Supermassive Black Holes at Cosmic Dawn

The origin and growth of the first supermassive black holes (SMBH) remains one of the most pressing unsolved problems in astrophysics.

SMBHs are present in the nuclei of most massive galaxies and impact many aspects of galaxy evolution, including feedback, morphological transformations, properties of galaxies in clusters, and possibly cosmic reionization.

Several scenarios can explain how the seeds of SMBHs may have been planted, but from the empirical perspective we simply do not know. This severely limits theoretical/computational studies of both galaxy growth and reionization.

The origins of SMBHs could lie in the collapse of very massive stars at very early times, or with runaway mergers of stars in dense clusters all the way up to the present day.

To distinguish between these scenarios, we must firmly establish their comoving number density at high-redshifts, where model predictions show the greatest difference in their abundance.

SMBHs undergo their period of most rapid growth at redshifts beyond 6, which is also when the Universe underwent its last major phase transition: the Epoch of Reionization (EoR).

We do not currently know if this is a coincidence, or whether SMBH accretion processes, giving rise to Active Galactic Nuclei (AGN), contributed significantly to reionization.

Thus black hole seeding, together with the EoR progression, are two of the most well-posed and central questions in astrophysics.

I propose a PhD project to answer these questions, and will give the student an ideal start in high-profile astronomical research.

We will perform two novel, unique, and creative surveys of high-redshift galaxies and AGN, which combine the power of our flagship space observatories, HST and JWST. Both will be built upon data from large awards of General Observer time to the project PI.

We will identify new faint AGN by (1.) the phenomenon of temporal variability and (2.) by identifying the highly ionized gas conditions that can only be generated by accretion onto SMBHs.

The project will make what is by far the most robust estimate of the comoving number density of SMBHs in the early universe.

The combined surveys will target high-redshift sources at volume densities sufficient to discriminate between black hole seeding scenarios, and will be sensitive enough to rule out late-time formation scenarios for SMBHs.

Combined with additional HST survey data of AGN at lower redshift, we will also determine the contribution of these high-z AGN to cosmic reionization.

To facilitate this project we have had to design a new observing strategy with JWST, that goes beyond the originally planned observing modes for Near Infrared Spectrograph (NIRSpec).

We are developing this during JWST Cycle 2, with the aim of contributing a new, official, supported observing mode for the observatory.

Several further applications can be envisaged, which should give significant visibility to both the project and to the graduating PhD student.