(Fred Heeren Interviews
If the standard cosmological
picture is true – if our universe’s 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 today – then 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 hasn’t simply
always been here (as in, for example, Hoyle’s 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 don’t
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. One of astronomy’s
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 diligently –
but unsuccessfully – for 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 dozens of primeval
galaxies, the farthest and earliest ones ever observed. Here’s
an edited 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 higher redshifts.
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
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 time.
Heeren: So how have astronomers
gone about seeking these primeval galaxies over the years,
and what did you do differently to find them now?
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
Heeren: And I’ve heard
it’s like trying to find a needle in a haystack.
Steidel: And it’s much
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.**
*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, can’t 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.
What’s 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 they’re right, astronomers are now looking into
a time when galaxies were fewer, and they’ve begun to
identify an earlier epoch yet when no galaxies appear at all.
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