INTERVIEW OF THE WEEK
CHUCK STEIDEL, Ph.D., head of the international team of astronomers that recently
discovered scores of primeval galaxies.
... We actually think we're seeing the central bulge regions of galaxies forming, that
is the round part in the middle of a spiral or an elliptical galaxy forming, where you
expect all of the star formation to be happening in a relatively small region.
For this interview, Day Stars president, Fred Heeren, visited the California
Institute of Technology in Pasadena, California to interview astronomer Chuck Steidel.
How Far Back in Time Can We Actually See?
|The deepest view of a Hubble Deep Field
image, with arrow pointing to a galaxy that may be the most distant yet known. These
galaxies are four billion times fainter than the limits of human vision. Caltechs
Chuck Steidel has identified the very earliest ones among themand found them to look
very different from the galaxies of recent times. Courtesy of NASA
Note from Fred Heeren:
One of astronomys long-sought goals has been to
fill in the great gap in knowledge of events between the initial decoupling of light from
matter (observed in the microwave background) and modern-day galaxies. For twenty years,
astronomers have searched diligentlybut unsuccessfullyfor a way to single out
a population of the very earliest galaxies. If the holy grail for physicists has become a
grand unified theory, then one of the biggest holy grails for astronomers has become the
finding of a collection of primeval galaxies.
Recently I visited Caltech to interview Chuck Steidel,
the young astronomer who led the team that found this holy grail. Using a method called
the ultraviolet dropout technique, Dr. Steidel and his colleagues have detected and
confirmed their finding of scores of primeval galaxies, the farthest and earliest ones
ever observed. Heres a portion of our conversation.
STEIDEL: Until very recently,
everything beyond a redshift of one was pure speculation, and there were essentially no
observations that could confirm or rule out any theory of what's going on with galaxies at
HEEREN: So you made your observations with a variety of
telescopes, and then used the Keck, the largest telescope in the world, to confirm that
they were really that distant.
STEIDEL: That's right.
HEEREN: What do these galaxies look like?
STEIDEL: ....We actually think we're seeing the central
bulge regions of galaxies forming, that is, the round part in the middle of a spiral or an
elliptical galaxy, where you expect all of the star formation to be happening in a
relatively small region. And those parts of galaxies we see today are also the parts that
we think are the oldest stars in those galaxies.
HEEREN: And you're saying that modern galaxies have the
oldest stars in the bulges, is that right?
STEIDEL: That's right.... It's still somewhat
controversial. But there isn't any doubt that we're finding a number of these things that
matches fairly closely to the number that you would expect to find if you were finding the
progenitors of the present-day, bright galaxies.
HEEREN: So all this adds up to looking like the
universe truly has changed with time, as opposed to having always been there?
STEIDEL: Oh, absolutely. It's absolutely changed with
HEEREN: So how have astronomers gone about seeking
these primeval galaxies over the years, and what did you do differently to find them
STEIDEL: The way that people have looked for these in
the past tended to be looking for particular, spectacular fireworks of stars going off all
at once. And so they were looking for relatively rare events, using, generally speaking,
narrow-band filters tuned to find an emission line that comes from hydrogen atoms. And you
have to have the filter exactly tuned to that wavelength to see it.
HEEREN: And I've heard it's like trying to find a
needle in a haystack.
STEIDEL: And it's much more difficult.
HEEREN: So rather than try to find something that stands
out you're trying to find something that drops out?
STEIDEL: That's correct. It's a very simple technique,
where we take pictures through different filters, very deep images of the sky with CCD
detectors,* and we take three filters, and we look for objects that are present through
two of those filters, and they completely disappear in the third. And the reason they
disappear is because they're at a high enough redshift.**
Three images demonstrate how Dr. Steidel finds distant galaxies, searching for those whose
ultraviolet light is absorbed by intervening hydrogen gas, so that they "drop
out" when viewed through a UV filter
|*CCD stands for charge-coupled
device, an electronic light-detector that contains a silicon chip covered with
light-sensitive pixels. These capture images in a fraction of the exposure time required
by a conventional camera and film, creating a digital image
**This ultraviolet dropout technique
(developed by Steidel and Donald Hamilton of the Max Planck Institute for Astronomy in
Germany) makes use of the fact that the most distant galaxies, while showing up at red and
green wavelengths, cant be seen in the ultraviolet, unlike nearer galaxies. Light
from the farthest galaxies has much more hydrogen gas to pass through before it reaches
us, and this intervening hydrogen gas absorbs ultraviolet light. The technique of
searching for ultraviolet dropouts now gives Steidel and his colleagues the
opportunity to systematically find a whole population of very early galaxies, rather than
relying heavily on luck to spot an occasional protogalaxy by more traditional methods
(such as searching for galaxies emitting a wavelength of light called Lyman-alpha, the
signal of newborn stars).
HEEREN: Tell us the latest on what you've been finding
when you try to find galaxies farther back in time yet, at redshifts beyond four, where we
expect to find the very beginning of the universe.
STEIDEL: We're probably seeing the period of time during which big
galaxies were first coming together.... It's absolutely clear that the number of things is
much lower at redshift four than it is at redshift two-and-a-half or three. And so I think
it's safe to say that the epoch between, say, a redshift of four and a redshift of
two-and-a-half, is a very important one in galaxy formation. And I think what we're seeing
is the galaxies coming onto the scene directly, which I think is fairly exciting.
|Whats exciting about this, and what Steidel only hints at here very cautiously
(and others are saying more explicitly from his evidence), is that telescopes are already
powerful enough to show us the time before galaxies began to light up. If theyre
right, astronomers are now looking into a time when galaxies were fewer, and theyve
begun to identify an earlier epoch yet when no galaxies appear at all.
If the standard
cosmological picture is trueif our universes history can be divided into
stages in which it began in a big bang, galaxies formed from clouds of gas, stars
gradually fired up first in the hubs of galaxies, and then galaxies matured relatively
quickly into the types we see todaythen the most convincing proof of all this to the
non-scientist would be to look out across the light-years to actually see back to a stage
when all galaxies were forming. Observing a universe of baby-looking galaxies would be the
most direct evidence possible to demonstrate that the cosmos hasnt simply always
been here (as in, for example, Hoyles early schemes of old galaxies being
continually replaced by new ones, born out of new "creation fields").
Because the standard cosmological picture is no longer controversial, astronomers today
dont spend their time trying to prove it. But in striving to improve their
understanding of galaxy evolution, their observations continue to add to the evidence.
You'll find the whole history of these twentieth century cosmological discoveries, and the
bigger implications for everyday life, in Day Star's new book, Show Me God.
DAY STARS DISCUSSION KICKER OF THE WEEK:
Though the big bang theory is no longer controversial among cosmologists, has the
evidence behind it gotten through to the public yet? Should the fact that our universe had
a beginning make any difference to the average person? What are the implications of a
universe that begins, as opposed to the concept of a static, eternal universe (which was
the scientific view from Aristotle to Einstein)? Go to the
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