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How Far Back in Time Can We Actually See?
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(Fred Heeren Interviews Charles Steidel)

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 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 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 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.**



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*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).


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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.

To read more interesting articles like this one, log onto http://www.ScienceDiscussion.com

 

 

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