Dust-induced reversal of the optical wavelength dependence of spiral-arm pitch angles
Abstract
The classical density-wave scenario predicts a blue-to-red age--colour gradient across the spiral arms of disc galaxies as a consequence of shock-induced star formation. However, observations of spiral-arm pitch angles at different wavelengths sometimes suggest the opposite trend, namely a red-to-blue ``colour jump''. The origin of this apparent contradiction remains unclear. In this work, we investigate the effects of dust attenuation on post-shock stellar emission, together with the streamline motions of gas and stellar populations. We find that dust attenuation in regions with relatively high optical depths, τ̂V ≳ 0.2, can produce an apparent red-to-blue spatial sequence, with respect to the shock location, by reversing the relative positions of the flux maxima of stellar populations observed in different bands. Moreover, streamline motions confine post-shock stellar populations near the spiral arms for longer than in a circular-orbit model, thereby modifying the spatial distribution of the emitted light. We conclude that the apparent red-to-blue colour jump does not necessarily contradict the classical density-wave scenario. Instead, it can arise from the same post-shock stellar populations that intrinsically produce a blue-to-red age--colour gradient if their emission is modified by dust attenuation.
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BibTeX
@misc{martinezgarcia2026dustinduced,
title = {{Dust-induced reversal of the optical wavelength dependence of spiral-arm pitch angles}},
author = {Eric E. Martínez-García and Rosa A. González-Lópezlira and Ivânio Puerari and Arturo I. Gómez-Ruiz},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2611.00208},
eprint = {2611.00208},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2611.00208},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Dust-induced reversal of the optical wavelength dependence of spiral-arm pitch angles
AU - Eric E. Martínez-García
AU - Rosa A. González-Lópezlira
AU - Ivânio Puerari
AU - Arturo I. Gómez-Ruiz
PY - 2026
DA - 2026/11/01
PB - arXiv
DO - 10.48550/arxiv.2611.00208
UR - https://arxiv.org/abs/2611.00208
AB - The classical density-wave scenario predicts a blue-to-red age--colour gradient across the spiral arms of disc galaxies as a consequence of shock-induced star formation. However, observations of spiral-arm pitch angles at different wavelengths sometimes suggest the opposite trend, namely a red-to-blue ``colour jump''. The origin of this apparent contradiction remains unclear. In this work, we investigate the effects of dust attenuation on post-shock stellar emission, together with the streamline motions of gas and stellar populations. We find that dust attenuation in regions with relatively high optical depths, τ̂_V ≳ 0.2, can produce an apparent red-to-blue spatial sequence, with respect to the shock location, by reversing the relative positions of the flux maxima of stellar populations observed in different bands. Moreover, streamline motions confine post-shock stellar populations near the spiral arms for longer than in a circular-orbit model, thereby modifying the spatial distribution of the emitted light. We conclude that the apparent red-to-blue colour jump does not necessarily contradict the classical density-wave scenario. Instead, it can arise from the same post-shock stellar populations that intrinsically produce a blue-to-red age--colour gradient if their emission is modified by dust attenuation.
ER -