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, left). 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 above, right), 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.
Objective 2A measures star formation and black hole growth with rest-frame mid-IR and far-IR tracers which are immune to dust obscuration. The program begins with an unbiased field-filling survey with the PRIMAger hyperspectral imager in 1 and 10 square degree tiers. The images provide R~8 spectrophotometry in the rest-frame mid-IR for tens of 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.
Objective 2B determines the importance of outflow winds in quenching star formation. For this, a subsample of the most luminous galaxies are targeted for high-resolution spectroscopy to assess the prevalence of massive molecular outflows. These outflow winds are believed to be an essential means of quenching star formation in the most vigorous galaxies; PRIMA will test this hypothesis.
PRIMA separates the systemic and outflow components with model fits at a resolving power of R=900. The example shown on the left above is based on Mrk 231 scaled to z=1.5 and a 120 mJy continuum at payload performance. Messier 82 (M82), shown on the right, is a spectacular example of galactic outflows.
