Who Needs Galaxies?
Astronomer Sandra Faber explains how galaxies brew the ingredients for life.
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We would not be here if it were not for our galaxy, says Sandra Faber, University Professor of Astronomy at the University of California, Santa Cruz. That's because the strong gravitational pull of galaxies like the Milky Way sucks in the heavy elements that go into making up planets and people and all other non-gaseous matter in the universe.
This is just one of many reasons Faber has dedicated her career to studying galaxies, most recently as a core member of the large-scale survey of distant galaxies known as DEEP (Deep Extragalactic Evolutionary Probe). Here, find out why she believes life could have arisen elsewhere billions of years before Earth even existed, what message she feels the DEEP Survey has for humankind, and why she says studying fewer than 65,000 galaxies as part of the survey simply will not do.
Galactic awe
What do you find so beautiful about galaxies?
Faber: They're magnificent in terms of their scope, their stretch, their luminosity. Their forms are lovely. They have spiral arms, regular in some cases, disturbed in others. Some of them are colliding. Some of them are sources of unimaginably powerful radiations from their nuclei. They're just a wonderful array of things that are all similar and yet different. They really merit a whole lifetime of study.
Do you have favorite galaxies?
Oh, definitely. For a galactic astronomer, the first galaxy that comes to mind is our partner in space, the Andromeda Galaxy, M31, which is the other big galaxy in our local group. It's a little bit bigger than our Milky Way, but in essence looking at Andromeda is rather like standing there and looking back at ourselves.
There are some galaxies that not only teach us things but are just gorgeously beautiful to look at. My favorite example is the Antenne, which is a pair of colliding galaxies. Inside, the collision is stimulating a fireworks of star formation. Beautiful pictures have been taken of this galaxy with the Hubble Space Telescope. This galaxy is not like ours or Andromeda, but someday, when our galaxies collide, as they will, we may very well look like the Antenne.
My favorite galaxy of all is called the Sombrero, NGC 4594. It's an amazing galaxy that is really two galaxies in one. It consists of a more or less spherical pile of stars that's probably the result of some early collision. But embedded in that is a rotating disk of gas and stars. It's an elliptical galaxy and a spiral galaxy all in one. And we see it beautifully just barely inclined, so that you see the disk silhouetted in this spherical pile of stars.
What is the role of galaxies on the road to life?
You need to collect matter into dense lumps just to make stars, and it's the gravitational field of galaxies that does that. You have to understand that the heavy elements, the metals that come out of supernovae—that's what we need in order to make planets. You're not going to make planets and people out of hydrogen and helium. Supernovae are constantly enriching the interstellar medium with their products, but their ejecta come out at speeds of up to 20,000 kilometers per second. Those ejecta would just whiz away—they would just dissipate. What they need is a very powerful gravitational force to retain them and pull them back. That's what a galaxy does.
So the galaxy is like a pot cooking up an ever richer stew?
You could think of the galaxy as a sort of stove with lots of pots on it, and the pots are the interstellar medium, like a chicken broth getting stronger with every reduction. Every time a supernova goes off and sends its heavy elements out into the interstellar medium, we cook up a stew that's better and better for solar systems and life.
Looking for life
What is the minimum criterion for life?
Our solar neighborhood! Why? Because we're here. We succeeded in forming right here. And the story of recent work, mine and other people's, is that there are places in lots of galaxies that really look like our solar neighborhood.
What do you mean by solar neighborhood?
I mean our part of the galaxy. We're about a half or two-thirds of the way out from the center of our galaxy. There's an inner part where the stars formed earlier and are older, and there's an outer part that is still mostly gas, where the stars are still forming. Our solar neighborhood is the medium region in between, which is still a mixture of stars and gas. We see a lot of stars in it that have the same age as our sun and kind of a similar composition. This is where we would look first to find other solar systems. And we're finding them.
Is it easy to find these places? As an astronomer who wants to look for life, where would you start?
One of the most important messages of astronomy vis-í -vis life of the last 20 to 30 years is that our solar neighborhood is a completely normal place. When you look around us, there are dozens, hundreds of galaxies pretty close to us that look rather like our Milky Way. Each one of them has these intermediate neighborhoods of stars and gas, with stars about the same age as the sun, same mass, same brightness, same composition, same amount of heavy elements.
"It's quite likely that planets and solar systems like ours could be forming in other galaxies in great numbers."
Also, there's a nice, orderly progression: stars in circular, calm, orderly orbits around the center of the galaxy. No violent collisions taking place there, but rather quiescent, steady star formation. That's how our sun formed. We know that's a good recipe for making solar systems. So it's quite likely that planets and solar systems like ours could be forming in other galaxies in great numbers.
What are you doing now to understand the evolution towards life?
It's important to realize that astronomically, the seeds of life on Earth were sown four and a half billion years ago when the sun and solar system formed. That's a long time back in the past. But can we see the seeds of life in other galaxies in great abundance back then, or maybe even perhaps earlier than that? What we need to do is make big surveys of thousands of galaxies and study their stellar content and their metal abundances, their heavy element composition, to find out whether or not and when galaxies became ripe for making life.
Spectral fingerprints
What are the tools you need to look for life?
To look for life, or the readiness for life, far back in the past, you need a huge telescope to gather lots of light, and a spectrograph to spread the light of galaxies out into a spectrum. From a spectrum, we can learn many, many things that are important about galaxies as habitats for life.
What exactly is a spectrum?
A spectrum is the light of a galaxy spread out into its colors. It's a rainbow—there's a red end and a blue end. And when you look at a spectrum in detail, you can see features. You can see bright regions and regions that are less bright. These spectral features are the signature of atoms and molecules in that galaxy, even if the galaxy is billions of light-years away.
Basically you can think of a spectrum as being a fingerprint of a galaxy. Every spectrum is unique because the galaxies are all slightly different. And each fingerprint tells you what the galaxy is made of, how it's spinning, how far away it is, and how big it is. [For more on how to interpret spectra, see Decoding Cosmic Spectra.]
"We're really looking back almost to the beginning of the universe."
So what you're saying is that if you have the right tools, you can actually look from Earth and learn what a galaxy is made of?
Using spectra as our tool, we can tell you what elements exist in that galaxy—oxygen, carbon, iron—and we can tell you whether the galaxy is rich in those elements. Has the broth cooked a lot, or is it still too dilute to make planets?
How do you make a spectrum?
Well, these sources are unimaginably faint, and it takes a lot of light to make a spectrum. So you've got to start with a big telescope like the Keck Telescopes, for example.
You bring the light to a focus, and it goes through a small hole into your device called the spectrograph. Inside the spectrograph is an optical element that spreads the light out into a rainbow. It could be as simple as a prism, like the crystals you hang in your window that spread sunlight out into a rainbow. You take a picture of this stripe of light, and in that stripe are spectral features whose presence tells you the existence and amount of these important heavy elements.
Back to the beginning
Can I see a galaxy with my naked eye when I look up at the sky?
You can see one galaxy with your naked eye, the Andromeda Galaxy, and it's two million light-years away. Its photons are quite old; they've been in transit for two million years. But two million years is a lot shorter than the photons we're measuring in our DEEP Survey, which get up to nine, even 12 billion years old. We're really looking back almost to the beginning of the universe. The galaxies we're looking at are about a million times fainter than anything you can see with your naked eye.
But you must have to look much further back to see galaxies born around the time our sun was born, right? What kind of equipment do you need to do that?
This is a special moment in astronomy when the size of the telescopes and detectors is big enough, for the first time, to look back billions of years. With the DEEP Survey, we are collecting the spectra of 65,000 galaxies, which will be the first census of the state of the universe at just about the epoch when galaxies became ready to harbor life.
Why 65,000?
We need 65,000 galaxies in order to get a really good idea about what is out there. All galaxies are different. It takes a huge census to understand what's there. It's like understanding the people on Earth. You need a lot of people in order to figure out who's living here.
How on Earth do you image 65,000 galaxies in a lifetime even? I remember when Hubble first went up, it was taking hours just to get one shot.
You have to be pretty clever! It's not enough just to have a big telescope. You have to put on it a spectrograph that's very efficient and that can observe lots of galaxies at one time. In a single exposure with our spectrograph called DEIMOS [Deep-Imaging Multiobject Spectrograph], we're taking 150 galaxies, and we're gathering a total of 1,000 galaxies per night. It's this high throughput that makes these vast surveys possible.
How difficult was it to build DEIMOS?
I would say it's the hardest thing I ever did. I'm just now trying to claw my way back into the science after spending six years building DEIMOS. It was the most anxiety-ridden thing I ever did in my life. It's not like writing a scientific paper. If a science project falls through, who fails, who's hurt? Just the author and maybe a couple of other people. But DEIMOS is a big project, $10 million, involving the work of many people. If it failed, it would have been a total catastrophe for our observatory, the scientists who hope to use it. A lot was riding on it.
Was there a moment when you thought it might fail?
Well, it was often hard to sleep at night. I would wake up at four in the morning, worrying about a particular problem that needed to be solved, and quickly. We were constantly running out of time and out of budget, worrying that our window of opportunity to build the spectrograph would close before we finished it.
Findings
Where does the DEEP Survey stand at the moment?
We've been working on the DEEP Survey now for about a year, and we're pretty much on schedule. We've collected about 25 percent of all our spectra, which means that we have about 15,000 spectra in the can. We've only analyzed about 5,000 of them. It's a terrible problem keeping up with this torrent of data. You go to the telescope; you have to keep observing. Meanwhile, the spectra keep coming in, and there's no time to really think about them.
"The habitat for life is everywhere."
We have managed to look at a few hundred spectra, though, in some detail. We're measuring the strength of these features that tell us the heavy element content of galaxies back in time. And there's a particular line ratio, the ratio of hydrogen to oxygen, that is a real bellwether for the formation of life, not only because oxygen is important for life, but because its production in supernovae is accompanied by carbon and nitrogen and the other elements that make up life. The same supernovae produce both.
So we're now just getting to the point with the first sample of a few hundred galaxies of looking back in time, and we see very interesting trends in the oxygen abundances of these galaxies as a function of time.
For instance?
What we're finding is very interesting. We didn't expect it, but in retrospect, we might have anticipated it. We can divide our galaxies into two rough categories, big and small. We're finding that the small galaxies are following the formula: the cooking pots are cooking up the broth, and nine billion years back in time, we see a lot less oxygen than we see today.
What's taken us by surprise is the big galaxies. Their oxygen is the same back in time as it is now. This flies in the face of theory, because we know the rate at which galaxies are cooking stars, making supernovae, and producing heavy element products. We know that there should have been a lot more heavy elements produced from then to now.
So how do you reconcile these two observations, that galaxies are constantly making stars over billions of years and yet at the same time don't seem to be increasing their oxygen? How is this possible?
Our present view is that these large galaxies are organized into rings, and they essentially form from the inside out. In a ring, it starts out as gas. The gas gets converted into stars. The heavy elements are cooked. We make stars like the sun—solar abundance. But then the gas gets used up, and that ring dies. The next ring farther out, which is gas-rich, goes through the same cycle.
So what we think in looking back eight or nine billion years in time is that we are looking at the inner rings of galaxies that are just going through their heavy-element production cycle. And they have the same oxygen abundance at their peak that all rings do when they go through their cycle of stars.
If this picture is true, we have a situation in which large galaxies produce habitats for life in a wave that extends from the center to the outside over billions of years. This explains our Milky Way, because our sun was formed relatively recently, four billion years ago. That was the point at which the wave of heavy-element enrichment hit our radius and triggered a level that was suitable for a solar system.
So if we think of a galaxy as a city of stars, like actual big cities, big galaxies seem to develop from the center outwards—an urban core, successive waves of Levittowns, and suburbia. Then, way out, we have farmland, which hasn't turned into houses yet. It's a wave of habitats for life that steadily expands as the galaxy ages.
Does this mean life could have gotten started elsewhere billions of years before our solar system even formed?
What the DEEP Survey is telling us is that, in a lot of galaxies, there may have been enough heavy elements to make life a lot earlier than the sun, as early as nine or ten billion years back in time.
The interesting thing is that those solar systems would be close to the centers of their galaxies. So if you wanted to look at a galaxy today, say our Milky Way, and you were determined to find really old solar systems that could harbor life, what this says is you should look towards the center of our galaxy. The center could be the cradle of really ancient solar systems.
A beautiful story
Is there a message in all of this for us here on Earth?
A message of the DEEP Survey for humankind is that our universe is hospitable to life, that there are billions and billions of galaxies everywhere cooking elements and making stars that are ripe for solar systems, that this process started early, and that, in most galaxies, you could have formed solar systems way before our own Milky Way formed. The habitat for life is everywhere.
The message of the DEEP Survey and all the other information that we're getting is a beautiful story. It's a new version of Genesis, a new version of the cosmic myth, only this time it's scientifically based, from the big bang to now. Big bang, formation of galaxies, formation of heavy elements in supernovae, sun, Earth, life—one unbroken great chain of being.
"We are the first generations of humans who are studying the universe billions of years ago as it formed."
And as we look out into the universe, we see this happening all over. It's as though the universe is a giant garden where flowers hospitable to life, habitats hospitable to life, are blooming all over. It remains for us to see if we can verify that these potentially powerful and favorable habitats are actually giving rise to life as we see it here.
So do you think there's life elsewhere, even intelligent life?
When an astronomer is asked, "Is there life elsewhere?" and "Is there intelligent life?," the first thought is to resort to the rule of large numbers. If you see 100 billion galaxies—and that's how many galaxies we can see with the Keck Telescopes—each with 100 billion stars in it, play the odds. Life could be rare, but by the time you multiply this huge number by however small a fraction of those might contain life, it looks like an attractive proposition that there is life out there, not only in our own Milky Way, but in all these other galaxies that we can see with the Keck Telescopes.
With so many galaxies to cover, does your work with the DEEP Survey ever seem like drudgery, or are you still as inspired as ever?
Well, when you first start to study a science, you can learn great things from one object. But then your next step, you probably need 10 objects. We are to the point in cosmology where we need thousands of objects. At each point in that process, you always feel the pull of the unknown. The next questions are just as interesting as the previous ones. Just because you need a massive effort, a production line, to get to the next step does not make those of us who are doing that feel any less inspired. We are the first generations of humans who are studying the universe billions of years ago as it formed. I think that's the most romantic scientific question you could envision.