Multiphase Gas Structure in JWST Little Red Dots

Abstract

JWST has uncovered a population of compact high-redshift galaxies, 'little red dots' (LRDs), exhibiting broad Balmer emission lines that often show narrow absorption features and strong Balmer breaks - spectral signatures attributed to dense gas surrounding rapidly growing black holes, but whose small-scale structure remains unknown. A striking feature of LRD spectra is that the H$β$ absorption is systematically deeper than H$α$, yet in any homogeneous gas this ordering is physically impossible. Here we model the H$α$ and H$β$ line profiles using radiative transfer simulations for 11 LRDs with JWST sensitive high resolution spectroscopy, and independently constrain the $n=2$ hydrogen column density, $N_{\rm HI,\mathit{n}=2}$, from the Balmer-break continuum. We find that the $N_{\rm HI,\mathit{n}=2}$ from H$α$ in a homogeneous medium cannot reproduce the observed H$β$ absorption in several objects, and that this $N_{\rm HI,\mathit{n}=2}$ typically falls two orders of magnitude below the lower bound set by the Balmer break. Both tensions are simultaneously resolved by a multiphase medium of dense clumps within diffuse hot ionized gas. This multiphase gas structure is a necessary ingredient for any viable LRD model, regardless of the nature of the central engine.