Webb’s Mid-Infrared Spectroscopy Will Help Unveil Molecules, Elements

The team behind NASA’s James Webb Space Telescope is still working through the 17 science instrument configurations. This week, they have completed the following tasks by numbers:

(5) NIRCam grism time series and (4) imaging time series, both used to study exoplanets and other time-variable sources; (12) NIRISS aperture masking interferometry mode, for direct detection of a faint object that is very close to a bright one; (11) NIRISS wide-field slitless spectroscopy, for studying distant galaxies; and (9) NIRSpec bright-object time series, for studying exoplanets. So far, seven modes have been accepted, with ten more to go.

We’re highlighting MIRI’s medium-resolution spectroscopy mode this week, as well as presenting our first spectroscopic engineering data. We asked David Law of the Space Telescope Science Institute (STScI) and Alvaro Labiano of the Centro de Astrobiologia (CAB) of the MIRI commissioning team to clarify this mode to us:

“The MIRI Medium Resolution Spectrometer is one of Webb’s most complicated instrument modes” (MRS). The MRS is an integral-field spectrograph that gives spectral and spatial data for the complete field of view at the same time. The spectrograph generates three-dimensional ‘data cubes,’ each of which includes a unique spectra for each pixel in a picture.

How Are They Efficient?

They combine the benefits of both traditional imaging and spectroscopy, such spectrographs are particularly useful tools for studying the composition and kinematics of celestial objects.

“The MRS has a spectrum resolving power of roughly 3,000 (observed wavelength divided by smallest detectable wavelength difference).” That level of resolution is sufficient to resolve crucial atomic and molecular characteristics in a number of situations. The MRS will be able to analyze hydrogen emission from the first galaxies at the highest redshifts.

What Are Its Objectives?

It will identify the brilliant spectral fingerprints of elements such as oxygen, argon, and neon that can teach us about the characteristics of ionized gas in the interstellar medium at lower redshifts, probing molecular hydrocarbon signatures in dusty neighboring galaxies.

Closer to home, the MRS will create maps of spectral characteristics caused by water ice and basic organic compounds in our own solar system’s large planets and planet-forming disks around other stars.

“The MRS integral field units are broken up into twelve independent wavelength bands, each of which must be calibrated individually, in order to cover the wide 5 to 28 micron wavelength range as efficiently as feasible.” The MIRI team (a big international group of astronomers from the United States and Europe) has spent the last few weeks largely calibrating the MRS imaging components.

They want to make sure that all twelve bands are spatially aligned with each other and with the MIRI Imager so that it can accurately position targets into the MRS’s smaller field of view.

How Can We Use This Observation?

Using observations of the brilliant K massive star HD 37122, we can demonstrate some early test results from this alignment method, exhibiting the image quality produced in each of the twelve bands (located in the southern sky near the Large Magellanic Cloud).

“Once the spatial alignment and image quality of the various bands have been thoroughly described, the MIRI team will focus on calibrating the instrument’s spectroscopic response.” Observations of compact emission-line objects and diffuse planetary nebulae produced by dying stars will be used to determine the wavelength solution and spectral resolution in each of the twelve areas of view.


With a tiny portion of a spectrum derived from recent engineering observations of the active galactic nucleus at the heart of Seyfert galaxy NGC 6552, we demonstrate the MRS’s outstanding spectral resolving capacity. Once these fundamental instrument features are determined, MRS may be calibrated to serve the plethora of Cycle 1 research programmes that will begin in just a few weeks.”


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