The James Webb Space Telescope team is still working through the 17 science instrument modes. This week, they checked off (5) NIRCam grism time series and (4) imaging time series, both of which are 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, so far, seven modes have been approved, with ten more to go.
We’re highlighting MIRI’s medium-resolution spectroscopy mode this week and sharing our first spectroscopic engineering data. We asked David Law of the Space Telescope Science Institute (STScI) and Alvaro Labiano of the Centro de Astrobiology (CAB) of the MIRI commissioning team to explain this mode to us:
“The MIRI Medium Resolution Spectrometer is one of Webb’s most complex instrument modes” (MRS). The MRS is an integral-field spectrograph that provides spectral and spatial information for the entire field of view at the same time. The spectrograph generates three-dimensional ‘data cubes’ in which each pixel in an image has its own spectrum. Because they combine the benefits of traditional imaging and spectroscopy, such spectrographs are extremely powerful tools for studying the composition and kinematics of astronomical objects.
“The MRS is designed to have a spectral resolving power of around 3,000 (observed wavelength divided by the smallest detectable wavelength difference). That is sufficient to resolve key atomic and molecular features in a wide range of environments. The MRS will be able to study hydrogen emissions from the first galaxies at the highest 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 at lower redshifts.Closer to home, the MRS will create maps of spectral features caused by water ice and simple organic molecules in our solar system’s giant planets and planet-forming discs around other stars.
“To cover the broad 5-to-28-micron wavelength range as efficiently as possible, the MRS integral field units are divided into twelve individual wavelength bands, each of which must be calibrated separately.” The MIRI team (a large international group of astronomers from the United States and Europe) has spent the last few weeks primarily calibrating the MRS imaging components.
They want to make sure that all twelve bands are spatially well aligned with one another and with the MIRI Imager, so that it can accurately place targets into the smaller MRS field of view.We present some preliminary test results from this alignment process, demonstrating the image quality obtained in each of the twelve bands using observations of the bright K giant star HD 37122. (Located in the southern sky near the Large Magellanic Cloud).
“Once the spatial alignment and image quality of the various bands have been well characterized, the MIRI team will prioritize calibrating the instrument’s spectroscopic response.” Observations of compact emission-line objects and diffuse planetary nebulae ejected by dying stars will be used to determine the wavelength solution and spectral resolution across each of the twelve fields of view.
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, we demonstrate the MRS’s exceptional spectral resolving power.Once these fundamental instrument characteristics are established, MRS can be calibrated to support the plethora of Cycle 1 science programs that will begin in a matter of weeks.”
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