How the James Webb Space Telescope will unlock the secrets of the first galaxies

We're going for a deep dive on the universe.
Brad Bergan
Webb in space (left), and a 3D-rendered galaxy (right).1, 2

NASA is preparing the James Webb Space Telescope's 17 science instrument modes, and while there are ten left to check off, we have a moment to review some that are already ready to go.

This week, the Webb team gave the go-ahead for the NIRCam grism time series, the imaging time series, the NIRISS aperture masking interferometry function, NIRISS wide-field slitless spectroscopy, and the NIRSpec bright-object time series.

That makes seven modes ready to go aboard the James Webb Telescope. So strap in for the future of space-based astronomy.

The James Webb Space Telescope will zoom in on the first galaxies

A recent NASA blog post features MIRI's medium-resolution spectroscopy (MRS) mode, and the prospect of sharing related engineering data. MIRI's MRS is one of the telescope's most sophisticated instrument modes, consisting of an integral-field spectrograph, "which provides spectral and spatial information simultaneously for the entire field of view," said Alvara Labiano of the Centro de Astrobiologica (CAB) and David Law of the Space Telescope Science Institute (STScI), in the NASA blog post.

"The spectrograph provides three-dimensional ‘data cubes’ in which every pixel in an image contains a unique spectrum," added Law and Labiano. "Such spectrographs are extremely powerful tools to study the composition and kinematics of astronomical objects, as they combine the benefits of both traditional imaging and spectroscopy."

The MRS was made to retain a resolving power — which is the observed wavelength divided by the smallest detectable wavelength difference — of roughly 3,000. "That is high enough to resolve key atomic and molecular features in a variety of environments," said Law and Labiano in the post. "At the highest redshifts, the MRS will be able to study hydrogen emission from the first galaxies."

MIRI_MRS 1536x472
Behold the first engineering data cubes for each of MRS' twelve spectral bands. Source: NASA / ESA / MIRI Consortium

Characterizing image quality and spatial alignment for Webb's MIRI

"At lower redshifts, it will probe molecular hydrocarbon features in dusty nearby galaxies and detect the bright spectral fingerprints of elements such as oxygen, argon, and neon that can tell us about the properties of ionized gas in the interstellar medium," added Law and Labiano. "Closer to home, the MRS will produce maps of spectral features due to water ice and simple organic molecules in giant planets in our own solar system and in planet-forming disks around other stars."

When the image quality and spatial alignment of several bands of information are "well characterized," the Webb team responsible for MIRI will work to calibrate the spectroscopic response of the instrument. This involves several steps, like narrowing down the correct wavelength and spectral resolution in all twelve fields of view for Webb — which will be accomplished using observations of "compact emission-line objects and diffuse planetary nebulae ejected by dying stars," explained Law and Labiano.

"We show the exceptional spectral resolving power of the MRS with a small segment of a spectrum obtained from recent engineering observations of the active galactic nucleus at the core of Seyfert galaxy NGC 6552," said the pair of scientists. Once the MRS is ready, it will play a vital role during "Cycle 1" science programs — which are the first several tasks the James Webb Space Telescope will execute in the service of genuine, space-based astronomy. And it's only a few short weeks away.

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