Supermassive black holes are ubiquitous in large galaxies today and some had huge masses even when the Universe was only one billion years old. How did black holes get so big so fast? How massive were they initially? How common were they in the early Universe and how did they evolve at later times?
Concepts for black hole seeds and their modes of growth are highly uncertain. Several models are depicted above (left). Lynx will have the sensitivity to detect and count black holes in the key regime of high look-back time and small seed masses reaching MBH = 104 Msun at redshift z=10 (X-ray flux of 1 to 4x10-19 erg-s-1-cm-2).
Above: Simulated 2'x2' regions from a deep JWST survey (left), a 4 Msec Lynx survey, and a 4 Msec ATHENA survey (right) highlighting the vast improvement afforded by Lynx's superior angular resolution. Colors in the X-ray images depict populations of active galactic nuclei hosting supermassive black holes (purple) and moderate-redshift normal galaxies (green).
To associate X-ray sources with unique JWST and WFIRST counterparts requires better than 1" angular resolution. Lynx will detect approximately 350 discrete sources in the region depicted above and can identify essentially all with unique optical/infrared counterparts. With its 5" angular resolution, ATHENA misses most of the population of normal galaxies which leads to very substantial source confusion and lack of associations with specific optical/infrared galaxies.
How does the growth of supermassive black holes proceed from cosmic dawn to z of 3 and how is the evolution of these SMBHs connected to that of their host galaxies? Do all SMBHs emerge at high redshifts or do some seeds form later or experience bursts of fast growth at lower redshift? Do relics of SMBH seeds still reside in nearby dwarf galaxies in the present day?
Lynx will answer these questions through deep extragalactic surveys that will reveal the population of X-ray-faint active galactic nuclei (down to luminosities of 1041 erg-s-1 as far away as z~3) and measure the Eddington ratio distribution, host galaxy characteristics, and the spatial clustering of these growing SMBHs.
Lynx and LISA will probe similar mass-redshift ranges for high-redshift black holes through accretion (X-rays) and mergers (gravitational waves). Predicted numbers of actively accreting supermassive black hole seeds and mergers at high z are very uncertain but, together, Lynx and LISA will reveal key details about formation and growth of SMBHs in the early Universe.
Lynx will be able to respond rapidly to high-significance LISA triggers on nearby merging SMBHs to provide details of pre- and post-merger X-ray emission from these powerful events.