JWST Integral Field Spectroscopy

JWST integral field spectroscopy (IFS) is the process of dissecting an astronomical scene into multiple spatial components and dispersing each component with a spectrograph in order to provide spatially resolved spectroscopic information.

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Why use an Integral Field Unit (IFU)?

See also: Introduction to IFU SpectroscopyIFU TerminologyIFU Example Science

Integral field units offer combined spatial and spectral information for an astronomical target; thought of another way, they provide a full spectrum for every spatial position in the field of view. This allows them to efficiently produce spatial maps of spectroscopic quantities such as kinematics and diagnostic line ratio strengths (see IFU Example Science). Additionally, IFUs are not affected by traditional slit loss problems (any light lost from one optical element is recovered by the adjacent element), reduce the need for complicated target acquisition procedures (since objects do not need to be carefully centered within a single slit), and provide a better estimate of the extended background/foreground structures surrounding the astrophysical object of interest. IFUs thus provide extremely dense information coverage for a single object, typically at the cost of a relatively small field of view (with some notable exceptions).

The IFUs aboard JWST are thus particularly useful for obtaining spectral maps of extended sources up to a few arcseconds in size (or larger, with mosaicing), or for obtaining spectra of point sources in which there is expected to be significant background emission which must be characterized and subtracted. Additionally, for mid-infrared wavelengths, the MIRI IFU is the only instrument capable of obtaining moderate resolution spectroscopy longward of 5 μm.



The JWST IFUs

Main articles: MIRI Medium Resolution Spectroscopy, NIRSpec IFU Spectroscopy

JWST has 2 IFUs: the MIRI medium resolution spectrometer (MRS) provides R ~ 1,500–3,500 spectroscopy from wavelengths of 4.9 to 28.8 μm over a contiguous field of view up to 7.2" × 7.9" in size, while NIRSpec provides R ~ 100, 1,000, and 2,700 spectroscopy from 0.6 to 5.3 μm over a contiguous field of view 3" × 3" in size. Details of the individual instruments are provided in the articles listed below.



Considerations for observing with the JWST IFUs

Main articles: NIRSpec IFU Dither and Nod Patterns, MIRI MRS Dithering, MIRI MRS Dedicated Sky Observations
See also: MIRI MRS Target Acquisition, NIRSpec Target Acquisition

Background observations

All astronomical scenes will contain signal from both the target of interest and background/foreground signal arising from zodiacal light, telescope thermal emission, or a variety of astrophysical sources. In many cases, it will be desirable to measure this signal so that it can be reliably removed from the spectra of the object of interest, especially longwards of 15 μm where telescope thermal emission dominates the background signal.

Traditional methods of dealing with background subtraction for IFS data have been two-fold, depending on the structure of the science target. For point source targets (or those that are small compared to the field of view), A/B style dithering is often performed to observe the target with 2 or more exposures widely spaced enough that each exposure can serve as the background for the other. This approach maximizes the on-source integration time of the telescope. For extended sources that fill an appreciable fraction of the field of view however, such simple in-scene dithering is insufficient to move the science target far enough on the detector. In such cases, a dedicated off-source background observation is typically obtained in order to provide a clean measurement of the background signal in a part of the sky known to be free of emission from the science target.

These different methods are reflected in the JWST Exposure Time Calculator as nod-in-scene (for point source targets) or nod-off-scene (for extended targets), as detailed in JWST ETC IFU Nod in Scene and IFU Nod off Scene Strategy.

Best practice procedures for both MIRI and NIRSpec background observations are currently under development and will be updated during commissioning.

Dithering

Since the JWST IFUs are spatially undersampled at most wavelengths, it is important to obtain dithered observations in order to optimize the image quality of the resulting data cubes. Detailed MIRI MRS and NIRSpec dither patterns have been designed that use sub-integer offsets to improve the sampling of the JWST point spread function. In addition to improving the spatial sampling (and corresponding image quality), dithering can also improve the spectral sampling of the IFUs, help mitigate the impact of bad pixels by sampling a source with redundant detector locations, and (in some cases) allow for measurement of the background signal in the IFU field of view.

PSF/LSF variations

In any integral field spectrograph, the size and shape of the spatial point spread function (PSF) and the spectral line spread function (LSF) (i.e., the spectral resolution) can change over the IFU's field of view. Any analyses working with these data should bear this in mind; estimates of the variability will become available during the commissioning and calibration process.

Target acquisition

The JWST IFUs can be used to observe both point sources (e.g., stars) and diffuse sources (e.g., nebulae). The absolute pointing accuracy of JWST is about 0.68" from the Fine Guidance Sensors; if more precise positioning of the object in the IFU field of view is required, then target acquisition should be performed using a bright point source. If the science target is diffuse, a nearby point source should be used for target acquisition instead.



Working with integral field data

Numerous tools and techniques have been developed for interacting with and visualizing IFS data; additional information will be added here as it becomes available.

  • Color maps: Since a major product of IFS data is two-dimensional maps of astrophysical quantities, color is often used to convey information in such plots. There is a literature available on best practices for greatest clarity in the presentation of such information.
  • Covariance: Since spectral data cubes are constructed from algorithms that reformat dispersed spectra, there is often significant covariance between adjacent spaxels in a data cube. There are a variety of methods that have been developed to account for this covariance in analyses of stacked spectra.
  • Voronoi binning: This is a classic technique used to construct spatial binning regions of varying size and shape where the corresponding spectra reach fixed continuum signal to noise.
  • Data cube construction: A variety of techniques exist to reformat pixel-level data into a convenient 3-D cube format, but the optimal method of constructing these cubes can vary depending on the science case.
  • JWST analysis tools: These tools allow the user to work with IFU data products within the JWST software ecosystem.



Additional resources

  • IFS Wiki: A shared resource for historical background and tips/tricks for working with IFS data
  • 3D Spectroscopy in Astronomy: An introductory text based on lectures at the Canary Island Winter School




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