Abstract

The population of active galactic nuclei (AGNs) newly discovered by the James Webb Space Telescope (JWST) exhibits peculiar properties that distinguish it from both local type I AGNs and high-redshift quasars. Most of these sources are compact, appearing as ‘little dots’; among them, the sub-class (10−30% of the total) characterised by significantly red optical colours is known as ‘little red dots’ (LRDs). In this work, we introduce the analogous term ‘little blue dots’ (LBDs) to characterise the remaining (bluer) sources (70−90%). We present a comparative analysis of two bright prototypical representatives, which we refer to as the Rosetta stones of the two classes: GN-28074 at z=2.26 as the red Rosetta stone and GS-3073 at z=5.55 as the blue Rosetta stone. Both Rosetta stones exhibit non-Gaussian broad Balmer lines, better described by exponential or double-Gaussian profiles rather than single Gaussians, similarly to normal low-redshift type I AGNs, indicating that non-Gaussian wings are not unique to LRDs. They are both extremely X-ray weak, showing strong auroral λ 4363 emission, weak hot dust mid-IR emission, and no time variability. However, they do differ in terms of excitation diagnostics: the λ 4686 line is undetected in the red Rosetta, but strongly detected in the blue Rosetta in both narrow and broad components, with the latter being much broader than hydrogen Balmer lines. This supports BLR stratification and disfavours the cocoon electron-scattering scenario in LBDs. An additional difference is the presence of prominent Balmer absorption in the red Rosetta, which is indicative of extremely dense gas along the line of sight and also happens to be absent in the blue Rosetta. Taken together, these results suggest that LRDs and LBDs share the same central engine as standard type I AGNs, while differing in terms of the amount and geometry of dense gas surrounding the accretion disk and/or in their accretion properties.