Beyond orientation: Evidence for distinct physical regimes among Little Red Dots and Little Blue Dots

Abstract

Little Red Dots (LRDs) and Little Blue Dots (LBDs) may represent the same population of compact active galactic nuclei (AGN) observed along different lines of sight. We test this scenario using a spectroscopic sample from the DAWN {\it JWST} Archive, selected based on common criteria for H$\alpha$ equivalent width, UV continuum slope, and compactness. We use the optical continuum slope to distinguish between samples of 89 LRDs and 191 LBDs spanning $1 \lesssim z \lesssim 7.5$, and compare their continuum properties, H$\alpha$ emission, and Balmer decrements. Consistent with previous studies, we find that LRDs exhibit broader H$\alpha$ profiles than LBDs systematically (with median FWHM values of $2319^{+71}_{-64}$ and $1424^{+56}_{-43}~\mathrm{km\,s^{-1}}$, respectively). We also confirm that LRDs show larger Balmer decrements, with median $\log_{10}(F_{\rm H\alpha}/F_{\rm H\beta})=1.01\pm0.03$, compared with $0.47\pm0.01$ for LBDs. Line-of-sight effects could explain both results. However, most significantly, we find that the LRD fraction increases strongly with H$\alpha$ line luminosity: LRDs are approximately six times more luminous in H$\alpha$ than LBDs. This robust finding is much harder to explain through unification with LBDs by orientation. Despite their higher line luminosities, LRDs have a moderately lower median H$\alpha$ equivalent width than LBDs, consistent with more of the raw emission being reprocessed into the rest-frame optical continuum in LRDs. These results, coupled with the systematically lower [O III]/H$\beta$ ratios found in LRDs and the decline of H$\alpha$ equivalent width towards the reddest optical slopes, disfavour a simple orientation-based link between LRDs and LBDs, and are instead more consistent with the predictions of gas-cocoon models in which increasing gas column density explains the apparent transition from LBDs to LRDs.