Microwave Emission in Interstellar Medium
Summary
Microwave emission from the interstellar medium arises from several distinct physical processes that overlap in frequency yet trace different components of the Galaxy. At low frequencies (≈1–30 GHz), synchrotron radiation generated by relativistic electrons spiralling in magnetic fields and free–free (thermal bremsstrahlung) emission from ionised gas dominate. Between 10 and 60 GHz, an additional component known as anomalous microwave emission (AME) emerges, widely attributed to electric dipole radiation from rapidly spinning very small dust grains. At higher frequencies (>60 GHz), thermal vibrational emission from large dust grains becomes the principal contributor. The combined spectral energy distribution encodes key information on grain size distributions, dust composition, magnetic field geometry and the relative proportions of ionised, neutral and molecular phases. Advances in instrumentation—ranging from spaceborne observatories to high-resolution ground-based surveys—have enabled separation of these components over large regions of sky, refining models of dust emissivity, polarisation behaviour and environmental dependencies. Improved understanding of microwave foregrounds not only illuminates the physical properties and lifecycle of interstellar dust but also underpins accurate extraction of cosmological signals in the cosmic microwave background.
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One study combined low-frequency data from satellite instruments within a Bayesian framework to place stringent upper limits on the polarisation fraction of anomalous microwave emission, demonstrating sensitivity to assumptions about the synchrotron spectral index and highlighting the need for complementary ground-based measurements. A separate large-scale survey at 30 GHz across the Galactic plane uncovered excess flux in multiple H ii regions, distinguishing an AME component from free-free and synchrotron emission and modelling its spectral energy distribution with multi-component spinning-dust frameworks. In parallel, full-sky analyses of the cold neutral medium fraction compared with mid-infrared tracers of polycyclic aromatic hydrocarbons (PAHs) reveal a strong link between PAH abundance and dense gas, yet show that AME intensity and peak frequency do not scale simply with PAH fraction, challenging current theoretical models of spinning dust in differing interstellar environments.
Microwave Emission in Interstellar Medium publication trend
The graph below shows the total number of articles in microwave emission in interstellar medium across all publications each year (not limited to Nature Index journals).
Technical terms
Anomalous Microwave Emission (AME): A component of Galactic microwave emission peaking near 20–30 GHz, attributed to electric dipole radiation from rapidly rotating very small dust grains.
Spinning dust: A theoretical mechanism whereby ultrasmall grains (nanometre scale) emit microwaves as they spin due to interactions with the interstellar environment.
Free–free emission: Thermal bremsstrahlung produced by electrons scattering off ions in ionised gas, with a relatively flat spectrum across centimetre wavelengths.
Synchrotron emission: Non-thermal radiation emitted by relativistic electrons spiralling in magnetic fields, characterised by a steep power-law spectrum at low frequencies.
Polycyclic aromatic hydrocarbons (PAHs): Large, planar organic molecules in the ISM whose mid-infrared emission and association with AME provide clues to dust grain composition and size distribution.
References
- Observations of the Polarisation of the Anomalous Microwave Emission: A Review. Advances in Astronomy (2012).
- COMAP Early Science. VI. A First Look at the COMAP Galactic Plane Survey. The Astrophysical Journal (2022).
- Polycyclic Aromatic Hydrocarbons, Anomalous Microwave Emission, and their Connection to the Cold Neutral Medium. The Astrophysical Journal (2022).
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