MIRI MRS and NIRSpec IFU Observations of Cassiopeia A
This example science program presents an application of the JWST integral field spectroscopy roadmap, showing how to create the cross-instrument MIRI/NIRSpec observing program to observe knots in Cassiopeia A.
The IFU Roadmap guides the reader, step by step, through the process of creating a JWST observing program that uses the MIRI medium resolution spectrometer (MRS) and/or the NIRSpec IFU. Whichever IFU the reader decides to use, there are a sequence of steps to follow for creating an observing program for the chosen IFU(s). Here we demonstrate this process, showing how choices are made at each step, for a cross-instrument science use case to observe the supernova remnant Cassiopeia A with MIRI MRS and the NIRSpec IFU.
Cassiopeia A ("Cas A") is a supernova remnant in our Galaxy. Supernovae are important when considering the life cycle of matter in a galaxy, so it is important to study a nearby supernova remnant in detail to learn how SNe fit in to the galactic life cycle of matter. Cas A has what have been termed "fast moving knots" (FMKs; Hurford & Fesen 1996), which make for interesting science targets. The knots are extended, so they are good targets for IFUs. These FMKs emit at both near- and mid-infrared wavelengths, so all of these wavelengths are of interest.
Steps for creating observations
Step 1 - Pick one or both of the JWST IFU observing modes based on needed wavelength coverage
In this case, since we want observations at both near-infrared and mid-infrared wavelengths, we will want to use both IFUs: MIRI MRS and the NIRSpec IFU. There are numerous emission lines of interest at both near- and mid-infrared wavelengths, and there is (potentially) interesting dust emission present at mid-infrared wavelengths. Below, we will present the steps necessary to create observations for both IFUs.
Step 2 - Pick wavelength setting(s)
We would like to use all 3 grating settings of the MRS in order to observe all emission lines present in the mid-infrared and dust emission from various types of dust. Rho et al. (2008) found multiple emission lines in the spectra of the Cas A supernova remnants, including [ArII] at 9.0 μm, [NeII] at 12.8 μm, and [OIV] at 25.9 μm, among others. In addition, Rho et al. note that their Cas A spectra suggest emission from solid-state dust of various kinds of compositions, including silicates and SiO2 (the latter has a prominent broad emission feature peaking near 21 μm), among others.
Some of the strongest lines from Cas A FMKs in the near-infrared that belong to elements such as S, Si, and Fe, are located near 1 μm wavelength (Hurford & Fesen 1996; Gerardy & Fesen 2001). A set of 4 strong emission lines belonging to [SII] were observed by Hurford & Fesen (1996) and Gerardy and Fesen (2001) as a blend of lines spanning the range 1.0287–1.0373 μm. Additionally, the greatest possible spectral resolving power is desired to resolve any velocity structure, if possible. Here, we use the G140H grating to achieve the highest spectral resolving power. We choose the F100LP filter; although the F070LP filter can be used with this grating, it is not advisable with the IFU, as the wavelengths <0.96 μm will fall off the detector.
Step 3 - (For MIRI MRS) Determine whether you should choose simultaneous imaging with the MIRI imager
See also: MIRI MRS Simultaneous Imaging
In this case, we would like to do simultaneous imaging, as it allows for deep imaging of nearby fields, including parts of the Cas A supernova remnants. It is not expected that anything in the imager field of view for these IFU observations will saturate the imager, and there are no data volume concerns, so there is no reason not to obtain the simultaneous imaging. There is a lot of interesting structure in the Cas A supernova remnant that will be recorded in such simultaneous imaging.
Step 4 - Decide whether you need to do a mosaic
The knots are extended, with full width at half maximum (FWHM) values ranging between 2"–6" as measured from Spitzer Space Telescope IRAC images, but the field of view for band 1A (channel 1 and grating setting A; the shortest wavelength band of the MRS) is only 3.2'' × 3.7'', though the field of view of the MRS grows larger for longer wavelength channels (up to 6.6'' × 7.7'' in channel 4). We therefore create a 2 × 2 mini mosaic for the MIRI MRS observation of each of the 3 knots, with 10% overlap of mosaic tiles in both mosaic rows and mosaic columns. This will produce mosaics of extent ~ 6" × 7", which should be large enough to image the FWHM range of all 3 FMKs at all MRS wavelengths. We further group the MRS observations of all three knots together as a target group in APT.
The NIRSpec IFU field of view is 3" × 3". This is fairly small compared to the extent of the Cas A FMKs, so we will create a 2 × 2 mini mosaic for the NIRSpec IFU observation of each of the 3 knots, with 10% overlap of mosaic tiles in both mosaic rows and mosaic columns. This will produce mosaics of extent comparable to the fields of view of MRS channels 2 and 3 (4" × 5" and 6" × 6", respectively), making for a more favorable comparison to the MRS observations of the FMKs. We further group the NIRSpec IFU observations of FMK2 and FMK3 together as a target group in APT.
Step 5 - Pick a dither pattern
Dithering improves both spatial and spectral sampling. Increased spectral sampling is useful because it better resolves velocity structure in the emission lines, and increased spatial sampling is useful because it resolves small spatial structure within the knots. A 4-point extended source dithering pattern for all wavelengths for the MRS is chosen, as it maximizes the common field of view between different pointings while still obtaining good sampling. Since this is an extended source dither pattern, APT offers the user no choice for the orientation on the sky of the MRS dither pattern—the only option in APT in this case is the negative dither pattern orientation.
Like for the MRS, dithering improves the spatial sampling of the NIRSpec IFU. In general, because the PSF for the NIRSpec IFU is so undersampled, it is always a good idea to dither with the IFU. This is important for finding small scale spatial structure in the knots of Cas A; therefore, dithering is included. The 4-point small cycling dither pattern (which is the same as the 4-point dither pattern; this dither pattern has the smallest footprint on the sky of all 4-point NIRSpec IFU dither patterns) is chosen and will be implemented at each tile in the 2 × 2 mini mosaics. The small extent for the pattern is chosen, as it is unnecessary for each tile of the mini mosaic to cover a large patch of sky (as from a larger dither pattern) since the mini mosaic is already providing this.
Step 6 - Determine whether you need a dedicated background observation
We include a dedicated background observation using the MRS. This background observation will be used for each of the three knots. Thermal background is significant for MIRI, so background subtraction is typically important at these wavelengths. The knots are expected to fill the MRS fields of view, so there is not expected to be any part of the MRS fields of view without science target emission for use in background subtraction. Therefore, a separate background observation for each knot is required. In general, dedicated background observations are recommended whenever an extended source dither pattern is used.
We provide dedicated background observations for the NIRSpec IFU observations. The ETC calculations for Cas A (see step 9 and the article Step-by-Step ETC Guide for MIRI MRS and NIRSpec IFU Observations of Cassiopeia A) provide the user with estimates of the background emission at the position of the science target. It was found that, at the half-power wavelengths of the 1.03 μm [SII] blend, at the edges of the 3" × 3" field probed by the ETC (which is within the spatial full width at half maximum of the knots), the background accounted for up to 10% or even a little more of the total surface brightness. To be safe, it was therefore decided to include dedicated background observations for the NIRSpec IFU observations of the FMKs.
Step 7 - For NIRSpec IFU: Decide whether you need to obtain "leakcal" observations to mitigate the effects of light that leaks through the NIRSpec micro-shutter assembly (MSA) shutters
There are 3 basic flavors of leakage through the MSA: bright spoilers whose radiation is transmitted through closed MSA shutters, leakage through stuck-open MSA shutters, and pile-up of extended dispersed background. Cas A does have extended emission that will be incident upon the MSA for almost any orientation as can be seen in Spitzer-IRAC images; however, the NIRSpec observations are at 0.95–1.9 μm, and near-infrared images, such as the [SIII] 0.9531 μm image shown by Hurford & Fesen (1996) and 2MASS broadband images, show this extended emission to be barely visible. For contamination from the bright spoilers, the Aladin Viewer in the Astronomer's Proposal Tool shows at most 6–7 bright (K < 11.5) stars, depending on the orientation. This is a small enough number of stars that it is anticipated that their contamination can be removed during cube building. Dispersed leakage is the leakage of concern for our Cas A observations. For dispersed leakage from Cas A, the signal is expected to be spatially smooth on scales of a few arcseconds, so the recommended leakage calibration exposure strategy is to obtain a "leakcal" observation at the same exposure time as the science observation, but only at one position.
Step 8 - Decide if you should do a target acquisition (TA)
The knots are fairly large, so there is no need to center the mini mosaics on any specific part of any knot. The absolute fine pointing accuracy of JWST is 0.19" (1-σ radial error), which is sufficient to center each knot adequately in the MRS field of view for each of the 4 channels. Therefore, we do not perform MRS TA.
Similar to the MIRI MRS observations, the knots are fairly large, and there is no need to center the mini mosaics on any specific part of any knot. The absolute fine pointing accuracy of JWST is 0.19" (1-σ radial error), which is sufficient to center each knot adequately in the NIRSpec IFU mini mosaic. Therefore, there is no need for a NIRSpec target acquisition.
Step 9 - Calculate the required exposure time and detector readout parameters using the Exposure Time Calculator (ETC)
To determine the exposure parameters for this observation using the Exposure Time Calculator (ETC), please see the article Step-by-Step ETC Guide for MIRI MRS and NIRSpec IFU Observations of Cassiopeia A.
Step 10 - Fill out the Astronomer's Proposal Tool (APT) for your observation
For details filling out the APT for this example science program, please see the article Step-by-Step APT Guide for MIRI MRS and NIRSpec IFU Observations of Cassiopeia A.
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