The universal thickness of mid-infrared bubbles
Abstract
Several hundred ring-like structures seen at 8μm in the Spitzer GLIMPSE surveys share a uniform morphology: the shell thickness ΔR is a nearly constant fraction of the outer radius R, ΔR/R∼0.3, across a factor of 40 in size and all exciting spectral types. This invariance is unexplained. We ask what sets this thickness and whether its near-constancy is physical or a measurement artifact. We derive analytic constraints on the location, temperature, and heating of the emitting grains, and confront the predicted scaling with radius against the GLIMPSE I and II catalogs (591 objects, 91 with kinematic distances). Grains in radiative equilibrium at the observed radii reach only 20-35 K and fall short of the required 8μm emissivity by more than 20 orders of magnitude; the emission must come from stochastically heated PAHs and very small grains, and therefore from the photon-dominated region, where the ring width is the far-ultraviolet penetration depth. This gives ΔR/R=AVPDR/AVcloud, independent of radius; a swept-up shell instead predicts ΔR/R∝(R/RS)3/2, with RS the initial Strömgren radius. We find a pronounced angular-resolution systematic that inflates the ratio for the smallest bubbles; once this is accounted for, ΔR/R=0.217, with a residual dependence on physical radius of 0.03±0.05, consistent with zero. A fixed ambient density is excluded at 20σ, the swept-up shell at 9-28σ. The implied columns are Ncloud≈8.7×1021 cm−2 with only 0.112 dex of scatter. The near-universal appearance of mid-infrared bubbles follows from the ratio of two nearly invariant column densities -- the dust-opacity-fixed far-ultraviolet shielding column and the Larson-like column of the parent molecular cloud -- and requires no reference to stellar winds.
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BibTeX
@misc{schulreich2026universal,
title = {{The universal thickness of mid-infrared bubbles}},
author = {Michael M. Schulreich and A. Beate C. Patzer},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.07361},
eprint = {2610.07361},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.07361},
}
RIS
TY - UNPB
TI - The universal thickness of mid-infrared bubbles
AU - Michael M. Schulreich
AU - A. Beate C. Patzer
PY - 2026
DA - 2026/10/05
PB - arXiv
DO - 10.48550/arxiv.2610.07361
UR - https://arxiv.org/abs/2610.07361
AB - Several hundred ring-like structures seen at 8μm in the Spitzer GLIMPSE surveys share a uniform morphology: the shell thickness ΔR is a nearly constant fraction of the outer radius R, ΔR/R∼0.3, across a factor of 40 in size and all exciting spectral types. This invariance is unexplained. We ask what sets this thickness and whether its near-constancy is physical or a measurement artifact. We derive analytic constraints on the location, temperature, and heating of the emitting grains, and confront the predicted scaling with radius against the GLIMPSE I and II catalogs (591 objects, 91 with kinematic distances). Grains in radiative equilibrium at the observed radii reach only 20-35 K and fall short of the required 8μm emissivity by more than 20 orders of magnitude; the emission must come from stochastically heated PAHs and very small grains, and therefore from the photon-dominated region, where the ring width is the far-ultraviolet penetration depth. This gives ΔR/R=A_V^PDR/A_V^cloud, independent of radius; a swept-up shell instead predicts ΔR/R∝(R/R_S)^3/2, with R_S the initial Strömgren radius. We find a pronounced angular-resolution systematic that inflates the ratio for the smallest bubbles; once this is accounted for, ΔR/R=0.217, with a residual dependence on physical radius of 0.03±0.05, consistent with zero. A fixed ambient density is excluded at 20σ, the swept-up shell at 9-28σ. The implied columns are N_cloud≈8.7×10^21 cm^−2 with only 0.112 dex of scatter. The near-universal appearance of mid-infrared bubbles follows from the ratio of two nearly invariant column densities -- the dust-opacity-fixed far-ultraviolet shielding column and the Larson-like column of the parent molecular cloud -- and requires no reference to stellar winds.
ER -