LRDs
Little Red Dots — and what is hiding inside them
Little Red Dots (LRDs) are the second extragalactic puzzle handed to us by JWST, after the UV-bright galaxies at high-z. Owed to their unique spectral shape and unresolved nature in the rest-frame optical, they are pretty easy to pick out in NIRCam images. This is, however, where the simplicity stops.
Over the last few years my work has followed this population from one interpretation to the next — from extreme black hole masses and surprisingly high number densities, to the emergence of the dense gas cocoon scenario, and most recently to the chemistry of the gas itself. What LRDs truly are remains an open question, and I suspect the answer will surprise us, which makes it all the more fun.
Below are my papers on the subject, most recent first.
The Ashes of Supermassive Stars
Kokorev et al. (2026)
Using deep NIRSpec spectroscopy from the SPURS program, we stack four LRDs and measure the elemental abundances of the gas around the central engine directly, from UV absorption lines. It comes out magnesium-depleted and aluminium-enhanced at a metallicity of only 1% solar — an anomaly otherwise seen only in globular cluster stars. Ionization, geometry and dust cannot mimic it, but hot hydrogen burning in a fully convective supermassive star of at least 104 M⊙ reproduces it, which is about 100 times more massive than any star in the present-day Universe. It ties LRDs to globular cluster formation and to the heavy seeds of the first black holes.
The Deepest GLIMPSE of a Dense Gas Cocoon
Kokorev et al. (2025)
Nearly 30 hours of NIRSpec time on a single lensed LRD behind Abell S1063. The depth buys us the full Paschen series, the He I triplet, Fe II multiplets pumped by Lyα, and broad lines whose profiles are exponential rather than Gaussian — the signature of electron scattering in dense gas rather than of Keplerian rotation. Together these pin down a stratified cocoon of dense gas around the central engine, and let us measure its density, column and covering fraction rather than assume them.
Silencing the Giant
Kokorev et al. (2024), ApJ
A z = 4.13 LRD in GOODS-N caught with the full set of NIRSpec medium-resolution gratings, which is rare enough to be worth the effort. Broad Hα (FWHM ~ 2500 km/s) and a broad Mg II doublet mark an accreting black hole — the first clear rest-UV AGN signature in an LRD — while the rest-optical continuum belongs to a massive (log M⋆/M⊙ ~ 10.6) galaxy that quenched about 200 Myr earlier. Unusually for this population the black hole is not overmassive: MBH/M⋆ ~ 0.04%, right on the local relation.
A Census of Photometrically Selected LRDs at 4 < z < 9
Kokorev et al. (2024), ApJ
A uniform colour- and size-based search across ~640 arcmin2 of blank JWST/NIRCam fields, which turned up 260 candidates at 4 < z < 9 and, more importantly, a number density. If the rest-optical light is AGN-dominated, these reddened AGN are roughly 100 times more abundant than UV-selected quasars of the same magnitude. One or two exotic objects can be waved away; hundreds of them cannot.
UNCOVER: A Broad-line AGN at z = 8.50
Kokorev et al. (2023), ApJL
The spectroscopic identification of a broad-line AGN only ~570 Myr after the Big Bang, lensed by Abell 2744. Hβ has FWHM = 3439 ± 413 km/s, implying log MBH/M⊙ = 8.17 ± 0.42 and accretion at ~40% of Eddington. With the host constrained to log M⋆/M⊙ < 8.7, that is a black-hole-to-host mass ratio of at least ~30% — orders of magnitude above local quasars. This “overmassive” problem is a large part of why the dense gas picture had to be invented.
