALMA, combined, might reveal details of these distant galaxies that we cannot even fathom today. (ALMA / HUBBLE / NRAO / NSF / AUI)

Up until now, scientists have been waiting for the James Webb Space Telescope — humanity’s next-generation, space-based infrared observatory — to peer through the light-blocking dust and solve the mystery of how our Universe grew up. While Webb will certainly teach us more about these early, growing galaxies and reveal details that remain unseen, we’ve learned that these obscured monsters really are out there, and might be the missing link in galaxy growth and evolution.

Either we’ve gotten incredibly lucky in finding a very rare type of galaxy in such a small region of space, or this new find is an indicator that these behemoths really are everywhere. For now, this new discovery should leave us all hopeful that ALMA will continue to find more of these galaxies, and that when James Webb comes online, one more piece of the cosmic puzzle might slide perfectly into place.


Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.

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