# A JWST Survey of the Supernova Remnant Cassiopeia A

Dan Milisavljevic Purdue University, Department of Physics and Astronomy, 525
Northwestern Ave, West Lafayette, IN 47907  Integrative Data Science
Initiative, Purdue University, West Lafayette, IN 47907, USA  Tea Temim
Princeton University, 4 Ivy Ln, Princeton, NJ 08544, USA  Ilse De Looze
Sterrenkundig Observatorium, Ghent University, Krijgslaan 281 - S9, B-9000
Gent, Belgium  Danielle Dickinson Purdue University, Department of Physics and
Astronomy, 525 Northwestern Ave, West Lafayette, IN 47907  J. Martin Laming
Space Science Division, Code 7684, Naval Research Laboratory, Washington, DC
20375, USA  Robert Fesen 6127 Wilder Lab, Department of Physics and Astronomy,
Dartmouth College, Hanover, NH 03755, USA  John C. Raymond Center for
Astrophysics $|$ Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138,
USA  Richard G. Arendt Center for Space Sciences and Technology, University of
Maryland, Baltimore County, Baltimore, MD 21250, USA  Code 665, NASA/GSFC,
8800 Greenbelt Road, Greenbelt, MD 20771, USA  Center for Research and
Exploration in Space Science and Technology, NASA/GSFC, Greenbelt, MD 20771,
USA  Jacco Vink Anton Pannekoek Institute for Astronomy & GRAPPA, University
of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands  SRON
Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, the
Netherlands  Bettina Posselt Department of Astrophysics, University of Oxford,
Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK  Department of
Astronomy & Astrophysics, Pennsylvania State University, 525 Davey
Lab,University Park, PA 16802, USA  George G. Pavlov Department of Astronomy &
Astrophysics, Pennsylvania State University, 525 Davey Lab,University Park, PA
16802, USA  Ori D. Fox Space Telescope Science Institute, 3700 San Martin
Drive, Baltimore, MD 21218, USA  Ethan Pinarski Purdue University, Department
of Physics and Astronomy, 525 Northwestern Ave, West Lafayette, IN 47907
Bhagya Subrayan Purdue University, Department of Physics and Astronomy, 525
Northwestern Ave, West Lafayette, IN 47907  Judy Schmidt Astrophysics Source
Code Library, Michigan Technological University, 1400 Townsend Drive,
Houghton, MI 49931, USA  William P. Blair The William H. Miller III Department
of Physics and Astronomy, Johns Hopkins University, 3400 N. Charles Street,
Baltimore, MD 21218, USA  Armin Rest Space Telescope Science Institute, 3700
San Martin Drive, Baltimore, MD 21218, USA  The William H. Miller III
Department of Physics and Astronomy, Johns Hopkins University, 3400 N. Charles
Street, Baltimore, MD 21218, USA  Daniel Patnaude Center for Astrophysics $|$
Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA  Bon-Chul
Koo Department of Physics and Astronomy, Seoul National University, Seoul
08861, Republic of Korea  Jeonghee Rho SETI Institute, 189 Bernardo Ave., Ste.
200, Mountain View, CA 94043, USA  Department of Physics and Astronomy, Seoul
National University, Gwanak-ro 1, Gwanak-gu, Seoul, 08826, South Korea
Salvatore Orlando INAF – Osservatorio Astronomico di Palermo, Piazza del
Parlamento 1, 90134 Palermo, Italy  Hans-Thomas Janka Max-Planck-Institut für
Astrophysik, Karl-Schwarzschild-Str. 1, 85748, Garching, Germany  Moira
Andrews  Purdue University, Department of Physics and Astronomy, 525
Northwestern Ave, West Lafayette, IN 47907  Michael J. Barlow Department of
Physics and Astronomy, University College London, Gower Street, London WC1E
6BT, United Kingdom  Adam Burrows Princeton University, 4 Ivy Ln, Princeton,
NJ 08544, USA  Roger Chevalier  Department of Astronomy, University of
Virginia, P.O. Box 400325, Charlottesville, VA 22904-4325, USA  Geoffrey
Clayton Department of Physics and Astronomy, Louisiana State University, Baton
Rouge, LA 70803, USA  Claes Fransson Department of Astronomy, Stockholm
University, The Oskar Klein Centre, AlbaNova, SE-106 91 Stockholm, Sweden
Christopher Fryer  Center for Theoretical Astrophysics, Los Alamos National
Laboratory, Los Alamos, NM 87545, USA  Department of Astronomy, The University
of Arizona, Tucson, AZ 85721, USA  Department of Physics and Astronomy, The
University of New Mexico, Albuquerque, NM 87131, USA  Department of Physics,
The George Washington University, Washington, DC 20052, USA  Haley L. Gomez
Cardiff Hub for Astrophysical Research and Technology (CHART), School of
Physics & Astronomy, Cardiff University, The Parade, Cardiff CF24 3AA, UK
Florian Kirchschlager Sterrenkundig Observatorium, Ghent University,
Krijgslaan 281 - S9, B-9000 Gent, Belgium  Jae-Joon Lee Korea Astronomy and
Space Science Institute, Daejeon 305-348, Republic of Korea  Mikako Matsuura
Cardiff Hub for Astrophysical Research and Technology (CHART), School of
Physics & Astronomy, Cardiff University, The Parade, Cardiff CF24 3AA, UK
Maria Niculescu-Duvaz  Department of Physics and Astronomy, University College
London, Gower Street, London WC1E 6BT, UK  Justin D. R. Pierel  Space
Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
Paul P. Plucinsky  Center for Astrophysics $|$ Harvard & Smithsonian, 60
Garden Street, Cambridge, MA 02138, USA  Felix D. Priestley  Cardiff Hub for
Astrophysical Research and Technology (CHART), School of Physics & Astronomy,
Cardiff University, The Parade, Cardiff CF24 3AA, UK  Aravind P. Ravi
Department of Physics and Astronomy, University of California, 1 Shields
Avenue, Davis, CA 95616-5270, USA  Nina S. Sartorio Sterrenkundig
Observatorium, Ghent University, Krijgslaan 281 - S9, B-9000 Gent, Belgium
Franziska Schmidt  Department of Physics and Astronomy, University College
London, Gower Street, London WC1E 6BT, United Kingdom  Melissa Shahbandeh
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218,
USA  Patrick Slane Center for Astrophysics $|$ Harvard & Smithsonian, 60
Garden Street, Cambridge, MA 02138, USA  Nathan Smith Steward Observatory,
University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA  Niharika
Sravan  Department of Physics, Drexel University, Philadelphia, PA 19104, USA
Kathryn Weil  Purdue University, Department of Physics and Astronomy, 525
Northwestern Ave, West Lafayette, IN 47907  Roger Wesson Cardiff Hub for
Astrophysical Research and Technology (CHART), School of Physics & Astronomy,
Cardiff University, The Parade, Cardiff CF24 3AA, UK  J. Craig Wheeler
Department of Astronomy, University of Texas at Austin, Austin, TX, USA

(Received December 22, 2023)

###### \par\LTX@newpageAbstract

We present initial results from a JWST survey of the youngest Galactic core-
collapse supernova remnant Cassiopeia A (Cas A), made up of NIRCam and MIRI
imaging mosaics that map emission from the main shell, interior, and
surrounding circumstellar/interstellar material (CSM/ISM). We also present
four exploratory positions of MIRI/MRS IFU spectroscopy that sample ejecta,
CSM, and associated dust from representative shocked and unshocked regions.
Surprising discoveries include: 1) a web-like network of unshocked ejecta
filaments resolved to $\sim$ 0.01 pc scales exhibiting an overall morphology
consistent with turbulent mixing of cool, low-entropy matter from the
progenitor’s oxygen layer with hot, neutrino and radioactively heated high-
entropy matter, 2) a thick sheet of dust-dominated emission from shocked CSM
seen in projection toward the remnant’s interior pockmarked with small ($\sim
1^{\prime\prime}$) round holes formed by $\lesssim$ 0$\farcs$1 knots of high-
velocity ejecta that have pierced through the CSM and driven expanding
tangential shocks, 3) dozens of light echoes with angular sizes between $\sim
0\farcs 1$ to $1^{\prime}$ reflecting previously unseen fine-scale structure
in the ISM. NIRCam observations place new upper limits on infrared emission
($\lesssim 20$ nJy at 3 $\mu$m) from the neutron star in Cas A’s center and
tightly constrain scenarios involving a possible fallback disk. These JWST
survey data and initial findings help address unresolved questions about
massive star explosions that have broad implications for the formation and
evolution of stellar populations, the metal and dust enrichment of galaxies,
and the origin of compact remnant objects.

supernovae – supernova remnants

††journal: ApJL††facilities: JWST(MIRI), JWST(NIRCam)

##  1 Introduction

Core-collapse supernovae (SNe) are among the most consequential phenomena in
the universe, and yet many key questions about their explosion mechanisms and
progenitor systems remain unanswered (see, e.g., Smartt 2009 and Eldridge et
al. 2013; Janka 2012 and Burrows & Vartanyan 2021). This uncertainty in our
understanding has broad and important implications: SNe collectively shape the
energy balance, chemistry, and structure of galaxies (Dalla Vecchia & Schaye,
2008); they produce neutron stars (NSs), black holes, and some gamma-ray
bursts (Woosley et al., 2002); they are a major site of nucleosynthesis
(Nomoto et al., 2013), dust production (Hoyle & Wickramasinghe, 1970), and
cosmic rays (Blasi, 2013); and they produce neutrinos (Hirata et al., 1987; Al
Kharusi et al., 2021), and gravitational waves (Murphy et al., 2009; Andresen
et al., 2017), that can now be studied with multi-messenger facilities
(Szczepańczyk et al., 2021; Vartanyan et al., 2023).

Figure 1: Composite images of our NIRCam (top) and MIRI (bottom) mosaics
obtained as part of our JWST survey of Cas A. Fields of view have been subtly
cropped and minor corrections have been made to compensate for gaps in
coverage. Corners not imaged by JWST have been filled in with archival Spitzer
data at comparable wavelengths. Mosaics of all individual filters showing the
entire fields of view are shown in the Appendix.

The superior resolution and sensitivity to near- and mid-infrared wavelengths
made possible with the successful launch of the James Webb Space Telescope
(JWST; Gardner et al. 2023) has opened new pathways to investigate fundamental
questions about the nature of SNe via observations of young SN remnants
(SNRs). While recent and upcoming all-sky surveys (Tonry et al., 2018; Bellm
et al., 2019; Ivezić et al., 2019) can find tens of thousands of extragalactic
SNe, analyses of these unresolved events face unavoidable limitations due to
their inability to provide clear and robust three-dimensional kinematic and
chemical information. In contrast, access to many of the specific and detailed
properties needed to advance our understanding of massive star explosions can
only be obtained through spatially-resolved observations of young ($\lesssim
2000$ yr) Galactic SNRs (Milisavljevic & Fesen, 2017).

The near- to mid-infrared spectral region uniquely accesses emission from
cool, unshocked SN debris, which enables unique insight into the total mass,
relative chemical yield, and kinematic distribution of various components of
the SN ejecta (Laming & Temim, 2020). It also enables investigations about how
much ejecta is transformed into dust and how much of that dust survives
passage through the reverse shock (Bianchi & Schneider, 2007; Dwek & Arendt,
2008; Williams & Temim, 2017; Priestley et al., 2021). Finally, the infrared
region also avoids much of the problem with foreground extinction towards
Galactic remnants, providing new opportunities to constrain processes
governing the formation and final fate of compact objects made in SN
explosions (De Luca, 2017).

Table 1: JWST Imaging Observation log

Instrument | Filter | $\lambda_{p}$ | BW | PSF | $t_{\rm exp}$ | Sources of strong emission
---|---|---|---|---|---|---
|  | ($\mu$m) | ($\mu$m) | (″) | (sec) |
NIRCam | F162M | 1.626 | 0.168 | 0.055 | 3350 | [Fe II] 1.644; [Si I] 1.645; synchrotron
| F356W | 3.563 | 0.787 | 0.116 | 1675 | [Ca IV] 3.207, [Si IX] 3.936; PAHs; synchrotron; dust
| F444W | 4.421 | 1.024 | 0.145 | 1675 | [Si IX] 3.936; [Ca V] 4.159; [Mg IV] 4.487, [Ar VI] 4.530;
|  |  |  |  |  | [K III] 4.618; CO; synchrotron; dust
MIRI | F560W | 5.6 | 1.2 | 0.207 | 1598 | [Mg V] 5.61; dust; synchrotron
| F770W | 7.7 | 2.2 | 0.269 | 1598 | [Ar II] 6.99; PAHs, dust
| F1000W | 10.0 | 2.0 | 0.328 | 1598 | [Ar III] 8.991; [S IV] 10.511; dust
| F1130W | 11.3 | 0.7 | 0.375 | 1598 | PAHs; dust
| F1280W | 12.8 | 2.4 | 0.420 | 1598 | [Ne II] 12.814; [Ne V] 14.32; dust
| F1800W | 18.0 | 3.0 | 0.591 | 1598 | [Fe II] 17.94; [S III] 18.713; dust; H2
| F2100W | 21.0 | 5.0 | 0.674 | 1598 | [S III] 18.713; dust
| F2550W | 25.5 | 4.0 | 0.803 | 1598 | [O IV] 25.89; dust

Note. — Values for filter pivot wavelength ($\lambda_{p}$), bandwidth (BW),
and full-width-half-maximum of the point spread function (PSF), have been
adopted from JWST User Documentation (https://jwst-docs.stsci.edu/). $t_{\rm
exp}$ is the total exposure time for the mosaic. Line identifications guided
in part by Smith et al. (2009) and Laming & Temim (2020).

Cassiopeia A (Cas A) is arguably the SNR that provides the clearest access to
the properties of a core-collapse SN (Koo & Park, 2017). Cas A is the youngest
Galactic core-collapse SNR known ($\approx 350$ yr; Fesen et al. 2006c); it is
among the closest ($3.4^{+0.3}_{-0.1}$ kpc; Reed et al. 1995; Alarie et al.
2014); it is the only core-collapse remnant with a secure SN classification
from light echo spectroscopy performed from multiple lines of sight (Type IIb;
Krause et al. 2008; Rest et al. 2008, 2011); it is one of the best case
studies to understand dust formation in SN ejecta and shock-processing of that
dust (Rho et al., 2009; De Looze et al., 2017); and its central X-ray point
source is a key object to understanding NS evolution models (Pavlov & Luna,
2009; Gotthelf et al., 2013; Posselt & Pavlov, 2022; Shternin et al., 2023).
Models for the remnant suggest that the 15–25 M⊙ zero-age-main-sequence
progenitor star lost the majority of its mass prior to explosion as a $\approx
4$–6 M⊙ star (Chevalier & Oishi, 2003; Hwang & Laming, 2012; Lee et al.,
2014), which was likely encouraged through interaction with a binary companion
(Young et al., 2006; Sato et al., 2020). Although claims have been made of
surviving OB companions in extragalactic SNe IIb (Maund et al., 2004; Ryder et
al., 2018), HST observations have ruled out this possibility for Cas A and to
date no surviving companion has been located (Kochanek, 2018; Kerzendorf et
al., 2019).

Figure 2: MIRI/MRS spectra obtained as part of the survey. P1 and P3 were
selected to sample two shocked ejecta knots of different compositions. P4
represents a ‘core sample’ of unshocked ejecta through the interior of the
remnant, and P2 was selected to help diagnose the emission arising in the
Green Monster (see Section 3.2). All spaxels of the overlapping fields of view
across all four channels have been integrated. Positions with respect to Cas A
are shown and labeled in Figure 3. Dominant emission lines are labeled.
Bandpasses of the MIRI filters are overlaid with throughputs normalized to
arbitrary units. Many of the same emission lines are seen at all four
positions, but offsets due to velocity are evident. Dust emission features are
seen at all positions and described in more detail in Section 4.1. Note the
large dynamic range on the vertical axis.

Here we present an overview of a JWST reconnaissance of Cas A made up of near-
and mid-infrared imaging mosaics and exploratory spectroscopy. This survey was
motivated by outstanding questions about the nature of Cas A’s progenitor
system, the explosion dynamics of the original SN, as well as the processes
influencing the formation and destruction of dust and molecules. These topics
are relevant for broader populations of SNe and their environmental impacts,
which in turn have consequences for the formation and evolution of stellar
populations (Eldridge et al., 2008; Smith, 2014), the metal enrichment of
galaxies (Vogelsberger et al., 2014; Nelson et al., 2019), and the origin of
planetary systems (Dwek, 1998; Nittler & Ciesla, 2016).

Our NIRCam, MIRI, and MIRI/MRS observations are described in $\S$2, followed
by $\S$3 & $\S$4 where we present the imaging mosaics and IFU spectra,
highlight the data quality, and our major findings with regard to mapping dust
and unshocked interior ejecta. We then discuss the serendipitous discovery of
a large, bright light echo that resolves the surrounding ISM in $\S$5, and
compare our JWST data to radio and X-ray observations in $\S$6. The use of the
NIRCam images to constrain possible infrared emission from the surviving NS is
discussed in $\S$7, and we review major findings and describe new science
opportunities enabled by our survey in $\S 8$.

##  2 Observations

Cas A was observed with JWST in Cycle 1 General Observers (GO) Program 1947
(PI: Milisavljevic). The observations reported here were carried out between
August and November 2022, using NIRCam (Rieke et al., 2023) and MIRI (Wright
et al., 2023). NIRSpec (Jakobsen et al., 2022) observations obtained as part
of this program that overlap with the MIRI/MRS positions are reported in De
Looze et al. (2024) and Rho et al. (2024). All JWST data used in this paper
can be found in MAST: 10.17909/szf2-bg42.

The NIRCam observations were obtained on 2022 November 5 using three filters,
as shown in Table 1. The F162M filter was repeated in the Short Wavelength
(SW) camera during both of the Long Wavelength (LW) camera exposures using
filters F356W and F444W. The remnant was covered using a $3\times 1$ mosaic
with 3TIGHT primary dithers, each with 4 subpixel dithers. The field center is
approximately $\alpha(\rm J2000.0)$ = 23:23:23.91, $\delta(\rm J2000.0)$ =
+58:48:54.0, with the entire field of view (FOV) spanning approximately
$6.3^{\prime}\times 7^{\prime}$ and rotated with position angle 206.8°. Some
of the resulting mosaics have gaps near the edges of the fields, which depend
on the camera. The BRIGHT1 readout pattern was used, with seven groups and one
integration per exposure, for 12 total dithers leading to a total exposure
time of 1675 s.

Figure 3: Important features of Cas A identified in our survey and discussed
in this paper. The composite image in the center panel combines NIRCam and
MIRI filters as indicated. Large boxes outlined with dashed white lines show
areas of interest enlarged in the surrounding panels that use the same filters
and color scheme, with the exception of panels 1b and 6 that only use NIRCam
filters. Small boxes outlined with solid white lines show the positions of the
four regions of MIRI/MRS IFU spectroscopy.

Imaging observations with MIRI were first carried out on 2022 August 4-5 using
eight filters and are also shown in Table 1. The remnant was covered by a
3$\times$5 mosaic with a 4-point dither pattern. The readout pattern was
FASTR1, eight groups and one integration per exposure, with four total dithers
leading to exposure times of 88.8 s. An additional location was selected well
away from the remnant to sample the background.

Unfortunately, there was an error with the tiling pattern of the spacecraft,
which introduced gaps in the mosaics and a gradual east-to-west drift between
the centers of the fields of view of approximately 16′′. We applied and were
approved for a return visit to cover the gaps with three additional MIRI
pointings per filter. These return observations were performed on 2022 October
26. The overlapping field center of all mosaics is approximately
$\alpha$(J2000) = 23h23m26$\fs$97, $\delta$(J2000) =
+58o$49^{\prime}$08$\farcs$6, with the entire FOV spanning approximately
$5.3^{\prime}\times 5.7^{\prime}$ and rotated with position angle
131.5$\arcdeg$. This return visit provided a serendipitous opportunity to
study variable light echoes in the region surrounding Cas A (see Section 5).

The imaging data were processed using the calibration pipeline version 1.8.4
and the calibration reference data system (CRDS) version 11.16.14, with the
CRDS context file $\mathtt{jwst\\_1017.pmap}$. Background images were produced
from dedicated sky observations and subtracted from the individual images in
the $\mathtt{calwebb\\_image2}$ pipeline step. We did not subtract background
images from the F770W, F1000W, F1130W, and F1280W data due to the variable
spatial structure of the sky background at these wavelengths. The level 2
images were astrometrically aligned using the JWST Alignment Tool (JHAT) (Rest
et al., 2023).

Mosaic images were then constructed using the default pipeline parameters but
with the $\mathtt{tweakreg}$ and $\mathtt{skymatch}$ steps turned off. Pixel
scales of the final mosaics are 0.031′′ per pixel for the NIRCam SW camera
images, and 0.063′′ for the NIRCam LW camera images. The pixel scale of the
MIRI mosaics is 0.111′′. Composite images made from the mosaics are shown in
Figure 1. Mosaics of individual filters showing the entire fields of view are
shown in the Appendix.

Spectroscopy was performed between August and November 2022 with MIRI and the
Medium Resolution Spectrograph (MRS), which is an integral field unit with a
field size that is variable with wavelength ranging from $3.2\arcsec\times
3.7\arcsec$ to $6.6\arcsec\times 7.7\arcsec$. Three locations were selected
after the MIRI imaging observations were available and could be inspected for
precise placement with respect to remnant emission: two positions targeted
bright ejecta knots in the main shell (P1 and P3), with one targeting newly-
identified emission towards the projected center of the remnant (P2). An
additional observation P4 was obtained on a location selected using archival
Spitzer observations targeting unshocked ejecta in the central region (DeLaney
et al., 2010; Isensee et al., 2010). For P1, P2, and P3, a four point dither
was used with the FASTR1 readout pattern, using 50 groups per exposure, for
total exposure times of 555 s. Simultaneous imaging was obtained in the F560W,
F770W, and F1500W filters. For P4, the total exposure time was doubled, and
simultaneous MIRI imaging was obtained in filters F1130W and F1500W.

We used version $\mathtt{1.11.0}$ of the $\mathtt{jwst}$ module, which
includes the 3 stages of the pipeline processing, and the
$\mathtt{jwst\\_1141.pmap}$ reference file over the course of the calibration.
Stage 1 and stage 2 processing was performed on all four cubes and the
designated background with default settings. All cubes ran through
$\mathtt{jwst.residual\\_fringe.ResidualFringStep}$ and then stage 3
processing. The designated background was used for the master background step
for P1, P2, and P3. The master background step was skipped for P4, since the
designated background was obtained 3 months after P4 was observed.

The stage 3 pipeline provides the processed data cubes, as well as a 1D
extracted spectrum over the entire field of view. We combined all 12 channel
and grating settings. A constant was added to each of the four fully-combined
spectra such that the integrated surface brightness over the F770W
transmission curve for each spectrum matches the average surface brightness at
the corresponding position measured in the F770W image within a region
corresponding to the MRS field-of-view at this wavelength.

Below we present and briefly discuss all four MRS spectra, shown in Figure 2,
with locations identified in Figure 3. We focus on P2 and P4 within the
context of investigating unshocked interior ejecta. De Looze et al. (2024)
provide an in-depth analysis of P2 within the context of interaction between
the remnant and the dusty circumstellar environment (see also section 3), and
Rho et al. (2024) investigate P1 and P3 within the context of molecule
formation and destruction.

##  3 Mosaic Images

Our mosaic images map thermal and non-thermal near- and mid-infrared emission
from Cas A with unsurpassed depth and sensitivity. Table 1 provides a complete
log of imaging observations along with sources of relatively strong emission
in each filter bandpass. Prominent emission features of these imaging mosaics
and discoveries of our survey are highlighted in Figure 3 and discussed below.

###  3.1 NIRCam

Three filters were selected for the NIRCam mosaics that could distinguish
between ejecta, CSM, dust, and the fundamental vibrational mode of CO centered
around 4.65 $\mu$m. The F162M filter was selected for its sensitivity to [Si
I] 1.645 $\mu$m and [Fe II] 1.644 $\mu$m emission present in ejecta (Koo et
al., 2018), both diffuse and clumped, and He-rich CSM (Koo et al., 2023). The
F444W filter is sensitive to multiple emission lines of the ejecta, including
magnesium and argon, along with CO emission, synchrotron radiation, and faint
dust continuum emission. The F356W filter, which largely serves as a continuum
reference for the F444W, is sensitive to relatively weaker ejecta lines,
including ones from calcium and silicon, along with dust and synchrotron
radiation.

The strongest emission in the NIRCam mosaic (Figure 1) is seen along the main
shell of ejecta that has encountered the reverse shock, and represents the
remnant’s densest material ($n\sim 10^{3-5}$ cm-3) with temperatures between
5000 to 10,000 K (Chevalier & Kirshner, 1978; Hurford & Fesen, 1996; Smith et
al., 2009; Lee et al., 2017). As seen in HST data (Fesen et al., 2001), the
ejecta are often grouped into large filament complexes of varying scales, and
are often increasingly dissipated in directions away from the reverse shock
(Morse et al., 2004).

The superb resolution of the NIRCam/F162M (0.055′′) compared to earlier HST
NIR images places a new constraint on the clump size as small as $<200$ A.U.
Furthermore, NIRCam observations detect numerous ejecta knots beyond the main
ejecta shell, many of which have not been previously observed either in HST
optical or ground-based NIR observations (Fesen & Milisavljevic, 2016; Koo et
al., 2018). Some knots are visible only in the F162M image, while others are
visible in both the F162M and F444W images, suggesting their elemental
compositions are different. The physical and chemical properties of the
outlying ejecta knots will be investigated by Koo et al. (2024).

Outside of the main shell, the remnant is enveloped in synchrotron emission,
seen most strongly around the periphery of the main shell (see representative
region enlarged in Figure 3, panel 1), but also interior to the main shell
(Figure 1). This is associated with the forward shock interacting with
surrounding CSM/ISM. Inside the main shell, diffuse emission in F162M is a
blend of synchrotron and [Fe II]+[Si I] line emission associated with
unshocked SN ejecta (Koo et al., 2018). We note that differences in the
distribution of interior emission between our F162M image and the long-
exposure ground-based image published in Koo et al. (2018) are most likely
because they removed H-band emission. Additional discussion of the synchrotron
radiation is provided in section 6.

Clumpy N- and He-rich CSM (aka “quasi-stationary flocculi” or QSFs; see van
den Bergh 1971, Koo et al. 2018, Koo et al. 2020) appear bright in the F162M
image due to their strong [Fe II] lines. This is evident because all prominent
QSFs identified in the 2013 ground-based [Fe II] 1.64 $\mu$m image by Koo et
al. (2018) remain observable in the new JWST data. JWST’s resolution exposes
intricate morphologies arising from interactions with the SNR shock. Some QSFs
have sharp boundaries with abrupt brightness drops implying the presence of
unshocked CSM that has not been processed by the shock yet. Scattered
throughout the interior are numerous never-before-seen complete and partial
rings approximately 1′′ in size, associated with emission from shocked CSM,
that overlap with holes observed in MIRI images (these features are discussed
more extensively below in section 3.2).

An unexpected result from our NIRCam mosaics was the discovery of abundant
light echoes within close proximity to the remnant, including a particularly
large and finely structured one $\sim 4$′ southwest of Cas A’s center (Figure
3, panel 6). These light echoes are also observed in our MIRI mosaics, in
places where the more limited MIRI field of view overlapped with NIRCam.
Dozens of light echoes are seen scattered around the periphery and even
projected within the main shell, with angular sizes between $\sim 0\farcs 1$
to $1^{\prime}$. Light echoes are discussed in more detail in Section 5.

###  3.2 MIRI

For MIRI imaging, eight filters were selected to provide a minimal set
required to sample and differentiate line emission associated with ejecta and
CSM from dust continuum emission. The bright main shell is made up of reverse-
shocked ejecta emitting lines associated with oxygen, sulfur, argon, and neon
(Ennis et al., 2006; DeLaney et al., 2010), and an overall much stronger
contribution from dust (Rho et al., 2008). Clumps of ejecta are seen down to
the resolution limit of each image. Dust emission is also present throughout
the surrounding CSM that has been heated by the forward shock. Particularly
conspicuous is the arc of dust continuum strongest in the F2100W and F2550W
filters seen stretching from NW to E outside of the main shell. Bow shocks
bright in dust emission are seen in the north, and in the northwest a
relatively large cloud of CSM has been overtaken and disrupted by the forward
shock (Figure 3, panels 1 and 2).

The strongest emission seen interior to the main shell is concentrated in what
we will refer to as the “Green Monster,” which extends across the west side of
the central region (Figure 3, panel 4). The emission is seen faintly in
earlier Spitzer IRAC and MIPS observations, and the general location overlaps
with the characteristic spectra identified by Arendt et al. (2014) as the
“South Spot” that was fit with dust compositions similar to ones found around
X-ray Fe ejecta emission. The emission is seen in all filters longward of
F1000W, suggesting a dominant dust continuum component. Our JWST observations
show, for the first time, the detailed structure of the Green Monster, which
is concentrated in long filaments among fainter sheets that together are
pockmarked with circular holes $\sim 1^{\prime\prime}$ in radius. Some of
these holes are outlined as rings (partial and complete) in the NIRCam
mosaics, especially in the F162M filter.

One possible interpretation of the Green Monster and its unusual pockmarked
morphology is that it is associated with unshocked ejecta that have been
expanded by small clumps of radioactive material (56Ni or 44Ti). Cas A is
already known to have been strongly shaped by large-scale Ni-bubbles that
expanded into O-, Ar-, and S-rich ejecta, forming large-scale rings as
observed in the shocked main shell (Lawrence et al., 1995; Blondin et al.,
2001; DeLaney et al., 2010; Milisavljevic & Fesen, 2013; Alarie et al., 2014),
and cavities as observed in the interior unshocked ejecta (Milisavljevic &
Fesen, 2015), with radii of 0.5′ to 1′. Hence, the holes in the Green Monster
could be a much smaller-scale version of this phenomenon, induced by extremely
finely clumped radioactive material, expanding within the central interior of
the remnant.

However, in light of low velocity ($\lesssim 100$ km s-1) emission lines of
[Fe II] 1.644 $\mu$m, H Br$\alpha$ 4.05 $\mu$m, [Ne II] 12.81 $\mu$m, and [Ne
III] 15.56 $\mu$m associated with the small rings (De Looze et al., 2024),
along with the X-ray properties of emission in this region that include
correlation with morphology observed in the infrared (Vink et al. 2024; see
also Section 6), we instead favor that the dominant emission in the Green
Monster region is due to circumstellar gas, rich in dust, that has been
excited by the forward shock and sculpted by stellar debris. In this scenario,
the central location within the main shell is simply a projection effect, and
the holes are the result of small knots ($\lesssim$ 0$\farcs$1) of high-
velocity ejecta that have punctured through the CSM and driven expanding
tangential shocks (e.g., Orlando et al. 2022).

Finally, an apparent blowout protruding outward from the main shell is seen in
the western portion of the remnant (Figure 3, panel 5). This blowout runs due
west and is different from the direction of the known southwest outflow (or
“jet”) (Fesen, 2001; Hwang et al., 2004). The morphology of this western
blowout shares some resemblance to the NE rupture in the main shell (Figure 3,
panel 3; see also Fesen & Gunderson 1996), and the general location is known
to host a large population of QSFs (Koo et al., 2018). The blowout could be
associated with interaction between the remnant and an abrupt density
discontinuity in the ISM, not unlike the breakout observed in the NE section
of the Galactic remnant CTB 1 (Fesen et al., 1997). Nonetheless, the MIRI
emission in this region has conspicuous overlap with i) Si-enriched X-ray
emitting ejecta (Vink, 2004) and ii) the projected trajectories of dozens of
high-velocity ($\gtrsim 8\times 10^{3}$ km s-1) S-rich ejecta knots moving
radially outward from the center of expansion (Fesen & Milisavljevic, 2016),
which together indicate that the blowout, like the NE rupture, may reflect
structure imprinted by the explosion dynamics of the original SN (Laming et
al., 2006).

##  4 IFU Spectroscopy

Our MIRI/MRS spectra only cover four very small fields of view (channel
dependent, ranging from $\sim$3.5″ to $\sim$7″111See the MIRI MRS page in the
JWST Documentation for details, https://jwst-docs.stsci.edu/jwst-mid-infrared-
instrument/miri-observing-modes/miri-medium-resolution-spectroscopy), but have
far superior spatial and spectra resolution compared to previous low-
resolution $5-38$ $\mu$m spectral maps of Cas A obtained by Spitzer with the
Infrared Spectrograph (IRS) (Smith et al., 2009). An overview of the spectra
of all four positions is shown in Figure 2. These MIRI/MRS positions sample a
diverse range of emission from the remnant and surrounding environment. P1 and
P3 were selected as regions covering different elemental compositions of
ejecta and dust excited by the reverse shock in the north and south; both the
shape of the dust continuum and the relative intensity of the emission lines
vary between the two positions. P2 is coincident with one of the small holes
of the Green Monster, but also samples a column of unshocked ejecta and dust
through the center of the remnant. P4 was a “deep drilling” IFU position
designed to be primarily sensitive to unshocked ejecta, especially iron,
through the center of the remnant. Selection of these areas was guided in part
by the Spitzer IRS spectral maps. Based on the in-flight performance report
from Argyriou et al. (2023), MIRI/MRS wavelength resolution is accurate to
2-27 km s-1 depending on wavelength, and has a spectrophotometric precision of
$\approx$5.6%.

###  4.1 Dust Emission

Cas A’s young age and proximity make it an ideal target for an in-depth
investigation of SN dust formation and destruction processes. Herschel
observations were able to probe the cold dust component in the inner unshocked
ejecta for the first time (Barlow et al., 2010; De Looze et al., 2017;
Priestley et al., 2019) and confirmed the high mass of newly formed dust
($0.2-1.0$ M⊙), but at a limited spatial resolution of $\sim 0.6^{\prime}$.
Similarly, the spatial resolution achieved with Spitzer IRS spectroscopy
(2.5′′ to 6′′) was insufficient to resolve individual ejecta knots, which made
it impossible to constrain the chemical pathways that lead to the formation of
specific warm dust species in Cas A. Spitzer IRS did, however, identify the
formation of many different dust populations around chemically distinct
regions in Cas A (Rho et al., 2009; Arendt et al., 2014). JWST can associate
spatially resolved warm dust knots with different chemical compositions (Ar,
Ne, S, O) and has the sensitivity to detect distinct dust emission features in
those individual knots. The detailed analysis of the MRS dust emission and
decomposition of the various dust species that condensed in the ejecta of Cas
A will be presented in a future publication. Here, we summarize the general
properties of the dust emission observed with MRS and compare them to previous
studies with Spitzer.

Figure 4: Mapping unshocked ejecta of Cas A. Top: Image resulting after
subtraction of a scaled version of the F2100W filter from F2550W, and masking
main shell emission (which appears as black). The emission inside the main
shell is mostly due to [O IV] 25.89 $\mu$m from unshocked oxygen material,
with potential minor contributions from cold dust, line emission from
filaments of shocked oxygen ejecta, and line emission from other
shocked/unshocked material including iron. The dashed box indicates the region
enlarged in the right panel, and the labeled boxes identify MIRI/MRS IFU
regions. Bottom: a 3D reconstruction of the MIRI/MRS [O IV] 25.89 $\mu$m
emission from P2 and P4 as a function of velocity showing the wide spread of
the ejecta in velocity space. Emission $>3\sigma$ above the continuum level is
represented.

The MIRI/MRS spectra in Figure 2 all show a continuum from warm dust grains
and various dust emission features. Position P1, which samples reverse-shocked
ejecta with particularly bright argon emission, shows prominent broad dust
features at 9, 12, and 21 µm. Arendt et al. (2014) attributed the 9 and 21 µm
features to $\rm Mg_{0.7}SiO_{2.7}$ grains characterized by a relatively low
magnesium-to-silicon ratio, while Rho et al. (2018) attributed the features to
SiO2 grains. Arendt et al. (2014) argued that the 12 $\micron$ feature could
arise from ellipsoidal SiO2 or SiC grains with a continuous distribution of
axial ratios, and that the dust with this spectral profile is likely a mixture
of silica and Mg silicates with Mg/Si $<$ 5\. The MRS spectra show that this
12 µm feature is likely present in all four positions in Cas A, while the 9
and 21 µm features are only apparent in the argon-rich ejecta. The narrow dust
feature at 11.3 $\micron$ that is likely produced by SiC grains (e.g. Jiang et
al., 2005) is also present in all four spectra shown in Figure 2. We note that
the same complex of dust emission features (9, 11.3, 12, and 21 $\micron$) is
present in the spectrum of newly condensed SN dust in SNR G54.1+0.3 (Temim et
al., 2010), which may be evidence that these features all arise from a common
grain species or that the multiple grain species that produce the set of
features are often found in the same environment.

Position P3 also samples reverse-shocked ejecta, but it is centered on a
region that is neon-bright. This region is characterized by a smooth and
featureless dust continuum that is clearly very different from position P1.
The continuum emission at positions P2 and P4 that sample the remnant interior
and the Green Monster region both show similar shapes with a “knee” in the
spectrum around $\sim$16 µm. Interestingly, the shape of the dust spectrum in
these interior positions is most similar to the Spitzer-observed spectrum
arising from Cas A’s CSM component, which Arendt et al. (2014) fitted with
either $\rm MgFeSiO_{4}$ or $\rm Mg_{2.4}SiO_{4.4}$ as the dominant dust
constituent.

In addition to providing a unique view of dust composition variations, the
extensive imaging of Cas A with JWST allows the small-scale structures of the
freshly condensed ejecta dust to be resolved at unprecedented spatial scales.
The high resolution of MIRI images allows ejecta knots to be resolved down to
sizes of $1-4\times 10^{16}$ cm (or 0.003-0.013 pc), with NIRCam pushing this
resolution limit to $6\times 10^{15}$ cm (or 0.002 pc) at 3.56 $\mu$m. Where
HST was able to resolve line-emitting ejecta knots down to similar scales,
JWST provides a complementary view of the dust-emitting structures in the
ejecta. Comparison of the specific dust compositions with the distribution and
elemental abundances of certain elements on resolved scales can provide us
with clues on how dust condensation proceeds. Future work can also use these
JWST observations of Cas A to address the predominant question of how much
freshly condensed dust will survive passage of the reverse shock. Model
estimates for Cas A predict a wide range of survival rates (1-50%; Bocchio et
al. 2016; Biscaro & Cherchneff 2016; Kirchschlager et al. 2019; Slavin et al.
2020). These estimates depend sensitively on their assumed pre-shock grain
size distribution and the clump sizes, along with the density contrast between
ambient ejecta and dense knots (Kirchschlager et al., 2023). JWST can resolve
the shocked dust emission in ejecta knots down to scales that constrain these
parameters with measurements that to date have been impossible to obtain.

###  4.2 Interior unshocked ejecta

State-of-the-art core-collapse simulations are having increased success
reproducing full-fledged explosions in 3D, and are beginning to evolve
explosions from core-collapse to the SNR phase to allow direct comparisons
with observations (Hungerford et al., 2003, 2005; Ellinger et al., 2012, 2013;
Wongwathanarat et al., 2013; Janka et al., 2016; Ono et al., 2020; Powell &
Müller, 2019; Burrows et al., 2020; Orlando et al., 2020; Vance et al., 2020;
Orlando et al., 2021). The specific explosion mechanism(s) and appropriate
treatment of physics enabling the forward shock to overcome the ram pressure
of infalling outer layers remain uncertain. A neutrino-driven explosion aided
by asymmetries is generally favored for Cas A (Hungerford et al. 2005; Young
et al. 2006; Wongwathanarat et al. 2017; Vance et al. 2020), but other
explosion mechanisms could participate or even dominate. For example, the two
Si-rich wide-angle jet-like outflows of Cas A in the NE and SW (Fesen, 2001;
Hwang et al., 2004), may be the result of magnetohydrodynamic jets (Khokhlov
et al., 1999; Maeda & Nomoto, 2003; Couch et al., 2009; Soker, 2018), or
reflect disk accretion of fallback matter by the new-born NS (see section 7).
A thermonuclear explosion (Kushnir & Katz, 2015), or quark nova (Ouyed et al.,
2011), have also been suggested.

The relative yields and distributions of elements, including radioactive
nuclei freshly synthesized during the SN such as 56Ni and 44Ti (and their
decay products) that originate from the innermost regions of the star where
the explosion initiated, provide valuable constraints for these simulations
(Grefenstette et al., 2014; Wongwathanarat et al., 2017; Grefenstette et al.,
2017; Fryer et al., 2023; Sieverding et al., 2023). Hundreds of years after
the SN, these nuclei continue to reflect the physical processes dominating the
physics of SN engines and probe the degree of explosion asymmetry.
Particularly important is the mass and distribution of Fe. The X-ray-bright
reverse-shocked Fe-rich ejecta of Cas A (which trace the original 56Ni
distribution) has velocities around and above 4000 km s-1 (Hughes et al.,
2000; DeLaney et al., 2010) and is associated with a mass of $\approx 0.13$ M⊙
(Hwang & Laming, 2012), which is a fair fraction of the total amount of Fe
expected to have been ejected in a neutrino-driven explosion of about $2\times
10^{51}$ erg as estimated for Cas A (Orlando et al., 2016). For this reason,
Cas A has been used as a clear example of how the ejecta asymmetries connected
to the neutrino-driven mechanism and the nature of the progenitor as a
H-stripped star can strongly influence the final distribution of ejecta by
seeding Rayleigh-Taylor instabilities that in turn encourage radial mixing
(Wongwathanarat et al., 2015, 2017).

Presently, uncertainties in theoretical models (due to both the unknown exact
mass and structure of the progenitor and uncertain degrees of freedom in the
neutrino physics of supernova calculations) leave it unclear how much
unshocked Fe is expected and observations have been unable to derive how much
is actually present in Cas A (e.g., Young et al., 2006; Eriksen et al., 2009).
Multiple lines of evidence support the notion that some Fe must still reside
in the center of Cas A (see, e.g., Hwang & Laming 2012; Milisavljevic & Fesen
2015; Grefenstette et al. 2017). However, how much and how it is distributed
are critical unknowns. By measuring the iron abundance, we obtain a better
understanding of the nature of the central engine. Among the best efforts to
date to investigate unshocked Fe was a deep 1.64 $\mu$m image of Cas A by Koo
et al. (2018), which likely traced interior [Si I] 1.645 $\mu$m line emission
as opposed to [Fe II] 1.644 $\mu$m line, and Laming & Temim (2020) that used
Spitzer IRS spectra of Cas A. Laming & Temim (2020) estimated the total
unshocked ejecta mass to be 0.47${}^{+0.47}_{-0.24}$ M⊙, which was broadly
consistent with previous estimates (Hwang & Laming, 2012; DeLaney et al.,
2014; Arias et al., 2018), and with mass fractions of 30% O, 60% Si, a few
percent S, and traces of Ne and Ar. No Fe was confidently detected in the
Spitzer data, but possible features at [Fe VIII] 5.446 and [Fe V] 20.85 $\mu$m
along with other considerations led to an upper limit Fe mass of $<0.07$ M⊙
(Laming & Temim, 2020).

We examined MIRI/MRS spectra of P2 and P4 closely for possible emission
features around the [Fe VIII] 5.446 and [Fe V] 20.85 $\mu$m emission lines.
The only possible candidate is faint emission peaked around 5.39 $\mu$m
observed only in the P4 region potentially associated with blue-shifted [Fe
VIII] 5.446 $\mu$m. A second weaker emission peak is also observed at 5.53
$\mu$m. The distribution of emission around these candidate lines is unlike
other ejecta lines that are narrower and better resemble what would be
expected from PAHs, though none are documented at these wavelengths.
Nonetheless, if this emission originates from truly diffuse, extended gas,
then the observed distribution is viable. Following assumptions in Laming &
Temim (2020) that emission lines come from density regions where their
emission is maximized, we estimate that the putative [Fe VIII] emission in the
P4 field of view (approximately $7\times 10^{-7}$ ergs cm-2 s-1 sr-1)
represents $\sim 10^{-5}$ M⊙ of Fe. If replicated similarly across the
remnant, this would imply $\sim 10^{-2}$ M⊙ of unshocked Fe material. However,
since the candidate emission feature is absent from P2 spectra, clearly the Fe
is not distributed uniformly. Future work can investigate the reality of this
candidate detection and carefully utilize all available images and spectra to
develop the best constraint possible of the total Fe yield from the supernova.

Another potential marker of unshocked ejecta available in the JWST range is
the relatively bright [O IV] 25.89 $\mu$m line emission (Figure 2). Even
though ejecta interior to the main shell are unshocked, they are still ionized
by the UV and X-ray emission from the surrounding reverse shock, resulting in
[O IV]. The F2550W filter is sensitive to [O IV], with additional contribution
from dust continuum emission. As seen in Figure 2, the F2100W filter primarily
samples dust continuum emission. By using the F2100W image as a continuum
reference, and scaling appropriately to match the general continuum level of
F2550W, the resultant of F2550W - F2100W, should primarily arise from [O IV].

We present this map of unshocked ejecta in Figure 4. A remarkable,
asymmetrical, web-like network of filaments resolved to 0.01 pc scales is seen
interior to Cas A’s main shell of reverse-shocked material. Many ejecta
streams directed outward from the center of expansion are seen, including a
thin filament that runs directly towards the center of the NE rupture (“NE
filament”), a branch of emission that runs almost due west (“Western branch”),
and multiple filaments extending in the SE direction towards the high-energy
ejecta plumes of iron observed in the X-ray (“SE filaments”) (Hughes et al.,
2000; DeLaney et al., 2010; Sato et al., 2021). A variety of rings/loops with
scales of $\approx 5$″ are embedded within these filaments. An especially
bright arc of looped filaments is seen east of the main shell center (labeled
“Central arc”). We interpret the significance of this structure in the context
of additional multi-wavelength comparisons in section 6.

We note that portions of our unshocked ejecta map may be contaminated by cold
dust embedded in the interior ejecta, as well as emission from filaments of
shocked ejecta. However, many arguments strongly support the view that the
dominant emission is indeed from unshocked ejecta. For example, the bright
“Central arc” is consistent with interior structure already mapped in [S III]
$\lambda\lambda$9069, 9531 by Milisavljevic & Fesen (2015) having a velocity
distribution of $-3000$ to $+4000$ km s-1. Likewise, the distribution of
emission in our unshocked ejecta map strongly correlates with features
interior to the main shell only seen in the [S III]-sensitive F1000W image.

The centrally-located P2 and P4 MIRI/MRS spectra provide a complementary probe
of the unshocked ejecta through the center of the remnant. We isolated [O IV]
emission in each spaxal by exploring the data cube with the STScI-developed
JDAViz tool (JDADF Developers et al., 2023) to find appropriate wavelength
regions to model the continuum. A 3rd order polynomial was used for the
continuum fit, which was then subtracted from the data cube. The cube was
cropped by $\sim 3$ pixels on each side to eliminate artifacts from the edge
of the detector. Coordinates in right ascension and declination were converted
into velocity using the scaling factor of 0.022′′ per km s-1 (Milisavljevic &
Fesen, 2013), with respect to the center of expansion (Thorstensen et al.,
2001). The data were linearly interpolated and resampled with a resolution of
22 km s-1 in all three dimensions, which is comparable to the original spatial
resolution.

The continuum-subtracted, cropped, interpolated [O IV] emission from P2 and P4
as a function of velocity is shown in the bottom of Figure 4 from two viewing
angles. Previous observations by Spitzer using the IRS presented by Isensee et
al. (2010) with spatial resolution of $2.5^{\prime\prime}$ and spectral
resolution of 0.05 $\mu$m indicated sheet-like structures and filaments,
including holes within the sheets at the scale of 30″. Our data reveal an even
more intricate distribution, with filaments seen at the scale of 1″. The
velocities range over $\pm$3000 km s-1, indicative of ejecta velocities and
not CSM. We interpret these MIRI/MRS as further confirmation that the interior
network of filaments seen in the top panel in Figure 4 traces unshocked ejecta
accurately.

Figure 5: Examples of light echoes around Cas A. Top panel shows an enlarged
section of the largest IR echo identified in the middle panels as E1.
Comparison of our NIRCam F356W image with archival Spitzer IRAC observations
(center) shows that the emission was not present in the past. The boxed region
E2 highlights an area where multiple epochs of MIRI observations are available
and the time variability of the light echoes can be tracked; this region is
enlarged in the bottom two panels. In the bottom right panel, sources with
negligible proper motion between the 109 days separating the MIRI F770W
observations appear white, whereas light echoes appear as red and teal between
the first and second observation.

The [O IV] emission is accompanied by fainter emission in the [S IV] 10.521
$\mu$m, [S III] 18.676 $\mu$m and [Ar III] 21.823 $\mu$m lines. Their spatial
structure within the IFU field and their velocity structure are nearly
identical to those of [O IV], implying that these elements are thoroughly
mixed down to the scale of the JWST resolution. In photoionization equilibrium
in Cas A, these ions exist at densities in the range of 1 - 20 amu/cc and
temperatures from about 20,000 K down to 600 K (Laming & Temim, 2020). This
gas fills around 20% of the volume. If higher density gas were present, the
[Ar II] 6.9842 $\mu$m line would be visible, but only a very narrow line from
the background or photoionized CSM is seen. There is no emission from Ne or
the [Fe II] and [Fe III] lines in the spectra of the unshocked ejecta, whereas
they are seen in the shocked ejecta at P1 and P3. Together this indicates that
substantial abundance variations occur on larger scales.

Figure 6: Top: The NIRCam F356W image compared to a VLA S-band image from 2012
(courtesy of T. DeLaney). Bottom: Left panel shows a composite image made
using the NuSTAR 44Ti map (Grefenstette et al., 2014), the Chandra 1 Msec
X-ray image of Cas A (Hwang et al., 2004), and our MIRI F1280W image. The
location of the NS (Fesen et al., 2006a) is marked with a black circle. The
right panel shows a composite image combining our unshocked ejecta map (see
Section 4.2), with the NuSTAR 44Ti map, and a Chandra Fe–K map (energy $\sim
6.7$ keV). The locations of the center of expansion (COE; Thorstensen et al.
2001) and the NS are marked with white and black circles, respectively.

##  5 Light echoes

Transient luminous sources such as novae and SNe can produce echoes of their
outbursts by way of reflected light from dusty regions in the surrounding ISM
(van den Bergh, 1965; Rest et al., 2005; Dwek & Arendt, 2008). Light echoes
have been recorded around Cas A and spectra of them have been used to classify
the explosion type (Krause et al., 2008), measure explosion asymmetry by
observing the original supernova from different lines of sight (Rest et al.,
2011), and probe the structure of the surrounding ISM itself (Kim et al.,
2008; Vogt et al., 2012; Besel & Krause, 2012). Generally, emission at
$\gtrsim 3$ $\mu$m is likely to be reradiated thermal emission from dust,
whereas the shorter wavelength emission is more likely to be scattered light
of the transient source itself. Light echoes can be recognized by their
structure (more sharply defined than ISM features), spectral energy
distribution (warmer than typical ISM), and most clearly by their dramatic
brightness changes (when data are available at multiple epochs).

Echoes of various brightness are found across the field of our JWST
observations. Figure 5 highlights two prominent regions of echoes. The region
labeled E2 shows typical bright echoes, which appear as bright clumps embedded
in fainter more filamentary structures. The additional JWST observations
discussed in section 2 were performed 109 days following the initial
observations, providing an unexpected opportunity to search for variations of
light echo features surrounding portions of Cas A. The lower right panel in
Figure 5 shows the dramatic changes seen in the MIRI F770W images between
these two epochs.

Even more surprising was the appearance of a particularly large and bright IR
echo in the corner of the NIRCam field (Figure 3, panel 6), enlarged in Figure
5. Comparison with archival Spitzer observations confirms that this emission
was not present previously, and detection across multiple filters ensures this
is a real feature. Recent HST imaging in December 2022 (GO 17210; PI: Fesen)
also weakly detected this new light echo, further verifying its reality. An
echo this close to Cas A ($\approx 4^{\prime}$ from the center of expansion)
means the reflecting surface is almost directly behind the remnant ($\sim 170$
lt-yr; see further Dwek & Arendt 2008). In this orientation, the echoing
ellipsoid provides a tomographic slice of that portion of the ISM, nearly in
the plane of the sky.

The incredible structure in this echo, showing many small isolated features
and concentric arclets, along with its overall well defined outer edge,
indicates a very complex ISM cloud surface. The thinnest strands remain
unresolved in width even in our NIRCam F162M image. To our knowledge, this is
the best view yet of ISM structure at the smallest of scales possible: 0.1′′
translates to $\approx 350$ A.U., or $\sim 2$ light-days, which is consistent
with the echoes changing completely on time scales of months (Dwek & Arendt,
2008), and implies that changes on time scales of hours are possible. The
F162M was observed in parallel with both the F356W and F444W observations,
which occurred 19.3 hours apart in the initial mapping. Comparison of the
F162M images of the large IR echo at these epochs does not reveal any very
short term changes in the echo brightness. This suggests that there were no
major changes in the illuminating SN emission on this time scale and/or that
the echoing structures are $\gtrsim 19$ lt-hr $\simeq 140$ A.U. in size.
However, F162M is not the band that is most sensitive to the echo emission.

It is worthwhile to note that Krause et al. (2005) had speculated that some of
the echoes may have been produced more recently ($\sim 1952$) by an unseen or
highly directional burst from the NS. Although Kim et al. (2008) and Dwek &
Arendt (2008) discounted this possibility based on geometry and energetics
required to fit the IR spectra, some suspicion lingers due to the locations,
morphology, and apparent motion of some echoes, which give the visual
impression that they lie within the circumstellar environment and thus must be
of recent origin. Optical (i.e., reflected) spectra of these echoes in the
immediate vicinity of Cas A would definitively determine if they are the
product of the Type IIb SN explosion (Krause et al., 2008), or a more recent
NS outburst of some kind.

##  6 Multi-wavelength comparisons

Cas A exhibits complex and overlapping sources of thermal and non-thermal
radiation, arising from ejecta, CSM, dust, molecules, and particle
acceleration. Comparing our JWST infrared observations with Chandra X-ray
Observatory and Jansky Very Large Array radio telescope data make it possible
to disentangle these many sources of emission.

Cas A is one of the brightest radio sources on the sky (Reber, 1944), having a
total flux density of $\sim 2000$ Jy in the 1-2 GHz band (Baars et al., 1977).
A relatively circular region with radius $\sim 100^{\prime\prime}$ is
associated with the reverse shock propagating into the expanding ejecta.
Weaker radio emission associated with synchrotron radiation (Ginzburg &
Syrovatskii, 1965) is seen at some azimuths extending out $\sim
150^{\prime\prime}$ to the location of the main forward shock, traveling at
4200 – 5200 km s-1 (DeLaney & Rudnick, 2003; Patnaude & Fesen, 2009).

Infrared emission shortward of about 5 $\mu$m is known to be largely
synchrotron emission from electrons accelerated in shocked regions (Rho et
al., 2003; Jones et al., 2003). Earlier Spitzer observations with IRAC at 3.6
$\mu$m were observed to resemble radio synchrotron images (Ennis et al.,
2006). A more recent study involving radio and Spitzer IRAC data revealed
evidence for spectral flattening from a spectral index of $\alpha\approx 0.77$
in the radio to $\alpha\approx 0.55$ for the radio to infrared comparison, in
particular for those regions close to shock regions (Domček et al., 2021).
However, in the southeast region the synchrotron spectra are relatively steep
$\alpha\approx 0.67$, perhaps indicating the onset of synchrotron cooling.
Synchrotron cooling should affect the synchrotron spectrum for frequencies
$\nu\gtrsim 7.2\times 10^{13}~{}B_{\rm mG}^{-3}t_{100}^{-2}$ Hz, with $B_{\rm
mG}$ being the magnetic-field strength in milliGauss (mG) and $t_{100}$ the
age of the shocked plasma in units of 100 yr. The F356W filter corresponds to
$\nu\approx 8.4\times 10^{13}$ Hz. This implies that signatures of synchrotron
cooling can be present for the plasma containing relativistic electrons that
have been accelerated early on ($t_{100}\approx 3$), and/or for $B\gtrsim 1$
mG. For comparison, estimates for the mean magnetic-field strength in Cas A
are $\sim 0.5$–1 mG (e.g. Rosenberg, 1970; Vink & Laming, 2003; Uchiyama &
Aharonian, 2008).

In Figure 6 (upper panels), we show the NIRCam F356W image alongside an S-band
VLA image of Cas A. The increase in sensitivity and spatial resolution of JWST
over Spitzer is clear in JWST’s ability to map the faint, extended synchrotron
radiation component. At least at a qualitative level, these images also reveal
patches of low infrared synchrotron surface brightness in the southeastern
region, in agreement with Domček et al. (2021). However, a more quantitative
analysis accounting for the variable background level is required to provide
solid evidence for synchrotron cooling effects in the southeastern region, and
perhaps elsewhere.

Other striking morphological features, most likely associated with synchrotron
radiation, are the conspicuous cell-like patterns concentrated in the central
southwest region (Figure 6). Although these features appear to have a radio
counterpart, the contrast between the patterns and more diffuse synchrotron
emission is lower. A preliminary explanation for the infrared pattern is that
it is the consequence of regions observed edge-on, perhaps associated with
blowout regions on the near- or far-sides of Cas A, or with protrusions in the
reverse shock (Blondin & Ellison, 2001; Mandal et al., 2023). If the infrared
synchrotron emission from these regions is affected by synchrotron cooling,
then the shells surrounding these blowout regions are narrower in the infrared
than in the radio, providing stronger limb-brightening effects. Portions of
the cell pattern align with X-ray synchrotron filaments, for which synchrotron
cooling effects are even more prominent.

In Figure 6 (lower left panel), we show a composite image using 0.3-10 keV
Chandra broadband observations (Hwang & Laming, 2012), a NuSTAR 44Ti map
(Grefenstette et al., 2014), and our MIRI F1280W image. A gap in the 44Ti
emission that runs north-south is approximately where the brightest ridge of
the Green Monster is located. Because we favor the interpretation that the
bulk emission of the Green Monster is associated with CSM on the front side of
the remnant (towards us), the anti-correlation with 44Ti (which is redshifted
overall; Grefenstette et al. 2017) should be coincidental. Absorption of 44Ti
emission ($\approx 70$ keV) from the Green Monster seems unlikely, since a CSM
column density of $\sim 10^{27}$ cm-2 would be needed to reduce the flux by a
factor of 10. Nonetheless, clear correlations in emission are seen between
Chandra and our JWST data, suggesting a shared origin. De Looze et al. (2024)
and Vink et al. (2024) investigate this X-ray–IR relationship in detail and
attribute it to interaction between the forward shock and relatively dense
CSM.

Another composite image is shown in Figure 6 (lower right) that compares our
unshocked ejecta map (section 4.2; Figure 4) with a Chandra Fe–K map (energy
$\sim 6.7$ keV; tracing where 56Ni was produced), and the NuSTAR 44Ti map. As
noted earlier, the SE filaments and Western branch of the unshocked ejecta
connect with Fe-rich locations. On the other hand, no strong relationship is
seen between [O IV] and 44Ti, except that 44Ti emission is located where [O
IV] is weak. Together, these comparisons support the view that the fine
structures in the interior volume are most likely rooted in turbulent mixing
of cool, low-entropy matter from the progenitor’s oxygen layer with hot,
neutrino and radioactively heated high-entropy matter.

##  7 Neutron Star

Figure 7: The JWST $3\sigma$ upper limits in the near-infrared (NIRCam F162M,
F356W, F444W) at the location of the Cas A CCO and the CCO’s X-ray spectrum
detected with Chandra ACIS. Power law spectra consistent with JWST upper
limits and an example blackbody spectrum with a temperature of 900 K are
plotted for an extinction of $A_{V}=8$, using the extinction curve by Gordon
et al. (2023). The Chandra ACIS X-ray data are from Posselt & Pavlov (2018),
the HST optical and NIR limits are from Fesen et al. (2006b), and the Spitzer
limits are from Wang et al. (2007).

First-light images of Cas A taken by Chandra revealed a central X-ray point
source (Tananbaum, 1999; Pavlov et al., 2000), that is presumably its remnant
NS. This NS is part of the family of enigmatic central compact objects (CCOs),
which are young, exhibit X-ray emission that is steady and predominantly
thermal, and lack surrounding pulsar wind nebulae and counterparts at other
wavelengths (Pavlov et al., 2004; Halpern & Gotthelf, 2010; De Luca, 2017).
CCOs have relatively low inferred magnetic fields, $B\sim(3$–$10)\times
10^{10}$ G for the three CCOs where it was estimated (i.e., two orders of
magnitude lower than that of a typical young NS), and X-ray luminosities
larger than their spin-down power (Gotthelf et al., 2013).

Deep HST NICMOS images of the Cas A center in NIR filters (Fesen et al.,
2006b) did not detect the CCO. The JWST NIRCam images enable a much deeper
look at the CCO of Cas A, which is regarded as a key object to understanding
NS evolution. The Cas A CCO is the youngest of the dozen currently known CCOs,
as well as the youngest of all the known NSs for which we can directly study
the surface emission. It is particularly interesting because it has possibly
shown a noticeable cooling during the 20+ years of Chandra observations
(Shternin et al., 2023, and references therein; but see Posselt & Pavlov 2022
and Alford & Halpern 2023).

Because ongoing accretion can be excluded for CCOs (Pavlov et al., 2004),
their thermal X-rays are likely emitted from the NS surface layers heated by
the heat transfer from the cooling NS interiors. The low magnetic fields of
CCOs can be explained by two scenarios: (1) these NSs are born with weak
magnetic fields (“anti-magnetars”) or (2) the normal ($\sim 10^{12}$ G) or
strong ($\gtrsim 10^{14}$ G) magnetic field of a newborn NS has been buried in
its crust by “fallback” debris after the SN explosion (Viganò & Pons, 2012).
Such fallback could also form a disk surrounding the NS. Comprehensive
searches at X-ray and radio wavelengths have failed to find evolutionary
descendants of these young CCOs (Gotthelf et al., 2013). Disk emission in the
IR would strongly support the re-emergence of magnetic field scenario and
explain why it is so difficult to identify CCO descendants.

Non-detections outside the X-ray range are usually attributed to a lack of
pulsar activity because of the low magnetic field or to high extinction
(Pavlov et al., 2004; Halpern & Gotthelf, 2010; De Luca, 2017). Our NIRCam
images offer the possibility to peer deeply enough through high extinction at
wavelengths other than X-rays to test hypotheses about the CCO’s nature. To
this end, we conducted forced photometry at the coordinates of the CCO
($\alpha$(J2000) = 23h23m27$\fs$943, $\delta$(J2000) =
+58o$48^{\prime}$42$\farcs$5; Fesen et al. 2006b), on the three NIRCam filter
images using the space_phot222See
https://app.soos.io/research/packages/Python/-/space-phot. package. The
photometry was measured using point-spread function (PSF) photometry on the
level 2 NIRCam “CAL” images, which preserves the PSF structure compared to
level 3 resampled images. The NIRCam pixel area map for the corresponding SW
and LW modules must also be applied to each exposure to correct for pixel area
variations across the images. Here we use the PSF models provided by WebbPSF
(version 1.2.1)333https://www.stsci.edu/jwst/science-planning/proposal-
planning-toolbox/psf-simulation-tool to represent the PSF, which have been
updated to better match the observed PSF in each filter, and take into account
temporal and spatial variation across the detector. We remove a constant
background in the fitting region, which is small enough that a constant
background is sufficient. The background is estimated as the median value in
an annulus centered at the position with an inner radius of 5 pixels and outer
radius 7 pixels. space_phot simultaneously constrains the common flux in all
CALs at the forced photometry position, within a $5\times 5$ pixel square. The
final measured flux is the integral of each full fitted PSF model, which
includes a correction to the infinite aperture flux. These total fluxes, which
are in units of MJy/sr, are converted to AB magnitudes using the native pixel
scale of each image.

No sources were detected confidently, and we estimate 3$\sigma$ AB magnitude
upper limits of 28.15, 28.31 and 27.90, corresponding to 20.0, 17.3 and 25.2
nJy for the F162M, F356W, and F444W images, respectively. There are three
faint source candidates within an arcsecond of the CCO coordinates with S/N
$\approx 2-3$ in the F162M image. However, these candidate sources are
consistent with detector noise patterns seen elsewhere, and are not present in
other filters, so we judge these to not be credible CCO candidates. We
performed the same procedure on a modified version of the F162M image where
the so-called $1/f$ noise had been removed by custom processing the stage 2
pipeline output and rerunning the stage 3 pipeline, but this exercise did not
yield a deeper limit.

Thus, although our limits are much deeper than those found by Fesen et al.
(2006b), as seen in Figure 7, the CCO remains undetected. We should note that
even these deep limits are much higher than the expected thermal emission from
the NS with $T_{\rm eff}=2\times 10^{6}$ K (e.g., the predicted flux density
at 3 $\mu$m is $\sim 1.4\times 10^{-13}$ Jy). However, the measured JWST
limits can be used to constrain some possible scenarios in which IR emission
could be expected.

Firstly, we can constrain the area of any circular disk with a blackbody
temperature similar to the one inferred from the Spitzer detection of the
magnetar 4U 0142+61 (Wang et al., 2006). Such a disk with temperature $\sim
900$ K must have an area smaller than $5.5\times 10^{20}(\cos i)^{-1}$ cm2
(i.e., $R_{\rm out}<0.2(\cos i)^{-1/2}R_{\odot}$), where $i$ is the disk
inclination, and $R_{\rm out}$ is the outer radius of the disk.

The temperature of a realistic fallback disk is expected to decrease outwards.
If the decrease can be approximated by a power law, $T(r)=T_{\rm in}(r/R_{\rm
in})^{-\beta}$, where $T_{\rm in}$ is the temperature at the inner disk radius
$R_{\rm in}$, then the spectrum of the disk emission is
$f_{\nu}\propto\nu^{3-(2/\beta)}$ at $kT_{\rm out}\lesssim h\nu\lesssim
kT_{\rm in}$, where $T_{\rm out}=T_{\rm in}(R_{\rm in}/R_{\rm out})^{\beta}$
is the temperature at the outer edge of the disk. For instance, the limiting
power-law spectra $f_{\nu}\propto\nu^{-1}$ and $\nu^{-2}$, shown in Figure 7,
correspond to $\beta=0.5$ and 0.4, respectively.

If the disk is heated by X-rays produced by the NS, one can expect its IR flux
to be proportional to the X-ray flux. For the Cas A’s CCO, the upper limit on
the IR to X-ray flux ratio is $F_{\rm F356W}/F_{\rm 0.6-6\,keV}<5\times
10^{-6}$. This limit is substantially lower than $F_{\rm F356W}/F_{\rm
0.6-6\,keV}\sim 4\times 10^{-5}$ for the magnetar 4U 0142+61, or $F_{\rm
F160W}/F_{X}=3\times 10^{-4}$ for the extended NIR emission around the much
older ($\sim 0.5$ Myr) X-ray thermal isolated neutron star RX J0806.4$-$4123
(Posselt et al., 2018).

A power law spectrum is also expected for non-thermal emission from the NS or
from a pulsar wind nebula. In the case of the Cas A CCO, the non-thermal X-ray
emission may be faint and overpowered by the strong thermal emission from the
NS surface, but nonthermal emission could in principle be detectable at IR
wavelengths, where the thermal NS emission is undetectable. The new JWST
limits, however, indicate that any non-thermal NS emission must be very faint.

##  8 Conclusions

We have presented results from an extensive JWST survey of the SN remnant Cas
A made up of NIRCam + MIRI imaging mosaics and exploratory MIRI/IFU
spectroscopy. Our observations provide the most detailed and comprehensive
mapping ever of its ejecta, CSM/ISM, and associated dust and molecules at NIR
and MIR wavelengths. Significant findings and implications made from this
survey include the following:

  1. 1.

We have uncovered a web-like network of unshocked ejecta filaments seen in [O
IV] emission and resolved to $\sim$ 0.01 pc scales reflecting turbulent mixing
processes and hydrodynamical instabilities that occurred shortly after core
collapse of the progenitor star. We weakly detect emission possibly associated
with [Fe VIII] 5.446 $\mu$m that had been predicted by Laming & Temim (2020),
and estimate upwards of $\sim 10^{-2}$ M⊙ of diffuse, unshocked Fe material.
Many aspects of this distribution have significant relevance for core-collapse
SN simulations, including a clear connection seen between the unshocked ejecta
filaments and Chandra Fe–K emission. Correlations with NuSTAR are less clear
but worth further investigation. This finding is also relevant for attempts to
simulate light curves and spectra of unresolved extragalactic SNe, which must
make assumptions regarding the fragmentation of the ejecta due to
hydrodynamical instabilities. Ni-bubble expansion reduces the effective
optical depth of non-radioactive material (Dessart et al., 2018; Dessart &
Audit, 2019), and inappropriate treatment could mean that the current ejecta
masses estimated for the majority of stripped-envelope SNe are systematically
underestimated (Ergon & Fransson, 2022; Ergon et al., 2023).

  2. 2.

A large structure called the “Green Monster,” interior to the main shell and
bright in MIRI images, is resolved for the first time. We conclude that this
is most likely associated with a thick sheet (or sheets) of dust-dominated
emission from shocked CSM seen in projection toward the remnant’s interior.
The structure is pockmarked with small ($\sim 1^{\prime\prime}$) round holes
that have most likely been formed by high-velocity N- and He-rich ejecta knots
that have pierced through the CSM and driven expanding tangential shocks.
Support for this conclusion, including detailed analysis of MIRI/MRS and
NIRSpec spectroscopy and correlated X-ray emission observed with Chandra, is
provided in De Looze et al. (2024) and Vink et al. (2024). The discovery
provides an exciting opportunity to probe mass loss of the progenitor system
some $10^{4-5}$ years prior to explosion.

  3. 3.

Dozens of light echoes with angular sizes between $\sim 0\farcs 1$ to
$1^{\prime}$ reflecting previously unseen structure in the ISM are found
throughout our NIRCam and MIRI images. Although light echoes had been expected
in the vicinity of Cas A, the brightness, size, and especially the complexity
of an echo seen in the SW corner of our NIRCam mosaics is quite startling. Its
fine-scale structure suggests relatively poor mixing, and raises questions of
how it came to be this way. Is the dust not mixed uniformly into the ISM gas,
or is the gas also structured this way? Was this once some sort of concentric
shells of outflow from a star that have been buffeted and twisted by
turbulence in the ISM? Are magnetic fields involved? If this structure is
typical of the large ISM clouds, do we see the imprint of this on older SNRs
in some way? A more detailed investigation of the large SW echo and other
light echoes may start to answer these questions.

  4. 4.

We do not detect the neutron star in the center of Cas A. Our NIRCam
observations place new upper limits on the CCO’s infrared emission ($\lesssim
20$ nJy at 3 $\mu$m) and tightly constrain scenarios involving a possible
fallback disk.

Additional epochs of JWST observations would make it possible to improve
characterization of the kinematic and elemental properties of the ejecta via
proper motion measurements, while also permitting further study of nearby
light echoes. Imaging mosaics with wider fields of view (reaching 5′ from the
center of expansion versus the $\approx$3′ reach of our existing data) would
increase opportunities for these measurements dramatically, and cover the
NE/SW outflows to more accurately constrain their total mass, composition, and
relationship to the original SN (Fesen & Milisavljevic, 2016). Additional
spectroscopy, including widefield NIRSpec MSA observations across the entire
SNR and targeted MIRI/MRS observations of regions interior to the main shell
(especially ones that overlap with 44Ti emission), would be invaluable to
improve estimates of chemical abundances and place the deepest limits possible
for the total Fe yield.

This work is based on observations made with the NASA/ESA/CSA James Webb Space
Telescope. The data were obtained from the Mikulski Archive for Space
Telescopes at the Space Telescope Science Institute, which is operated by the
Association of Universities for Research in Astronomy, Inc., under NASA
contract NAS 5-03127 for JWST. These observations are associated with program
#1947. Support for program #1947 was provided by NASA through a grant from the
Space Telescope Science Institute, which is operated by the Association of
Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127.

D.M. thanks the Munich Institute for Astro-, Particle and BioPhysics for
hosting the “Interacting Supernovae” program where interpretations used in
this manuscript were developed. We are grateful for the Lorentz Center for
hosting the workshop “Supernova Remnants in Complex Environments” where some
ideas used in this manuscript were developed. Tracey DeLaney provided Spitzer
IRS and VLA S-band data that were inspected as part of this paper. Brian
Grefenstette provided the NuSTAR 44Ti map used in this paper. Una Hwang
provided the Chandra 1 Msec image used in this paper. D.M. is grateful for
conversations with Luc Dessart, who contributed ideas used in this paper. D.M.
acknowledges invaluable support from Mark Linvill and Chris Orr who help
manage computational resources used to complete this research.

D.M. acknowledges NSF support from grants PHY-2209451 and AST-2206532. T.T.
acknowledges support from the NSF grant 2205314 and the JWST grant JWST-
GO-01947.031. J.M.L. was supported by JWST grant JWST-GO-01947.023 and by
basic research funds of the Office of Naval Research. I.D.L. acknowledges
funding from the Belgian Science Policy Office (BELSPO) through the PRODEX
project “JWST/MIRI Science exploitation” (C4000142239). I.D.L., F.K., and
N.S.S. have received funding from the European Research Council (ERC) under
the European Union’s Horizon 2020 research and innovation programme DustOrigin
(ERC-2019-StG-851622). M.J.B. acknowledges support from the ERC grant SNDUST
ERC-2015-AdG-694520. M.M. and R.W. acknowledge support from STFC Consolidated
grant (ST/W000830/1). S.O. acknowledges support from PRIN MUR 2022
(20224MNC5A). B.-C.K. acknowledges support from the Basic Science Research
Program through the NRF of Korea funded by the Ministry of Science, ICT and
Future Planning (RS-2023-00277370). Work by R.G.A. was supported by NASA under
award number 80GSFC21M0002. The research of J.C.W. is supported by NSF
AST-1813825. H.-T.J. is grateful for support from the German Research
Foundation (DFG) through the Collaborative Research Centre “Neutrinos and Dark
Matter in Astro- and Particle Physics (NDM),” Grant No. SFB-1258-283604770,
and under Germany’s Excellence Strategy through the Cluster of Excellence
ORIGINS EXC-2094-390783311.

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The color composite images shown in the body of this paper are interesting for
showing the relative differences between multiple filters of data, but they
can also hide details that are present within the individual data sets,
especially for our MIRI imaging. As indicated in Figure 2, some bandpasses are
dominated by continuum emission and others contain both continuum and line
emission. In this Appendix, we show images of each filter of the NIRCam and
MIRI imaging mosaics individually to allow these details to be assessed on a
filter by filter basis.

Figure 8: Three NIRCam imaging mosaics obtained as part of our Cas A survey.
The F162M filter was used as the SW channel for both the F356W and F444W
observations, so the exposure time is twice as long. Gaps are seen in the
F162M mosaic because of the dithering 3-point pattern that prioritized the LW
filters and introduced gaps in the SW channel detectors. The images are shown
at the same scale and orientation, with North and East indicated at upper
right. In the color version at lower right, the blue, green and red show the
images in increasing wavelength, respectively. Figure 9: The four shorter
wavelength MIRI imaging mosaics obtained as part of the survey. The full
spatial coverage (including Lyot coronagraph) is shown. All panels are shown
on the same flux scale, and all except F560W have a maximum intensity set to
50 MJy sr-1. The common scale and orientation are shown in the upper left
panel. Figure 10: Same as Figure 9, except showing the four longer wavelength
MIRI imaging mosaics.