2026/07/21 by Debosmita Pathak, Karin M. Sandstrom, Adam K. Leroy +27
#astro-ph.GA
paper · pdf · doi:10.3847/1538-4357/ae8ae4
We present 50-100 pc-resolution JWST/MIRI and NIRCam measurements of mid-infrared (mid-IR) color variations in the diffuse interstellar medium (ISM) of 71 nearby star-forming galaxies from the PHANGS-JWST survey. Mid-IR emission traces the dust column density, intensity (U) and hardness of the interstellar radiation field, and the physical state (charge, size) and abundance of polycyclic aromatic hydrocarbons (PAHs). Mid-IR colors that trace PAH band-ratios remain fairly constant in the diffuse ISM of star-forming disks. However, they show stark variations in extreme environments: highly star-forming central molecular zones (CMZs) and star-formation deserts/quiescent bulges. In CMZs, PAH-to-continuum (3.3/21, 7.7/21, and 11.3/21 μm) and the 10/21 μm continuum colors are 0.2-0.4 dex lower than in normal disks. We attribute this to higher U based on the far-IR dust colors and the high 21 μ\rm m/Σ\rm Mol, which we suggest to be a good tracer of U outside star-forming regions. Meanwhile, star-formation deserts show low 7.7 μm PAH emission, resulting in low 7.7/21 μm and 7.7/11.3 μm, while all other mid-IR colors remain typical. This suggests the presence of more neutral PAHs in star-formation deserts, where low 7.7 μm likely reflects ISM conditions similar to early-type and elliptical galaxies. All environments form part of a continuous trend in 7.7/11.3 μm vs. specific star-formation rate.