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Hubble hits the Red Limit. Next up: Webb Telescope

January 26th, 2011

hubble space telescope in orbit
The day had to come, and we all knew it. Hubble Space Telescope has been squinting for years, and now it’s reached the limit of its power to see back to the earliest epochs of cosmic time. As in Cosmic Time, or the amount of time elapsed since the Big Bang.

Today, a team of scientists made this exciting announcement:

SANTA CRUZ, CA–Astronomers studying ultra-deep imaging data from the Hubble Space Telescope have found what may be the most distant galaxy ever seen, about 13.2 billion light-years away. The study pushed the limits of Hubble’s capabilities, extending its reach back to about 480 million years after the Big Bang, when the universe was just 4 percent of its current age.

A story on Bad Astronomy explains the details, as does a NASA press release and one from the University of California, Santa Cruz. And the First Galaxies website provides even deeper scientific background in plain English.

As light from a distant galaxy speeds toward us, it gets stretched, or “redshifted,” by the expansion of space itself. Astronomers measure redshift with a quantity called “z.” The paper in Nature reports a redshift of z=10. 3. The first galaxies probably formed 200 to 300 million years post-Big Bang, which is more z’s than Hubble can deliver. To get to that redshift, Hubble would need instruments that can see even redder — more redshifted — light than it can now. So, in short, Hubble is at the “red limit” of what it can see.

I asked Jason Tumlinson, a galaxy researcher at the Space Telescope Science Institute to explain:

“My opinion is that we’re very near the limits of what HST can do in terms of pushing back the redshift frontier, and in fact have been operating at HST’s limits for several years. Everything depends on the performance of the cameras, and the major upgrade provided by the new Wide Field Camera 3 (WFC3) in 2009 has made a big difference.”

The upgraded WFC3 was installed on Hubble during the final servicing mission in May 2009.

I asked Amber Straughn, a Goddard astrophysicist in the Observational Cosmology Laboratory and a member of the James Webb Space Telescope (JWST) team, to explain why Hubble has reached the “red limit” of its seeing ability:

“The short answer is, at z~10, we are AT the limit of HST’s ability to look back in time. The reason for this is simply due to HST’s wavelength coverage. The light from these very distant galaxies is very, very red — and HST’s (Wide Field Camera 3) filters cut off at around 1.7 microns. . . .That’s the ‘red limit’ of HST.”

See Dr. Straughn talk to a TV reporter about the Webb Telescope.

Another issue, Tumlinson says, is the amount of Hubble telescope time available. The light-sensing detectors on Hubble contribute a certain amount of electronic “noise” that can swamp the signal from whatever you happen to be observing. To overcome this, astronomers have to schedule enough “Hubble time” to make sure the signal from the astronomical target is sufficiently stronger than the background noise – sort of like the way you have to raise your voice to be heard in a noisy room.

Tumlinson explains:

“The detector itself adds noise to the measurement — called readout noise, generally — which is an important factor in setting the faintest observable source. Of course, HST users could go deeper and push further with longer observations so that they collect more source counts relative to this noise term, but only so much time is available. “

NASA and the scientific community saw Hubble’s red limit coming. So they invented the James Webb Space Telescope. With its huge collecting mirror — 6.5 meters (21.3 feet) in diameter — and ultrasensitive infrared detectors, Webb can see longer, redder wavelengths of light, and “redder” translates to “more distant.”

Tumlinson explains:

“Discovering galaxies at high redshift is one of the top reasons NASA is building JWST. Being much larger and optimized for this sort of work, Webb should make z ~ 10 detections routine, and could push the frontier to z = 12, 15, or even higher.”

Z=15 is around 275 million years after the Big Bang — the sweet spot for observing the first stars and galaxies forming. Stay tuned!

OH AND DID I MENTION? All opinions and opinionlike objects in this blog are mine alone and NOT those of NASA or Goddard Space Flight Center. And while we’re at it, links to websites posted on this blog do not imply endorsement of those websites by NASA.

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