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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
NOVA: 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.
NOVA: Do you have favorite galaxies?
Faber: 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.
NOVA: What is the role of galaxies on the road to life?
Faber: 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.
NOVA: So the galaxy is like a pot cooking up an ever richer
stew?
Faber: 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
NOVA: What is the minimum criterion for life?
Faber: 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.
NOVA: What do you mean by solar neighborhood?
Faber: 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.
NOVA: Is it easy to find these places? As an astronomer who
wants to look for life, where would you start?
Faber: One of the most important messages of astronomy
vis-à-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.
NOVA: What are you doing now to understand the evolution
towards life?
Faber: 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
NOVA: What are the tools you need to look for life?
Faber: 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.
NOVA: What exactly is a spectrum?
Faber: 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.]
NOVA: 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?
Faber: 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?
NOVA: How do you make a spectrum?
Faber: 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. [Editor's
note: Built atop the dormant Hawaiian volcano Mauna Kea, the two
Keck Telescopes each have a diameter of 33 feet and are among the
largest telescopes ever built.]
“We’re really looking back almost to the beginning of
the universe.”
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
NOVA: Can I see a galaxy with my naked eye when I look up at
the sky?
Faber: 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.
NOVA: 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?
Faber: 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.
NOVA: Why 65,000?
Faber: 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.
NOVA: 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.
Faber: 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.
NOVA: How difficult was it to build DEIMOS?
Faber: 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.
NOVA: Was there a moment when you thought it might fail?
Faber: 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
NOVA: Where does the DEEP Survey stand at the moment?
Faber: 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.
NOVA: For instance?
Faber: 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.
NOVA: 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?
Faber: 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.
NOVA: Does this mean life could have gotten started elsewhere
billions of years before our solar system even formed?
Faber: 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
NOVA: Is there a message in all of this for us here on Earth?
Faber: 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.
NOVA: So do you think there's life elsewhere, even
intelligent life?
Faber: 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.
NOVA: 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?
Faber: 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.
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