The Kavli Foundation Q&A: How Do Galaxies Die?

All galaxies are fated to "die" and stop making stars. But why do they stay that way, when even dead galaxies eventually accumulate all the gassy ingredients they need to restore them to their star-forging glory days?

Kavli IPMU

In this artist’s impression, gas is pulled from the galaxy at left into the galaxy at right. The stolen gas triggers energetic winds from the galaxy at right’s central, supermassive black hole. These winds heat and disturb available gas that would otherwise settle to form new stars. The galaxy at right has entered what astronomers have newly described as a “red geyser” phase in its evolution.
Kavli IPMU

The reason galaxies die, according to new research, lies with the supermassive black holes that lurk at the centers of dead galaxies. In a recent paper, scientists revealed these black holes are spewing out energy that heats up gas in lifeless galaxies—gas that otherwise would cool and condense into new generations of stars. Like a geyser, these energetic eruptions happen periodically, and at a rate that ensures dead galaxies remain full of old, red stars, never getting a second chance at life. Astrophysicists have therefore nicknamed galaxies undergoing this phase “red geysers.”

The finding sheds light on the life cycles of the billions of galaxies populating our universe, including what we might anticipate as the fate for our own galaxy, the Milky Way.

The Kavli Foundation recently spoke with three astrophysicists about these newly discovered red geysers and how they keep galaxies dead and buried.

The participants were:

The following edited transcript of their roundtable discussion is provided courtesy of The Kavli Foundation. The participants have been provided the opportunity to amend or edit their remarks.


THE KAVLI FOUNDATION: How well do we understand the life cycle of a galaxy — from birth, when it forms lots of stars, to death, when star formation has ceased? 

EDMOND CHEUNG: Here’s the general trend of what we think galaxies are doing. We start off with galaxies that form stars in the early universe. Over time, they somehow change their structure and their shape, going from spiral galaxies like the Milky Way to big, bloated, old galaxies that have an elliptical shape. These old galaxies somehow stop forming stars, and become what we call quiescent, or “dead.” They are kept from forming stars for many billions of years. This overall transformation is still quite a mystery to us.

KEVIN BUNDY: We don’t have the whole picture, but clearly there are some basic patterns to how galaxies evolve. We have a sense of those patterns, based on how the observable properties of galaxies like their colors, shapes, and so on are distributed. But we really don’t know what physical mechanisms drive those patterns. That’s what all of us are interested in discovering.

CHRISTY TREMONTI: As Edmond and Kevin just explained, we have an excellent, broad-brush outline of galaxies. But connecting the dots and really understanding the physics of galaxies is what’s at the frontier.

TKF: Let’s talk about these new “red geyser” galaxies you discovered. How are they helping us understand why quiescent galaxies no longer form stars and stay dead?

CHEUNG: It’s weird that old, quiescent galaxies have lots of gas in them and yet do not form stars. After all, all the ingredients are there, just as they are for younger galaxies. Now with red geysers, the evidence is getting stronger that supermassive black holes in the centers of quiescent galaxies are preventing star formation because of outflowing winds of gas.

TREMONTI: “Wind” is a tough term because we have our terrestrial interpretation for what it means. It might be better to think of these winds as outflows, originating from the centers of galaxies, where supermassive black holes are located. These outflows can have a big impact on galaxy evolution because they can carry away a significant amount of gas from a galaxy that would otherwise form stars.

In the cases of the red geysers, the winds are adding a lot of energy to the galaxy. That energy keeps the gas nice and hot and turbulent so it can’t cool down and collapse into the clouds of dust and gas that serve as stellar nurseries.

BUNDY: What Christy just said is exactly why we think these red geysers are so exciting. We’ve been able to estimate the amount of energy contained in the wind and the kind of heating it can achieve. But there’s still more work to be done. One of the things that we need to figure out is exactly how the energy in the wind transfers to the galaxy’s gas. We’ve found a very promising mechanism, but we need to study further how it actually works.

TREMONTI: That’s the missing piece that astronomers are still hammering away at.

CHEUNG: Something to note is that we don’t directly see the effects of supermassive black holes in our red geysers because, as much as we’d love to, we can’t see those black holes. What we’re doing is inferring that these winds have to be from a central source, and the only source that seems to make sense is a supermassive black hole.

TKF: The supermassive black holes found in the centers of nearly all galaxies are extremely small compared to the galaxies themselves--only about a billionth of the galaxy’s size. Are you surprised that something so relatively miniscule can determine the fate of star production in an entire galaxy, as your new research describes?

BUNDY: I vividly remember being taught in first-year grad school, around 15 years ago, that, there was no way a supermassive black hole would have any influence on its galaxy because there is such a difference in scale, as you just pointed out. That was the standard textbook response. Since then, things have really changed dramatically. We’ve recognized that supermassive black holes are very common in the centers of galaxies. We’ve also realized that if you put some of the energy derived from material falling into black holes back into the surrounding galaxy, you can do big things to that galaxy.

CHEUNG: I’m a little younger than Kevin, and I heard pretty much the same thing in my grad studies—that supermassive black holes are just so tiny, how could they affect their galaxies? It sounds pretty ridiculous.

TREMONTI: The scales are surprising, but there’s just such a huge amount of energy associated with these black holes, we think they must be the driver.

In fact, biologists were already doing something like that by tagging biomolecules with small molecules that that emit fluorescent light when illuminated. This let them locate tagged molecules inside cells and see what they are doing. But if there are a lot of molecules in a small space, all they could see through a microscope was one big clump.

So I wondered if I could turn the fluorescence of some molecules off, just briefly. Then I would turn the bright ones off and turn the dark ones on. This way, I would make sure that molecules next to one another would not emit light simultaneously. By separating their emission, I could tell densely packed features apart. This improved our ability to see details by an order of magnitude.

BUNDY: I think we have to be a little careful, though. Anytime we recognize a new, exciting phenomenon, there’s a tendency to blame all the things that we don’t understand on that new phenomenon. Maybe supermassive black holes can solve all of our problems in galaxy evolution. There’s certainly strong evidence of the link between black holes and galactic life cycles, but nevertheless it’s been funny to see this idea play out in the science community over recent years.

TKF: Kevin, you are leading the Mapping Nearby Galaxies at Apache Point Observatory, or MaNGA survey. What distinguishes the MaNGA survey from previous efforts to study nearby galaxies and enabled this brand new discovery of red geysers?

BUNDY: The dataset that many astronomers have used to understand nearby galaxies is from the Sloan Digital Sky Survey. It contains information about more than a million galaxies but we only have the technological means to sample the central regions of the galaxies. So we have to infer what a whole galaxy is like from its center. That’s a problem because galaxies aren’t uniform.

What we would really like to do is something like a CT scan for every galaxy, like doctors do for patients. We’d like to obtain a spectrum at every point across the galaxy’s face. That’s what MaNGA can do.

With MaNGA, we’ve got almost 3,000 galaxies now, so we’re already the largest by far of what we call integral field surveys that study galaxies in this detailed manner. We’re on track to reach our goal of 10,000 by the middle of 2020.

Importantly, we’re capturing all of the galaxy classes in the nearby universe, which is why MaNGA was able to find a lot of red geysers. Seeing all these red geysers gave us the confidence they are a broad phenomenon worth pouring our lives into, as Edmond and I have the last year and a half.

TKF: How often do we think galaxies become red geysers?

CHEUNG: Theorists, working with computer simulations, have proposed red geyser events, when the black hole winds kick up, happen once every 100 or 200 million years or so, which is fairly frequent in a cosmic sense, and often enough to keep dead galaxies from forming new stars.

BUNDY: With the MaNGA survey, we get a snapshot of galaxies, so we see them only at one specific point in their life cycle. Roughly 10 percent of the quiescent galaxies in our current sample appear to be in this red-geyser phase. Our hypothesis is that red geysers are a fairly short-lived phase that could be going on-and-off in all the quiescent galaxies we see.

That figure of a red geyser breaking out in a galaxy every 100 to 200 hundred million years is interesting. It’s similar to how often you might expect trickles of gas to come in from outside the galaxy, maybe triggering a red geyser, or existing gas in galaxies to flow into the central black hole, and likewise trigger a red geyser.

CHEUNG:So this is why we think we have found a mechanism that can keep a quiescent galaxy, well, quiescent!

TKF: What got you interested in studying the life cycles of galaxies?

CHEUNG: To be honest—it was because galaxies just look so pretty. When I first started grad school, I really wasn’t sure what I wanted to do in astronomy. But my advisor showed me some galaxies, and when I realized I can look out into the sky with telescopes and actually see these objects, that got me hooked.

BUNDY: That takes me back to when I used to be an amateur astronomer. I lived in Southern California and I would go out to the desert east of Los Angeles with my telescope. Some of the most amazing things you can see are nearby galaxies. I found them beautiful and interesting. Then in graduate school, I found I really enjoyed working with data and connecting it to the observations of galaxies, like we do with big surveys such as the Sloan Digital Sky Survey that Christy has worked on, and MaNGA [Mapping Nearby Galaxies at Apache Point Observatory], that Edmond and I work on.

BeckmanUCI

A picture of the Sloan Foundation 2.5-meter Telescope at Apache Point Observatory in southeastern New Mexico, where the MaNGA program is being conducted.
SDSS

TREMONTI: It’s funny, my path was actually almost the opposite of Edmond’s and Kevin’s. I remember at my first colloquium in grad school, someone showed lots of spectra of galaxies. Spectra are measurements of the brightness of objects as a function of their wavelength or color. They look like wiggly lines, sort of like cardiograms. And I thought, who could work on spectra? Why would anybody love those?

But then I started working on the Sloan Digital Sky Survey in the late 1990s. I learned that each spectrum kind of tells its own story. Now I can look at them and instantly know things about the galaxies they were obtained from, such as approximately how many stars the galaxy is making, and how old it is. So for me, the hook was spectra and all this hidden information they contained.

TKF: It sounds like we have a lot of the ‘what,’ but we’re not very clear on the ‘how’ when it comes to the life cycles of galaxies.

TREMONTI: Yeah, I think that’s putting it perfectly!

TKF: What else do you hope to learn about the life cycles of galaxies from the data you’re collecting with the MaNGA survey?

CHEUNG: These red geysers are my priority. I hope to further explore how their different aspects relate to galaxy evolution. That said, there’s so much out there that the MaNGA survey could discover. It’s an amazing dataset that is making a lot of things possible.

BUNDY: We’ve just rolled out a splash of publications in various stages of peer-review or acceptance at scientific journals. They provide a smattering of the science that is being done with MaNGA. A lot of work is focusing on the dynamics inside galaxies, studying differences between how the stars and gas are rotating. Some MaNGA team members are also trying to determine when and how rapidly stars form in different locations within a galaxy. That can that tell us about how the galaxies were assembled over time.

Another powerful thing MaNGA can do because of its large sample is look at the cosmic environment galaxies are sitting in—are they next to other galaxies or by themselves—and compare how this affects their structure and internal makeup. Because there’s just so many things being explored, it’s hard to say what the most exciting result will be that MaNGA eventually uncovers.

TREMONTI: With the original Sloan Digital Sky Survey, I did a lot of work on chemical evolution in galaxies—how the heavy elements created in stars scatter into a galaxy and enrich its gases over time. But these investigations were always incomplete because, again, as Kevin said, we were just sampling the centers of galaxies. That doesn’t tell you the whole story. Now with MaNGA, we have this more complete view of galaxies.

TKF: Let’s close the conversation with our own galaxy, the Milky Way. It’s still forming stars at a moderate rate. But could the Milky Way one day go through a red geyser phase?

CHEUNG: I think it’s a fair conclusion that eventually the Milky Way is going to become full of old, red stars. It is definitely on its way. Current studies have found that the average color of all the Milky Way’s stars places it in what astronomers refer to as the “green valley.” We think this is a transition phase between a young galaxy forming lots of new stars, which gives it a bluish color, and an old galaxy with a preponderance of red stars. After the Milky Way shuts down star formation, and we’re still not sure how black holes, or something else, do that, we think that the red geyser process is going to be critical in keeping this newly dead Milky Way from forming future stars.

Fermi bubbles of Milky Way

This artist's conception shows an edge-on view of the Milky Way galaxy, with the gamma-ray bubbles that Fermi deteted in pink.
NASA GSFC

BUNDY: From a much more speculative point of view, there’s no reason why winds from supermassive black holes couldn’t occur in galaxies that are actively forming stars. Though these winds would be harder to detect and verify, it leads one to wonder if there may be black hole-induced winds in the Milky Way nowadays.

In 2015, researchers discovered a pair of so-called Fermi bubbles, which are these giant lobes of energetic gamma rays emanating from the center of our galaxy. They probably formed because of the Milky Way’s supermassive black hole feeding on matter. [Read more about Fermi bubbles in a separate Kavli Roundtable.] Maybe this is setting the stage for when the Milky Way’s star formation is shut down and our galaxy becomes a red geyser.

TREMONTI: Those Fermi bubbles are so interesting and there’s so much we don’t know about them. Maybe they are a sign of what’s to come for the Milky Way.

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