PRIMA's core science program addresses three themes:

• Origin of Planets and their Atmospheres
• Co-Evolution of Galaxies and Supermassive Black Holes Since Cosmic Noon
• Buildup of Dust and Metals

The observatory is designed to deliver high-confidence results in these themes, elaborated below. All data resulting from the core science observations will be made available to the community as soon as possible.

We know planetary systems form from disks of gas and dust. ALMA imaging provides exquisite spatial detail in many planet forming disks, but the dust and carbon monoxide (CO) datasets are subject to uncertain depletion and gas-phase abundance. The ALMA images do not connect directly to the most fundamental basic properties of these disks: their masses and elemental abundances.

PRIMA connects water, oxygen, and carbon in protoplanetary disks with compositions of exoplanet atmospheres to understand how and where planets form.

The approach is full-band high-resolution spectra of 200 protoplanetary disks spanning a range of ages and stellar types. Each disk spectrum provides:

• Key water lines only accessible in the far-infrared to quantify the role of water in driving planet formation.
• The HD 112 micron line to provide accurate total gas masses and carbon (C) and oxygen (O) abundances in disks to reveal where exoplanets form.

No such complete far-IR disk spectra exist at present.

Explore Simulated Disk Spectrum

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Star formation and black hole growth are perhaps the two most important aspects of galaxy evolution. While galaxies and their central black holes operate on vastly different spatial scales, the present Universe shows a remarkable scaling relationship between galaxy stellar mass and central black hole mass (figure above). How did this come to be? Did they grow together, or via episodes where one is dominant and the other suppressed?

PRIMA will provide the first integrated census of star formation and black hole growth when galaxy evolution was most active, between ~ 9 billion and 3 billion years ago. PRIMA's far-IR spectrophotometry simultaneously measures black hole accretion rate (BHAR) of, star formation rate (SFR) of, and outflows from massive star-forming galaxies to establish how they are linked. This simultaneous measurement is only possible in the mid- and far-infrared because dust obscures UV and optical radiation which is often used to track star formation (exemplified by Centaurus A images below), and gas obscures X-rays typically used to track black hole growth. Both processes create signatures in the mid-infrared, which is redshifted into PRIMA's far-IR bands.

The program begins with an unbiased field-filling survey with the PRIMAger hyperspectral bands, giving R=10 spectrophotometry in the rest-frame mid-IR for thousands of galaxies. Each galaxy spectral energy distribution reveals the ratio of black hole growth to stellar growth.

Individual galaxies are then targeted for detailed FIRESS spectroscopic follow-up, providing robust nebular gas-phase measures of black hole accretion, star formation, and molecular outflow rates.

Centaurus A is a local galaxy with a rapidly accreting black hole at its center. Dust obscures the central active region from optical observatories, as is true for most galaxies across cosmic time. Far-IR light (right image) penetrates the dust and probes the active core as well as jets . PRIMA builds on Herschel with vast improvements in sensitivity to reach the early universe and provide full-band spectroscopy.

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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.

While dust absorbs optical radiation, (including in our own Milky Way galaxy - dark regions in image), PRIMA's far-IR instruments measure dust emission directly, with spectrophotometry and polarization. PRIMA's spectroscopy also catalogs the gas-phase heavy elements themselves. Both measurements have sensitivity and wavelength coverage to reach back across most of the Universe's history, and the combination will test basic theories of how the Universe's dust came to be.

PRIMA quantifies the dust properties and metal content of dusty galaxies from cosmic noon to diverse environments in galaxies today.

• PRIMA measures both polycyclic aromatic hydrocarbon emission and gas-phase heavy element abundances in galaxies at cosmic noon, and linking small grains to metallicity when the Universe was most vigorously creating heavy elements and dust.
• PRIMA determines the fraction of interstellar dust that is freshly ejected by stars (stardust) versus dust grown in the interstellar medium (ISM-mixed dust) by measuring the wavelength dependence of far-IR dust polarization in nearby galaxies.

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