A little red dot up close

Taking their name from their extremely red and compact emission, little red dots (LRDs) do not yet have a physical explanation that is consistent across their full spectral energy distribution (SED): whilst some spectral features indicate the presence of dusty active galactic nuclei (AGN), others are more naturally interpreted as stellar light. Observations and modelling are also often hampered by the high redshifts (z ≲ 5) at which most LRDs have been found. Bingjie Wang and colleagues present multiwavelength data for a likely LRD analogue at redshift z ~ 3, along with a feasible model for its structure.

Combining JWST observations from multiple instruments with HST imaging and upper limits from Spitzer and Herschel data, the authors construct a range of spectrophotometric models with flexible treatment of the putative AGN and galaxy light. Though they cannot rule out a galaxy-only model, the high stellar mass implied by such a model would require an extreme stellar surface density and is an order of magnitude larger than the object’s estimated dynamical mass. Their joint galaxy–AGN model, on the other hand, depends on the dust in the AGN torus being unusually — but not unprecedently — cold. They also detect a blueshifted He I absorption feature, suggestive of an ionized outflow from the supermassive black hole (SMBH).