Observing the first stars and black holes

The high sensitivity of JWST will open a new window on the end of the cosmological dark ages. Small stellar clusters, with a stellar mass of several × 106 M⊙, and low-mass black holes (BHs), with a mass of several $× 105 M⊙ should be directly detectable out to redshift z = 10, and individual supernovae (SNe) and gamma ray burst GRB afterglows are bright enough to be visible beyond this redshift. Dense primordial gas, in the process of collapsing from large scales to form protogalaxies, may also be possible to image through diffuse recombination line emission, possibly even before stars or BHs are formed. In this article, I discuss the key physical processes that are expected to have determined the sizes of the first star–clusters and black holes, and the prospect of studying these objects by direct detections with JWST and with other instruments. The direct light emitted by the very first stellar clusters and intermediate-mass black holes at z > 10 will likely fall below JWST’s detection threshold. However, JWST could reveal a decline at the faint-end of the high-redshift luminosity function, and thereby shed light on radiative and other feedback effects that operate at these early epochs. JWST will also have the sensitivity to detect individual SNe from beyond z = 10. In a dedicated survey lasting for several weeks, thousands of SNe could be detected at z > 6, with a redshift distribution extending to the formation of the very first stars at z ≳ 15. Using these SNe as tracers may be the only method to map out the earliest stages of the cosmic star–formation history. Finally, we point out that studying the earliest objects at high redshift will also offer a new window on the primordial power spectrum, on ∼100 times smaller scales than probed by current large-scale structure data.