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First observations test JWST capabilities

17 November 2022

Although the telescope’s early science results have exceeded expectations, even the most seasoned observers must use caution until calibrations are complete.

Pillars of Creation, as seen by HST and JWST.
Images of the Pillars of Creation taken by Hubble (left) and JWST (right) reveal differences in the telescopes’ resolution and target wavelengths. Credit: NASA, ESA, CSA, STScI; Joseph DePasquale (STScI), Anton M. Koekemoer (STScI), Alyssa Pagan (STScI)

For UCLA extragalactic astronomer Tommaso Treu, the opportunity to lead one of the James Webb Space Telescope’s (JWST’s) first observing programs wasn’t just a chance to live out his undergraduate dreams of glimpsing the oldest galaxies in the universe. It was also an opportunity to try out the most sophisticated telescope ever sent into space.

JWST’s technical capabilities have, on many levels, exceeded expectations. “It’s right to be excited and raving about the facility,” says James Dunlop, a University of Edinburgh extragalactic astronomer whose large-scale galaxy survey program will take place primarily in early 2023. But he and his colleagues will need to correct those upcoming measurements using calibrations that don’t yet exist—and doing those calibrations will take time. “We have this fantastic telescope that we want to push to the limits,” Treu says. “But we can’t yet because it’s not fully calibrated.”

Greater expectations

“We’ve been spoiled,” Treu says. For 32 years, astronomers using the Hubble Space Telescope (HST) have accumulated the knowledge, experience, and software to calibrate their data exquisitely and to understand nearly every case of noise and distortion caused by the telescope’s detectors. “Working with HST data is the easiest job you can have in astronomy,” he says. In contrast, data from JWST’s first observation programs still lack that level of certainty.

Program managers at the Space Telescope Science Institute (STScI) in Baltimore decided not to wait for perfect calibrations before beginning scientific observations. According to Massimo Stiavelli, JWST mission head at the STScI, the decision was strategic. “The alternative would not be acceptable,” he says. “People are eager to do science and always want it to be faster.” As a result, preliminary data are being made available to investigators and to the public before final on-orbit calibrations are complete. Stiavelli says that some astronomers have had unreasonably high expectations about the level of calibration that could be achieved for the telescope’s earliest science observations.

The calibration process involves observing well-studied objects, such as stars with precisely known spectral properties. Then, the spectra obtained by each instrument is reverse-engineered to match established values. As a placeholder while on-orbit work is being done, initial calibrations derived from a combination of ground-based observations and computer simulations were provided by the teams that built the instruments. Going forward, observing programs that are partly designed to calibrate the telescope’s instruments will run alongside research programs, though the STScI team expects the time dedicated to calibration to decrease over the years.

Thousands of galaxies appear in this JWST Near Infrared Camera image, which was released in July. Credit: NASA, ESA, CSA, STScI

Some of JWST’s early scientific observations have purposely involved measurements that are not extremely sensitive to calibration uncertainties. Many of them use the Near Infrared Camera (NIRCam), which counts photons from targets using 10 detectors and 29 filters to capture different wavelengths of light with extreme sensitivity. However, accurately measuring the flux of photons from a distant galaxy is not that simple. The measured flux varies from one detector to another and depends on the incoming light’s wavelength and the selected filter. That issue was relevant for one of JWST’s first discoveries: two star-forming galaxies at redshifts 10 and 12, observed as they existed around 300–500 million years after the Big Bang. Their identification was based on a sharp decline in flux between two of NIRCam’s filters.

The analysis of those two galaxies revealed that some measurements are robust even without a full calibration. A star-forming galaxy emits very blue light from young stars, but the gas between the galaxy and JWST is opaque to certain photons. The result is a spectrum that’s bright above a certain wavelength and nonexistent below another. Repeatedly analyzing NIRCam data assuming different correction factors for matching observed flux magnitudes to target standard ones consistently revealed that flux decline, leading observers to conclude that those young galaxies are brighter and more abundant than expected. “We can make meaningful and correct statements even if we have, say, 20% uncertainty on the flux,” says Treu. That 20% figure should be greatly reduced after calibrations are complete.

Spectrographic studies require better calibration. One example is an observing program that examines how star formation and evolution depend on chemistry in the Magellanic Clouds and in the Milky Way. That work uses the Near Infrared Spectrograph (NIRSpec) to take spectra of many stars at once. NIRSpec uses a grid of 248 000 small microshutter cells that can be opened and closed to transmit or block light from different parts of the sky, allowing simultaneous capture of spectra from 100 points. Calibration requires understanding how much light that leaks from one microshutter may find its way into the spectra from an adjacent one. “The flux calibration for each slit is not easy to make using observational data,” Stiavelli says. As a result, “NIRSpec has relied primarily on computer modeling for calibration.”

Another challenge is that the target stars in the Magellanic Clouds and Milky Way are often in dense star-forming regions, and light from the surrounding nebulas can flood the field of view and fluctuate in brightness over small scales. “This makes estimating how much nebular light versus starlight contributes to our spectra a tricky art,” says Ciaran Rogers, a PhD student at Leiden University who is analyzing the stellar data. Addressing the problem also involves looking at past data. “We’ve relied on data from Hubble to tune the processing parameters,” Rogers says. But because of the telescopes’ differing target wavelengths, “there’s been a lot of trial and error.”

NASA engineers examine the JWST Fine Guidance Sensor/Near Infrared Imager and Slitless Spectrograph in 2012. Credit: NASA/Chris Gunn

Another spectrographic study uses JWST’s Near Infrared Imager and Slitless Spectrograph (NIRISS), which can take simultaneous spectra of thousands of objects within a wide field of view, to look for dust in spiral galaxies that have little or no star formation. Gaining confidence in those spectra requires precise calibration to distinguish spectral lines that appear very close to each other and to determine the uncertainty in the difference in wavelengths between far-apart lines. “The spectrographs are lagging because there is less data so far, and analysis is more complex,” says Adriano Fontana, research director at the Astronomical Observatory of Rome and a co-investigator on two JWST observing programs. Meaningful spectroscopy should be possible in the coming months.

More excitement ahead

Despite calibration challenges, observers report few complaints. “The resolution of the images, especially in shorter wavelengths, exceeds expectations,” says Fontana. Even as the JWST hardware and electronics settled into the conditions of outer space, the telescope’s diffraction limit exceeded specifications. Additionally, “the launch was so perfect that there’s extra fuel,” says Dunlop. Those reserves will power the small rockets that hold JWST in position, potentially doubling the telescope’s lifetime to 10–20 years.

The combination of promising new cosmic targets and ever-improving calibration has astronomers eager to begin the next cycle of observing programs. For example, multiple Cycle 1 programs, including early observations from Dunlop’s large-scale survey, have identified a redshift 17 galaxy that formed about 230 million years after the Big Bang. Once calibrations are further along, reliable measurements regarding that galaxy’s composition and star formation rates will be possible. “We expect new proposals around objects that we didn’t even know about and can now observe in detail,” says Fontana. The call for Cycle 2 proposals opened on 15 November.

Now that astronomers have a taste of what the instruments can actually do, Fontana expects increasingly ambitious proposals. Cycle 1 has been a proving ground. Cycle 2 could provide tantalizing new surprises as the telescope settles in for the long haul.

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