Two houses with identical facades but different interior layouts: from the outside the V-shaped spectrum looks the same, but inside there are different light sources and different amounts of dust. That is exactly how the two populations of distant galaxies that astronomers have long confused appear. The first is the little red dots (LRDs), compact reddish galaxies of the early universe discovered by the James Webb Space Telescope. The second is the blue hot dust-obscured galaxies (BHDs), rare quasars with a blue excess that reside at closer distances. Both have a V-shaped spectrum: a red optical continuum and a blue ultraviolet excess. But as it turns out, the resemblance is deceptive.

To sort things out, the researchers compared 326 LRDs from the COSMOS-Web and PRIMER surveys with 26 confirmed BHDs. Both datasets were fitted with the same active galactic nucleus and host galaxy templates, varying the dust attenuation for two components: one heavily obscured and one nearly transparent. The result was unexpected. In BHDs, the blue excess is scattered light from the active nucleus breaking through a thick dusty shroud with attenuation AV~20–60 mag. In LRDs, the attenuation is much more modest: AV~2.6±1.4 mag. With such weak obscuration, the fraction of scattered light would be negligible, but observations show that in LRDs it is comparable to that in BHDs (~1%). This means that the blue excess in LRDs has a different origin, likely starlight from the compact host galaxy.

Further differences only deepen the divide. LRDs lack the hot dust characteristic of BHDs: in the mid-infrared (MIRI F1800W), LRDs are barely detected, and their F770W–F1800W color is bluer by ~3 mag. X-ray observations also diverge: among LRDs, only two objects out of hundreds are detected in the soft X-ray band with moderate absorption, whereas for BHDs five out of 44 observed are detected with column densities above $10^{23}$ cm^-2. Finally, LRDs are compact (effective radius ~100 pc), while BHDs are extended (1–4 kpc). All this indicates that LRDs are a distinct population, not a highly reddened analog of BHDs.

The study closes the hypothesis of a direct evolutionary link between LRDs and BHDs. If in BHDs the blue excess is light from the active nucleus scattered by a dusty outflow, in LRDs it most likely comes from stars in the compact host galaxy, not from an accreting black hole. This means that the growth mechanisms of supermassive black holes in the early universe differ from those at later epochs: LRDs probably represent a phase of rapid accretion onto black hole seeds in gas-rich but dust-poor systems, whereas BHDs are a later stage of massive galaxy mergers with a powerful dusty cocoon. This distinction is important for understanding how the first black holes gained mass during the first billion years.

Further observations with JWST/MIRI in the mid-infrared and deep X-ray surveys (for example, with the future AXIS mission) will allow a more precise measurement of the fraction of LRDs with hot dust and X-ray emission. High-resolution spectroscopy will help separate the contribution of stars and the active nucleus to the ultraviolet excess. Systematic searches for highly reddened analogs of BHDs (similar to the object Virgil at z=6.6) using mid-infrared color selection criteria will also be needed to understand how rare such systems are in the early universe and how they relate to LRDs. Two houses with identical facades turned out to be built from different blueprints, and now we know how to look inside.