2025ApJ...992....3Z

Together with gas, stars, and supermassive black holes, dust is crucial in stellar and galaxy evolution. Hence, understanding galaxies' dust properties across cosmic time is critical to studying their evolution. In addition to photometric constraints on the absorption of blue light and its reemission at infrared wavelengths, dust grain properties can be explored spectroscopically via polycyclic aromatic hydrocarbon (PAH) emission bands in the mid-IR. The new SPHEREx space telescope conducts an all-sky spectrophotometric survey of stars and galaxies at wavelengths of 0.75-5 μm, making it ideal for studying the widespread presence of the 3.3 μm PAH emission across galaxy populations out to z ∼ 0.4. In this paper, we simulated galaxy spectra to investigate SPHEREx's capability to study PAH emission in such galaxies. We find that for the all-sky survey the PAH 3.3 μm emission band flux can be measured to 30% accuracy at ${\mathrm{log}}\,(M/{M}_{\odot })\gt 9.5$ and star formation rate (SFR) > 1 M_⊙ yr^–1 at z = 0.1, ${\mathrm{log}}\,(M/{M}_{\odot })\gt 10.5$ and SFR > 10 M_⊙ yr^–1 at z = 0.2-0.3, and ${\mathrm{log}}\,(M/{M}_{\odot })\gt 11$ and SFR > 100 M_⊙ yr^–1 at z = 0.4. For deep SPHEREx fields, a factor of ∼10 deeper sensitivity limits can be reached. Overall, SPHEREx will enable the measurement of the 3.3 μm PAH band emission in several hundred thousand galaxies across the sky, providing a population study of the smallest dust grains (“nano grains”) and radiation properties in massive galaxies in the nearby Universe.