The JWST NIRSpec integral field unit (IFU) provides spatially resolved imaging spectroscopy over a 3" × 3" square region. Each spatial element in the resulting IFU data cube is 0.1" × 0.1".

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The JWST integral field spectroscopy mode enables spectral imaging over small fields of view. The NIRSpec integral field unit (IFU) is designed for studies of astronomical targets that are extended over a few arcseconds, or for sampling small regions of more extended objects. The NIRSpec IFU use cases range from the study of spatially resolved kinematics and emission lines in distant galaxies to atmospheric or topological features in extended moons or planets within the solar system.

The primary benefit of an IFU is the ability to obtain the spectrum of a contiguous, extended area on the sky (Figure 1). Optical realignment of the spectra is accomplished by an “image slicing” technique using mirrors in the IFU. The resulting slit images are dispersed without confusion from neighboring spatial elements. The 3" × 3" square field of view is dissected into 30 slices (Figure 2) and mapped onto optics to create 900 spatial elements within the field of view. Each of the NIRSpec spatial IFU elements is 0.1" × 0.1" on the sky, and can be dispersed using any of the NIRSpec gratings or the prism to acquire spectral imaging data cubes over the 0.6–5.3 μm wavelength range where NIRSpec is sensitive.

For further details on the IFU optics, please see the NIRSpec IFU instrument page.

*Click on the figure for a larger view.*
NIRSpec IFU observation of the nucleus of Seyfert galaxy NGC 7469 (Lai et al., 2023; Bianchin et al., 2023; see also Armus & Evans), from Early Release Science (ERS) Program 1328, "A JWST Study of the Starburst-AGN Connection in Merging LIRGs." (a) The dithered IFU field of view overlaid on a cropped JWST NIRCam image of the galaxy (Credit: ESA/Webb, NASA & CSA, L. Armus, A. Evans). (b) One slice of the IFU cube at the 1.08 µm He I line (observed frame, redshift z = 0.016). (c) A portion of the integrated NIRSpec IFU (G140H) spectrum. The data cube was visualized using the Jdaviz Cubeviz application on standard pipeline (build 9.2, ver 1.10.1) data products in MAST (panels b and c). Bad pixels show up as narrow spikes in the spectrum.

Properties of the IFU mode

See also: NIRSpec Integral Field Unit

Figure 2 shows the 3" × 3" IFU field of view and how each slice maps to a different position on the detector. This is how the slices would appear in imaging mode on the detector. Each of the 30 slices are 0.1" wide and 3" in length on the sky, which corresponds to a height of 30 pixels on the detector. The spectral dispersion direction is also indicated.

*Click on the figure for a larger view.*
The JWST NIRSpec IFU aperture as seen on the sky (upper). Each slice maps to a different virtual slit on the detector (lower). The numbers on both panels indicate the slice on the sky (upper) and its corresponding location on the detector (lower. This can best be seen by clicking on the figure to expand it. As shown in this figure, there are 2 NIRSpec detector arrays: NRS1 on the left, and NRS2 on the right, separated by a physical gap. The spectra from each slice can be extracted and combined to form a three-dimensional "data cube" that contains the source image for each spectral resolution element. The gap is responsible for some missing wavelengths in the extracted spectra. See Figure 4 for an example.

The NIRSpec IFU entrance aperture is in the same focal plane as the micro-shutter assembly (MSA) shutters and the NIRSpec fixed slits (FSs). This arrangement is shown in a diagram of the NIRSpec entrance apertures in Figure 3. The IFU entrance aperture is a square cutout in the metal mounting of the MSA. The aperture is opened during IFU operations, and closed when the IFU is not in use. In imaging mode, however, the IFU remains open (e.g., for verification images and for TA).

*Click on the figure for a larger view.*
The location of the IFU entrance aperture in the MSA focal plane is shown as the small red square. A photograph of the zoomed view is presented to the left.

In IFU mode, the NIRSpec MSA shutters are configured "all closed," and the IFU aperture is opened. Light from the NIRSpec IFU is dispersed onto the detector across the same pixel region as spectra obtained in the NIRSpec MSA mode. There are, however, 2 MSA instrument characteristics that affect IFU data and sensitivity:

Failed open MSA shutters:
The MSA has a population of shutters that are permanently or intermittently stuck in an open configuration (for an example of how this affects the data, see Figure 4). These failed open shutters always produce spectra of the sky, no matter the state of the MSA. For an IFU mode observation in a low intensity region of the sky, these shutters might only contain sky background flux from a blank field. However, in very crowded fields, the IFU spectra will likely be contaminated by the spectra of field objects through these failed open MSA shutters. If this characteristic is considered problematic for science data quality, the IFU aperture can be closed and a MSA leakage correction "leakcal” exposure or set of exposures can be acquired. Such observations can be used in post-processing to subtract the spectral contamination from failed open MSA shutters.
MSA contrast:
MSA shutters have finite contrast and are not completely opaque. Flux contamination in IFU observations from finite contrast MSA shutters is referred to as "print-through." This problem is described more completely in the MSA instrument article. MSA shutter contrast is defined as the flux through an open MSA shutter divided by the flux through a closed MSA shutter; higher contrast numbers characterize more opaque shutters. MSA contrasts range from patchy regions of 2,000–3,000 in MSA quadrant Q3, to contrasts of more than 10,000 (very opaque) seen in many regions across MSA quadrant 4. While the print-through is generally small for an individual shutter, it can be significant across all 365 MSA shutters in a row in one quadrant. All the shutters in the row contribute in a cumulative way to the flux at a given pixel on the detector. This can result in increased background and a loss of sensitivity in IFU mode observations. The effect can be problematic for very faint IFU targets and IFU targets in crowded regions or with strong nebulosity. If this is likely to affect the sensitivity of science exposures, the MSA leakage correction "leakcal” exposures (as described above) can also be used to correct this contamination from the finite contrast MSA shutters.

Note: Dithering can help mitigate the effects of individual failed open MSA shutters on IFU science exposures, but it will not improve the effect of the cumulative MSA print-through from the finite contrast shutters. Beside dithering, there are strategies that can help to mitigate or remove MSA leakage originating from failed open or poor contrast MSA shutters. More information can be found in NIRSpec MSA Leakage Subtraction Recommended Strategies and NIRSpec Bright Spoilers and the IFU Recommended Strategies.

Spectral configurations

Words in bold are GUI menus/
panels or data software packages;
bold italics are buttons in GUI
tools or package parameters.

All grating-filter combinations and the prism can be used in the NIRSpec IFU mode. The instrument configurations, spectral resolutions, and wavelength ranges that can be used are outlined in Table 1. For the F070LP filter + G140 grating settings, the minimum measured wavelength is shifted and the blue-ward spectra lie off the detector. This is because the IFU aperture is located on the short wavelength side of the NIRSpec focal plane (see Figure 3).

Table 1. Instrument configurations, spectral resolutions, and wavelength ranges for the NIRSpec IFU

Disperser-filter combination	Nominal resolving power	Wavelength range (μm)
G140M/F070LP	~1,000	0.90–1.27
G140M/F100LP		0.97–1.89
G235M/F170LP		1.66–3.17
G395M/F290LP		2.87–5.27
G140H/F070LP	~2,700	0.95–1.27
G140H/F100LP		0.97–1.89
G235H/F170LP		1.66–3.17
G395H/F290LP		2.87–5.27
PRISM/CLEAR	~100	0.6–5.3

The NIRSpec IFU 0.1" spatial elements are projected onto the detector with a factor of 2 magnification in the dispersion direction. As a result, the undispersed IFU slice images are 2 pixels wide, even though they project to an area of 0.1" x 3.0" on the sky. The magnification helps to match the spectral sampling of the IFU to that of the MSA shutters and the 0.2" wide fixed slits.

Detector wavelength gaps

There is a physical gap between the 2 NIRSpec detectors in the focal plane array (Figure 2). This affects NIRSpec IFU observations with the high resolution (R = 2,700) gratings because the spectra are long enough to span both NIRSpec detectors. Unfortunately, the wavelengths of the spectra which fall in the detector gaps are not recoverable in IFU mode. The NIRSpec IFU Wavelength Ranges and Gaps article discusses in detail these gaps and provides figures that show the the gaps for dispersers G140H, G235H, and G395H. The G140H/F070LP configuration in the NIRSpec IFU mode does not have a gap in wavelength because its spectra fall onto only one of the 2 detectors (NRS1). Each IFU slice has a very slightly different wavelength gap range.

Subarrays

NIRSpec IFU exposures are only acquired in full frame 2048 × 2048 detector pixel readout; no subarrays are used.

Exposure specification

NIRSpec IFU exposure times are tied to the timing of the detector readout patterns. There are 4 readout patterns available for NIRSpec IFU observations:

NRSRAPID
NRS
NRSIRS2RAPID
NRSIRS2

The readout patterns are split over 2 readout modes: (1) traditional and (2) improved reference sampling and subtraction (IRS²). The traditional mode, which is used for the NRSRAPID and NRS readout patterns, is similar to the detector readout for NIRCam and NIRISS. The IRS² mode, used by NRSIRS2RAPID and NRSIRS2 readout patterns, intersperses reference pixels within the science pixel reads to improve the noise characteristics during data processing. These IRS² readout patterns improve performance and sensitivity in long exposure IFU observations of faint objects.

Additional information on NIRSpec IFU exposure specification and how this translates to exposure time and sensitivity can be found using the JWST Exposure Time Calculator (ETC). The NIRSpec Detector Recommended Strategies article provides guidance about the selection of appropriate readout patterns to use in different science cases.

Options for target acquisition

The target acquisition method selected should reflect the desired pointing accuracy, which in turn depends on the science goals. If the blind pointing accuracy of the telescope is sufficient, a TA Method of None should be selected. The default option, WATA, will suffice for most cases where a better pointing accuracy is required. If there are regions of interest at the edge of the field of view or a large nod or dither pattern is used, then WATA may be necessary. The MSATA method is sometimes a viable alternative when WATA is not feasible due to a lack of suitable reference stars. The MSATA process requires an aperture position angle assignment and the selection of reference stars which in turn may require pre-imaging, or the availability of HST imaging to obtain accurate coordinates of nearby stars in the right brightness range. Considerations for the different methodologies for IFU target acquisition are discussed in NIRSpec Target Acquisition Recommended Strategies.

Options for dithering

Most observations with JWST will require dithering to improve PSF sampling and mitigate detector effects such as bad pixels. Since the NIRSpec PSF is undersampled at most wavelengths, dithering is required to achieve nominal spectral and spatial resolution. For observations with the NIRSpec IFU, there are several options available, including dithers and nods. Both dither and nod options move science sources on the detector, but nods differ because the offsets are also pairwise subtracted in data processing to remove astrophysical background flux. IFU dithers are optimized to provide a factor of 2 sub-sampling of the PSF in both directions.

Table 2. Summary of NIRSpec IFU dither and nod options

No dither or nod	No dithering or nodding is performed. The target is positioned at the IFU aperture center.
Two-point nod	Two points separated by ~1.6" in both X and Y directions.
Four-point dither	Four points forming a box that's ~0.4" on a side. These 4 points are separated by a small fraction of the size of the IFU field of view, so they share large areas in common, and the data will be co-added in the calibration pipeline.
Four-point nod	Four points forming a box that's ~1.6" on a side. The data from the 4 nod positions will be paired and subtracted by the calibration pipeline.
Cycling dither pattern	Up to 60-point cycling dither pattern (with either ~1", 0.5" and 0.25" extents)
Sparse cycling dither pattern	Up to 60-point cycling dither pattern (with a variety of separations).

The NIRSpec Dithers and Nods article provides an in-depth view of the available options briefly described in Table 2. The NIRSpec Dithering Recommended Strategies article provides dithering advice tailored to different science goals. When choosing dithers, one should take into account target acquisition uncertainties, as described in the NIRSpec Target Acquisition Recommended Strategies article.

Options for background subtraction

The JWST Background Model describes the different components of the background. The JWST ETC can be used to model and estimate the background in IFU mode, to determine whether background subtraction is needed and what observing parameters will yield the desired S/N on the science source. Bright science sources may not need this correction since the background is expected to be low at NIRSpec operational wavelengths.

Users have the option to do in-frame nodding or off-source backgrounds for background removal. All available options are covered in NIRSpec IFU Dither and Nod Patterns. The article NIRSpec Background Recommended Strategies can help the user decide which options to use for their science case.

What do NIRSpec IFU data look like?

Figure 4 shows NIRSpec IFU mode data acquired with a ground calibration test lamp using the high resolution (R = 2,700) disperser G140H and filter F100LP. Spectra of the 30 IFU slices are the horizontal bands in the figure with labels 1 to 30 (on the left). The fixed slit apertures are always open, so data is acquired through them even in IFU mode. They are shown and labeled "Fixed Slits" in the figure.

Spectra produced by the IFU are dispersed onto the same region of the detectors used by the NIRSpec MOS mode, so the IFU and MOS modes are not used simultaneously. The red arrows in Figure 4 point to the spectra of failed open MSA shutters that contaminate the IFU spectra.

Pipeline algorithms transform the data read out from the detectors into 3-D data cubes. One wavelength slice of a processed cube of simulated data is shown in Figure 4 in the lower left corner. Example wavelength and flux calibrated spectra of individual pixels are presented in the lower right corner of the figure.

*Click on the figure for a larger view.*
An example of a NIRSpec IFU mode calibration showing the detector view (top), the cube view (bottom left), and the spectrum view (bottom right). In the detector view, 30 IFU data slices are shown as horizontal bands that span the 2 NIRSpec detectors. Spectra in the middle of the detector view are from the NIRSpec fixed slits. The fixed slits are always open and spectra are always acquired on the sky. The red arrows point to spectra from example failed open MSA shutters. The cube view represents one wavelength slice of the pipeline processed cube from a simulated dataset. The spectrum view shows the wavelength and flux calibrated spectra obtained for three of the cube pixels.

References

Armus, L. and Evans, A.S., 2023, Special issue of ApJL
Focus on Early Results from GOALS-JWST

Bianchin, M., et al. 2024, ApJ, 965, 103
GOALS-JWST: Gas Dynamics and Excitation in NGC 7469 Revealed by NIRSpec

Böker, T. et al. 2022, A&A, 661, 82
The Near Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope III. Integral Field Spectroscopy

Closs, M., Ferruit, P., Lobb, D. R. et al. 2008 SPIE, 7010, 701011-1
The Integral Field Unit on the James Webb Space Telescope's Near-Infrared Spectrograph

Lai, T., et al. 2023, ApJL, 957, 2
GOALS-JWST: Small Neutral Grains and Enhanced 3.3 μm PAH Emission in the Seyfert Galaxy NGC 7469

Lutzgendorf, N. 2016 ESAC JWST "On Your Mark" Workshop
The NIRSpec IFU

Notable updates	27 Oct 2022 Major change to target acquisition recommendations 26 Mar 2018 Minor change to indicate frame times for IRS2 patterns 23 Mar 2018 Minor change to clarify Figure 1 caption 30 Jun 2017 Table 1 updated
Originally published	29 Dec 2016

JWST User Documentation

NIRSpec IFU Spectroscopy