## Appendix A: Elbow plots

The elbow plots generated for each of the 3.2–3.6, 5.95–6.6, 7.25–8.95, and 10.9–11.63 µm wavelength regions are given in Fig. A.1. The elbow plots generated prior to the initial and secondary rounds of clustering involving in the Atomic-PDR-focused clustering experiments are given in Fig. A.2. We also report the silhouette scores and elbow points for each experiment in Table A.1.

<table><caption>**Table A.1.** Average silhouette scores and elbow points for each of the clustering experiments are given.</caption><thead><tr><th>Δλ<br>[μm]</th><th>#<br>Clusters</th><th>Silhouette<br>Score</th><th>Elbow<br>Point</th></tr></thead><tbody><tr><td>3.2-3.6</td><td>2</td><td>0.57</td><td>4</td></tr><tr><td>3.2-3.6</td><td>3</td><td>0.457</td><td>4</td></tr><tr><td>3.2-3.6</td><td>4</td><td>0.328</td><td>4</td></tr><tr><td>3.2-3.6</td><td>5</td><td>0.286</td><td>4</td></tr><tr><td>3.2-3.6</td><td>6</td><td>0.268</td><td>4</td></tr><tr><td>3.2-3.6</td><td>7</td><td>0.271</td><td>4</td></tr><tr><td>5.95-6.60</td><td>2</td><td>0.521</td><td>4</td></tr><tr><td>5.95-6.60</td><td>3</td><td>0.404</td><td>4</td></tr><tr><td>5.95-6.60</td><td>4</td><td>0.306</td><td>4</td></tr><tr><td>5.95-6.60</td><td>5</td><td>0.273</td><td>4</td></tr><tr><td>5.95-6.60</td><td>6</td><td>0.26</td><td>4</td></tr><tr><td>5.95-6.60</td><td>7</td><td>0.212</td><td>4</td></tr><tr><td>7.25-8.95</td><td>2</td><td>0.403</td><td>5</td></tr><tr><td>7.25-8.95</td><td>3</td><td>0.416</td><td>5</td></tr><tr><td>7.25-8.95</td><td>4</td><td>0.420</td><td>5</td></tr><tr><td>7.25-8.95</td><td>5</td><td>0.423</td><td>5</td></tr><tr><td>7.25-8.95</td><td>6</td><td>0.306</td><td>5</td></tr><tr><td>7.25-8.95</td><td>7</td><td>0.228</td><td>5</td></tr><tr><td>10.90-11.63</td><td>2</td><td>0.612</td><td>4</td></tr><tr><td>10.90-11.63</td><td>3</td><td>0.476</td><td>4</td></tr><tr><td>10.90-11.63</td><td>4</td><td>0.469</td><td>4</td></tr><tr><td>10.90-11.63</td><td>5</td><td>0.456</td><td>4</td></tr><tr><td>10.90-11.63</td><td>6</td><td>0.405</td><td>4</td></tr><tr><td>10.90-11.63</td><td>7</td><td>0.397</td><td>4</td></tr><tr><td>&lt;13.2<sup>(a)</sup></td><td>2</td><td>0.471</td><td>5</td></tr><tr><td>&lt;13.2</td><td>3</td><td>0.493</td><td>5</td></tr><tr><td>&lt;13.2</td><td>4</td><td>0.349</td><td>5</td></tr><tr><td>&lt;13.2</td><td>5</td><td>0.336</td><td>5</td></tr><tr><td>&lt;13.2</td><td>6</td><td>0.33</td><td>5</td></tr><tr><td>&lt;13.2</td><td>7</td><td>0.33</td><td>5</td></tr></tbody></table>

**Notes.** (a) Here are silhouette scores for the secondary round of clustering applied to the pixels belonging to the cluster from the initial round of clustering that coincides most closely to the atomic PDR.
## Appendix A: Elbow plots

The elbow plots generated for each of the 3.2–3.6, 5.95–6.6, 7.25–8.95, and 10.9–11.63 µm wavelength regions are given in Fig. A.1. The elbow plots generated prior to the initial and secondary rounds of clustering involving in the Atomic-PDR-focused clustering experiments are given in Fig. A.2. We also report the silhouette scores and elbow points for each experiment in Table A.1.

<table><caption>**Table A.1.** Average silhouette scores and elbow points for each of the clustering experiments are given.</caption><thead><tr><th>Δλ<br>[μm]</th><th>#<br>Clusters</th><th>Silhouette<br>Score</th><th>Elbow<br>Point</th></tr></thead><tbody><tr><td>3.2-3.6</td><td>2</td><td>0.57</td><td>4</td></tr><tr><td>3.2-3.6</td><td>3</td><td>0.457</td><td>4</td></tr><tr><td>3.2-3.6</td><td>4</td><td>0.328</td><td>4</td></tr><tr><td>3.2-3.6</td><td>5</td><td>0.286</td><td>4</td></tr><tr><td>3.2-3.6</td><td>6</td><td>0.268</td><td>4</td></tr><tr><td>3.2-3.6</td><td>7</td><td>0.271</td><td>4</td></tr><tr><td>5.95-6.60</td><td>2</td><td>0.521</td><td>4</td></tr><tr><td>5.95-6.60</td><td>3</td><td>0.404</td><td>4</td></tr><tr><td>5.95-6.60</td><td>4</td><td>0.306</td><td>4</td></tr><tr><td>5.95-6.60</td><td>5</td><td>0.273</td><td>4</td></tr><tr><td>5.95-6.60</td><td>6</td><td>0.26</td><td>4</td></tr><tr><td>5.95-6.60</td><td>7</td><td>0.212</td><td>4</td></tr><tr><td>7.25-8.95</td><td>2</td><td>0.403</td><td>5</td></tr><tr><td>7.25-8.95</td><td>3</td><td>0.416</td><td>5</td></tr><tr><td>7.25-8.95</td><td>4</td><td>0.420</td><td>5</td></tr><tr><td>7.25-8.95</td><td>5</td><td>0.423</td><td>5</td></tr><tr><td>7.25-8.95</td><td>6</td><td>0.306</td><td>5</td></tr><tr><td>7.25-8.95</td><td>7</td><td>0.228</td><td>5</td></tr><tr><td>10.90-11.63</td><td>2</td><td>0.612</td><td>4</td></tr><tr><td>10.90-11.63</td><td>3</td><td>0.476</td><td>4</td></tr><tr><td>10.90-11.63</td><td>4</td><td>0.469</td><td>4</td></tr><tr><td>10.90-11.63</td><td>5</td><td>0.456</td><td>4</td></tr><tr><td>10.90-11.63</td><td>6</td><td>0.405</td><td>4</td></tr><tr><td>10.90-11.63</td><td>7</td><td>0.397</td><td>4</td></tr><tr><td>&lt;13.2<sup>(a)</sup></td><td>2</td><td>0.471</td><td>5</td></tr><tr><td>&lt;13.2</td><td>3</td><td>0.493</td><td>5</td></tr><tr><td>&lt;13.2</td><td>4</td><td>0.349</td><td>5</td></tr><tr><td>&lt;13.2</td><td>5</td><td>0.336</td><td>5</td></tr><tr><td>&lt;13.2</td><td>6</td><td>0.33</td><td>5</td></tr><tr><td>&lt;13.2</td><td>7</td><td>0.33</td><td>5</td></tr></tbody></table>

**Notes.** (a) Here are silhouette scores for the secondary round of clustering applied to the pixels belonging to the cluster from the initial round of clustering that coincides most closely to the atomic PDR.
<figure><img src="image_12.png" /><figcaption>**Fig. A.1.** Distortion scores for clustering applied in the 3.2-3.6, 5.95-6.6, 7.25-8.95, and 10.9-11.63 µm region plotted as a function of the number of clusters used. The inflection point, or ‘elbow’, in the distortion curve is highlighted with the dashed black line. As is shown, four clusters are optimal for each wavelength regime.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Fig. A.2.** Distortion scores for clustering on the first (left) and second (right) round of clustering for the 4.9-13.2 µm region in the MIRI MRS dataset as a function of the number of clusters used. The inflection point, or ‘elbow’, in the distortion curve is highlighted with the dashed black line. In the case of the first round of clustering across the entire, cleaned MIRI MRS footprint, four clusters are optimal. In the case of the clustering focused to pixels belonging to the cluster coincident with the atomic PDR, five clusters are optimal.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>**Fig. A.1.** Distortion scores for clustering applied in the 3.2-3.6, 5.95-6.6, 7.25-8.95, and 10.9-11.63 µm region plotted as a function of the number of clusters used. The inflection point, or ‘elbow’, in the distortion curve is highlighted with the dashed black line. As is shown, four clusters are optimal for each wavelength regime.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Fig. A.2.** Distortion scores for clustering on the first (left) and second (right) round of clustering for the 4.9-13.2 µm region in the MIRI MRS dataset as a function of the number of clusters used. The inflection point, or ‘elbow’, in the distortion curve is highlighted with the dashed black line. In the case of the first round of clustering across the entire, cleaned MIRI MRS footprint, four clusters are optimal. In the case of the clustering focused to pixels belonging to the cluster coincident with the atomic PDR, five clusters are optimal.</figcaption></figure>
**Appendix B: Further clustering applications**

Averaged spectral profiles for clusters calculated on each of the 3.2–3.6 µm, 5.95–6.6 µm, 7.25–8.95 µm and 10.9–11.63 µm wavelength regions were calculated on a number of clusters ranging from three to seven, inclusively. These average cluster profiles along with the corresponding spatial maps for the cluster zones, are given in Figs. B.1, B.2, B.3 and B.4, respectively, and for the 4.9-13.2 µm region in Fig. B.5. The mean spectral profile and corresponding cluster zone map for the clustering application on the 3.29 µm feature (from 3.2–3.36 µm) are given in Fig. B.6.
**Appendix B: Further clustering applications**

Averaged spectral profiles for clusters calculated on each of the 3.2–3.6 µm, 5.95–6.6 µm, 7.25–8.95 µm and 10.9–11.63 µm wavelength regions were calculated on a number of clusters ranging from three to seven, inclusively. These average cluster profiles along with the corresponding spatial maps for the cluster zones, are given in Figs. B.1, B.2, B.3 and B.4, respectively, and for the 4.9-13.2 µm region in Fig. B.5. The mean spectral profile and corresponding cluster zone map for the clustering application on the 3.29 µm feature (from 3.2–3.36 µm) are given in Fig. B.6.
<figure><img src="image_14.png" /><figcaption>**Fig. B.1.** Average spectral profile (left) and spatial footprint (right) for clusters determined in the 3.2 – 3.6 µm region. Each cluster is labelled with a number (in an arbitrary manner). Shading and normalisation (left), and labels and masked pixels (right) are the same as in Fig. 3.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>**Fig. B.1.** Average spectral profile (left) and spatial footprint (right) for clusters determined in the 3.2 – 3.6 µm region. Each cluster is labelled with a number (in an arbitrary manner). Shading and normalisation (left), and labels and masked pixels (right) are the same as in Fig. 3.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>**Fig. B.2.** Average spectral profile (left) and spatial footprint (right) for clusters determined in the 5.95 – 6.6 µm region. Each cluster is labelled with a number (in an arbitrary manner). Shading and normalisation (left), and labels and masked pixels (right) are the same as in Fig. 4.</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>**Fig. B.2.** Average spectral profile (left) and spatial footprint (right) for clusters determined in the 5.95 – 6.6 µm region. Each cluster is labelled with a number (in an arbitrary manner). Shading and normalisation (left), and labels and masked pixels (right) are the same as in Fig. 4.</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>**Fig. B.3.** The average spectral profile (left) and spatial footprint (right) for clusters determined in the 7.25–8.95 µm region. Each cluster is labelled with a number (in an arbitrary manner). Shading and normalisation (left), and labels and masked pixels (right) are the same as in Fig. 5.</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>**Fig. B.3.** The average spectral profile (left) and spatial footprint (right) for clusters determined in the 7.25–8.95 µm region. Each cluster is labelled with a number (in an arbitrary manner). Shading and normalisation (left), and labels and masked pixels (right) are the same as in Fig. 5.</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>**Fig. B.4.** Average spectral profile (left) and spatial footprint (right) for clusters determined in the 10.9 – 11.63 µm region. Each cluster is labelled with a number (in an arbitrary manner). Shading and normalisation (left), and labels and masked pixels (right) are the same as in Fig. 6.</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>**Fig. B.4.** Average spectral profile (left) and spatial footprint (right) for clusters determined in the 10.9 – 11.63 µm region. Each cluster is labelled with a number (in an arbitrary manner). Shading and normalisation (left), and labels and masked pixels (right) are the same as in Fig. 6.</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>**Fig. B.5.** Spatial footprint of the MIRI MRS FOV is colour-coded by cluster assignment for three (top left), five (top right), six (bottom left) and seven (bottom right) clusters applied to the 4.9 – 13.2 µm region. Cluster assignment is calculated on the secondary round of clustering applied only to those pixels belonging to cluster 3 in the initial round of clustering (see Fig. 7). Cluster and FOV labels are the same as in Fig. 7.</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>**Fig. B.5.** Spatial footprint of the MIRI MRS FOV is colour-coded by cluster assignment for three (top left), five (top right), six (bottom left) and seven (bottom right) clusters applied to the 4.9 – 13.2 µm region. Cluster assignment is calculated on the secondary round of clustering applied only to those pixels belonging to cluster 3 in the initial round of clustering (see Fig. 7). Cluster and FOV labels are the same as in Fig. 7.</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>**Fig. B.6.** Average spectral profile (left) and spatial footprint (right) for clusters determined in the 3.2 – 3.36 µm region. Each cluster is labelled with a number (in an arbitrary manner). Shading and normalisation (left), and labels and masked pixels (right) are the same as in Fig. 3.</figcaption></figure>

**Appendix C: Further comparison of clustering results**

Comparisons of the mean spectral profiles and cluster zone maps for each of the 7.25 – 8.95 and 3.2 – 3.6 µm regions with those based on clusters from the 10.9 – 11.63 µm region were also performed. The results from these experiments are given in Figs. C.1 and C.2, respectively.
<figure><img src="image_20.png" /><figcaption>**Fig. B.6.** Average spectral profile (left) and spatial footprint (right) for clusters determined in the 3.2 – 3.36 µm region. Each cluster is labelled with a number (in an arbitrary manner). Shading and normalisation (left), and labels and masked pixels (right) are the same as in Fig. 3.</figcaption></figure>

**Appendix C: Further comparison of clustering results**

Comparisons of the mean spectral profiles and cluster zone maps for each of the 7.25 – 8.95 and 3.2 – 3.6 µm regions with those based on clusters from the 10.9 – 11.63 µm region were also performed. The results from these experiments are given in Figs. C.1 and C.2, respectively.
<figure><img src="image_20.png" /><figcaption>**Fig. C.1.** Average spectral profiles (left) for the 7.25–8.95 µm region obtained from clustering assignment based on the 7.25–8.95 µm region and the corresponding cluster zones (right) are given in the top two frames. The average spectral profiles (left) for the 7.25–8.95 µm region obtained from clustering assignment based on the 10.9–11.63 µm region and the corresponding cluster zones (right) are given in the bottom two frames. Cluster and FOV labels are the same as in Fig. 8.</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>**Fig. C.1.** Average spectral profiles (left) for the 7.25–8.95 µm region obtained from clustering assignment based on the 7.25–8.95 µm region and the corresponding cluster zones (right) are given in the top two frames. The average spectral profiles (left) for the 7.25–8.95 µm region obtained from clustering assignment based on the 10.9–11.63 µm region and the corresponding cluster zones (right) are given in the bottom two frames. Cluster and FOV labels are the same as in Fig. 8.</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>**Fig. C.2.** Average spectral profiles (left) for the 3.2 – 3.6 µm region obtained from clustering assignment based on the 3.2 – 3.6 µm region and the corresponding cluster zones (right) are given in the top two frames. The average spectral profiles (left) for the 3.2 – 3.6 µm region obtained from clustering assignment based on the 10.9 – 11.63 µm region and the corresponding cluster zones (right) are given in the bottom two frames. Note that due to differences in the spatial resolution of the NIRSpec and MIRI MRS datasets, there are small discrepancies in the pixel masks, such are apparent in the lower right panel. Cluster and FOV labels are the same as in Fig. 8.</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>**Fig. C.2.** Average spectral profiles (left) for the 3.2 – 3.6 µm region obtained from clustering assignment based on the 3.2 – 3.6 µm region and the corresponding cluster zones (right) are given in the top two frames. The average spectral profiles (left) for the 3.2 – 3.6 µm region obtained from clustering assignment based on the 10.9 – 11.63 µm region and the corresponding cluster zones (right) are given in the bottom two frames. Note that due to differences in the spatial resolution of the NIRSpec and MIRI MRS datasets, there are small discrepancies in the pixel masks, such are apparent in the lower right panel. Cluster and FOV labels are the same as in Fig. 8.</figcaption></figure>