SPHEREx 3.3 Micron Aromatic Emission in the Magellanic Clouds: Separating Dust Column, Excitation, and Environmental Suppression
- 1Tsinghua University
Abstract
Bright emission in the 3.3 μ m unidentified infrared band (UIB), often described as an aromatic infrared band, can trace dust column, radiative excitation, or the survival and emissivity of its carriers. We combine SPHEREx maps of the Magellanic Clouds with HERITAGE dust columns, a TIR heating proxy, Br α , H α , H ii -region catalogs, and SAGE 8 μ m imaging. We measure continuum-subtracted 3.3 μ m feature excess, X 3.3 , and Br α excess, X Br α , in matched cells. The Clouds are analyzed symmetrically but sample different regimes: the LMC supplies most high- X Br α active-region dynamic range, whereas the SMC probes a lower-metallicity, lower-surface-brightness regime. In each Cloud, a quiescent diffuse baseline depending on dust surface density, Σ d , and TIR/Σ d predicts X 3.3 with R ² = 0.628 in the LMC and 0.288 in the SMC. Relative to this expectation, LMC H ii interiors are underluminous by − 0.22 3 − 0.040 + 0.035 dex; local-background measurements give − 0.15 3 − 0.043 + 0.014 dex despite a total- X 3.3 enhancement of +0.749 dex. SMC H ii -region residuals are consistent with zero over its lower- X Br α regime. A matched comparison with stellar-subtracted 8 μ m dust intensity, I 8,dust , shows that column and heating predict I 8,dust more tightly than X 3.3 , and that the high- X Br α LMC deficit is stronger for X 3.3 than for I 8,dust . The deficit is therefore selective to the isolated 3.3 μ m feature, not uniform in all aromatic-band-dominated emission. This result suggests that the small aromatic/aliphatic carbonaceous carriers traced by the isolated 3.3 μ m feature are more susceptible to processing in active regions than the larger or differently emitting carbonaceous-grain population dominating the 8 μ m aromatic complexes.
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BibTeX
@article{li2026spherex,
title = {{SPHEREx 3.3 Micron Aromatic Emission in the Magellanic Clouds: Separating Dust Column, Excitation, and Environmental Suppression}},
author = {Cheng Li},
journal = {The Astrophysical Journal},
year = {2026},
month = oct,
doi = {10.3847/1538-4357/aea6bd},
url = {https://iopscience.iop.org/article/10.3847/1538-4357/aea6bd},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - SPHEREx 3.3 Micron Aromatic Emission in the Magellanic Clouds: Separating Dust Column, Excitation, and Environmental Suppression
AU - Cheng Li
PY - 2026
DA - 2026/10/19
JO - The Astrophysical Journal
DO - 10.3847/1538-4357/aea6bd
UR - https://iopscience.iop.org/article/10.3847/1538-4357/aea6bd
AB - Bright emission in the 3.3 μ m unidentified infrared band (UIB), often described as an aromatic infrared band, can trace dust column, radiative excitation, or the survival and emissivity of its carriers. We combine SPHEREx maps of the Magellanic Clouds with HERITAGE dust columns, a TIR heating proxy, Br α , H α , H ii -region catalogs, and SAGE 8 μ m imaging. We measure continuum-subtracted 3.3 μ m feature excess, X 3.3 , and Br α excess, X Br α , in matched cells. The Clouds are analyzed symmetrically but sample different regimes: the LMC supplies most high- X Br α active-region dynamic range, whereas the SMC probes a lower-metallicity, lower-surface-brightness regime. In each Cloud, a quiescent diffuse baseline depending on dust surface density, Σ d , and TIR/Σ d predicts X 3.3 with R ² = 0.628 in the LMC and 0.288 in the SMC. Relative to this expectation, LMC H ii interiors are underluminous by − 0.22 3 − 0.040 + 0.035 dex; local-background measurements give − 0.15 3 − 0.043 + 0.014 dex despite a total- X 3.3 enhancement of +0.749 dex. SMC H ii -region residuals are consistent with zero over its lower- X Br α regime. A matched comparison with stellar-subtracted 8 μ m dust intensity, I 8,dust , shows that column and heating predict I 8,dust more tightly than X 3.3 , and that the high- X Br α LMC deficit is stronger for X 3.3 than for I 8,dust . The deficit is therefore selective to the isolated 3.3 μ m feature, not uniform in all aromatic-band-dominated emission. This result suggests that the small aromatic/aliphatic carbonaceous carriers traced by the isolated 3.3 μ m feature are more susceptible to processing in active regions than the larger or differently emitting carbonaceous-grain population dominating the 8 μ m aromatic complexes.
SN - 0004-637X
SN - 1538-4357
ER -