Just after the Big Bang, the Universe had only hydrogen, helium and a trace of lithium. But since the first stars and for the last 95% of cosmic time, the Universe has produced a growing complement of heavy elements (metals), including those necessary for life: carbon, oxygen, nitrogen, and more. Much of these heavy elements appear to have been converted to dust shortly after their formation, based on the ubiquity of dust in even very early galaxies, but this process remains a mystery.

Dust absorbs optical-wavelength radiation, as exemplified by the above visible-light image in the direction of the center of our Milky Way galaxy. Visible light from the Milky Way's center is completely obscured by the dust (dark regions in image). PRIMA's far-IR instruments, however, measure dust emission directly, at wavelengths where it streams freely through the interstellar medium.

PRIMA quantifies the dust properties and metal content of dusty galaxies from cosmic noon (z~2) to diverse environments in galaxies in the present-day Universe.

• To accomplish Objective 3A, PRIMA measures both polycyclic aromatic hydrocarbon emission and gas-phase heavy element abundances using wideband spectroscopy in a sample of galaxies at cosmic noon (z~2). The study links small grains to elemental abundances quantitatively when the Universe was most actively forming stars and building heavy elements and dust.
• To accomplish Objective 3B, PRIMA uses far-IR polarimetry in nearby galaxies to determine the fraction of interstellar dust that is freshly ejected by stars (stardust) versus dust grown in the interstellar medium (ISM-mixed dust). The ratio of these two basic dust types is revealed in the wavelength dependence of far-IR dust polarization (see Hensley and Draine, 2023 and Dowell, Hensley and Sauvage, 2025).

Modeled spectra of a star forming galaxy from redshift 2, when the Universe was only a quarter of its current age. Green shows a model galaxy rich in polycyclic aromatic hydrocarbons (PAH, the small dust grains), while blue shows a model for which PAHs have not formed or been rapidly destroyed. PRIMA measurements of hundreds of such spectra constrain theories of dust production and distribution through the epoch of peak activity in galaxies.