ATLAS. I. A Scaling Relation of LRDs between Broad H$\alpha$ and Bolometric Luminosities: Enhanced Broad H$\alpha$ Emission Relative to Low-$z$ Type 1 AGN

Abstract

We investigate the demography of little red dots (LRDs) using 37 objects at $z\sim 3$-$7$ with JWST/NIRSpec PRISM and grating spectra compiled from various JWST programs. We focus on spectroscopic quantities of the broad H$\alpha$ luminosity $L_{\mathrm{H}\alpha ,\mathrm{broad}}$ (and the broad H$\beta$ luminosity $L_{\mathrm{H}\beta ,\mathrm{broad}}$ where available) and the bolometric luminosity $L_{\mathrm{bol}}$ represented by modified blackbody emission, avoiding quantities contaminated by host-galaxy emission (e.g., total H$\alpha$ luminosity). We identifiy a tight scaling relation between $L_{\mathrm{H}\alpha ,\mathrm{broad}}$ and $L_{\mathrm{bol}}$, supporting the interpretation that these emissions are primarily powered by the central engine. Interestingly, the $L_{\mathrm{H}\alpha ,\mathrm{broad}}$-$L_{\mathrm{bol}}$ scaling relation of LRDs is enhanced by a factor of $\sim 40$ in $L_{\mathrm{H}\alpha ,\mathrm{broad}}$ relative to that of low-$z$ Type 1 AGN. A similar trend is found in the $L_{\mathrm{H}\beta ,\mathrm{broad}}$-$L_{\mathrm{bol}}$ relation, although the enhancement in $L_{\mathrm{H}\beta ,\mathrm{broad}}$ is smaller, only by a factor of $\sim 10$. We explore the physical origin of these enhancements and find that \textsc{Cloudy} photoionization modeling within the classic locally optimally-emitting cloud (LOC) framework can explain them through an increase in the covering factor from $\sim 20$\% (Type 1 AGN) to $\sim 100$\% (LRDs), together with an increase in the hydrogen column density from $N_{\mathrm{H}}\sim 10^{23}{\mathrm{cm}}^{-2}$ to $\gtrsim 10^{24}{\mathrm{cm}}^{-2}$, with a preferred gas density of $\sim 10^{10}{\mathrm{cm}}^{-3}$, successfully reproducing the modified blackbody emission. Such a nearly unity covering factor without requiring a gas density increase may result from a significant increase in the BLR filling factor or size, corresponding to a ``stuffed BLR" or ``giant BLR," respectively.

17 pages, 9 figures. Minor revisions