Little red dots -- JWST uncovers a new population of dust-obscured AGN in the epoch of reionization

Active galactic nuclei (AGN) at high redshifts, observed when the Universe was only a few hundred Myr old, are crucial for our understanding of black hole growth in the early Universe and their co-evolution with galaxies. In particular, if gravitationally lensed and multiply imaged by an intervening strong lensing galaxy cluster, distant AGN can yield precious insight into AGN properties. We can even use them to constrain cosmological parameters by studying the time delay between the multiple images through the varying AGN activity. The advent of the JWST and its phenomenal near-infrared sensitivity and spatial resolution has initiated a new era in observations of strong lensing galaxy clusters and the lensed background sources. JWST observations allow us to push the frontier of observability towards fainter magnitudes and higher redshifts -- and thus uncovered a new population of dust-obscured red AGN at high redshifts.

In my talk I will present these red point-source objects observed in strong lensing fields with JWST (and archival HST data) that represent new and hitherto unobserved populations of AGN at high redshift. For one case in particular, at z=7 and which is multiply-imaged by the massive galaxy cluster Abell 2744, we obtained the deepest JWST/NIRSpec spectrum taken of a single object to date and detected its broad emission lines. This object opens fascinating prospects for future studies of its black hole properties through reverberation-mapping and even cosmological parameters through strong lensing cosmography.