Mid-Infrared Emission in Star-Forming Galaxies

Mid-infrared (mid-IR) emission from star-forming galaxies arises primarily from thermal radiation of small dust grains and characteristic vibrational modes of polycyclic aromatic hydrocarbons (PAHs). In the wavelength range 3–25 µm, the spectrum is punctuated by broad PAH features at 3.3, 6.2, 7.7, 8.6 and 11.3 µm, superimposed on a warm dust continuum whose intensity traces the ultraviolet heating by young massive stars. High angular-resolution observations, most recently enabled by the James Webb Space Telescope, have resolved individual H II regions and molecular clouds in nearby galaxies, revealing that the strength and shape of PAH bands vary systematically with local physical conditions such as radiation field intensity, metallicity and the ionisation state of the grains. Quantitative analyses show that mid-IR luminosity at 21 µm provides a robust obscured star-formation tracer on scales down to tens of parsecs, while shorter-wavelength PAH bands require careful calibration owing to molecule destruction in harsh environments. Statistical descriptions of mid-IR surface-brightness distributions further connect the dense, star-forming component to the diffuse interstellar medium (ISM), enabling empirical models that link global star-formation rates to dust-reprocessed starlight across a diversity of galactic environments. Such insights underpin both local–universe studies and the interpretation of high-redshift infrared surveys, informing our understanding of galaxy evolution and the cosmic star-formation history.

Research from Nature Portfolio

No recent Nature Portfolio content available.

Mid-Infrared Emission in Star-Forming Galaxies publication trend

The graph below shows the total number of articles in mid-infrared emission in star-forming galaxies across all publications each year (not limited to Nature Index journals).

Technical terms

Polycyclic aromatic hydrocarbons (PAHs): Carbon-ring molecules whose vibrational modes produce prominent mid-IR emission features.

H II region: Ionised gas cloud surrounding young, massive stars, characterised by strong ultraviolet radiation.

Interstellar medium (ISM): Gas and dust in the space between stars, serving as the reservoir for star formation.

Mid-infrared (mid-IR): The portion of the electromagnetic spectrum from approximately 3 to 25 micrometres, sensitive to warm dust and molecular features.

Continuum emission: Broad, featureless radiation from dust grains heated by stellar ultraviolet and optical photons.

Probability distribution function (PDF): Statistical description of the distribution of surface brightness or column density values in a region.

References

1. The PHANGS–JWST Treasury Survey: Star Formation, Feedback, and Dust Physics at High Angular Resolution in Nearby GalaxieS. The Astrophysical Journal Letters (2023).
2. Calibrating mid-infrared emission as a tracer of obscured star formation on H II-region scales in the era of JWST. Astronomy & Astrophysics (2023).
3. A Two-Component Probability Distribution Function Describes the Mid-IR Emission from the Disks of Star-forming Galaxies. The Astronomical Journal (2023).
4. PHANGS–JWST First Results: Destruction of the PAH Molecules in H ii Regions Probed by JWST and MUSE. The Astrophysical Journal Letters (2023).