Little Red Dot \(-\) Host Galaxy \(=\) Black Hole Star: A Gas-Enshrouded Heart at the Center of Every Little Red Dot

The central engines of Little Red Dots (LRDs) may be “black hole stars” (BH*s), early stages of black hole growth characterized by dense gas envelopes. So far, the most direct evidence for BH*s comes from a handful of sources where the host galaxy is completely outshone as suggested by their remarkably steep Balmer breaks. Here we present a novel scheme to disentangle BH*s from their host galaxies assuming that the [OIII]5008Å line arises exclusively from the host. Using a sample of 98 LRDs (\(z\approx2-9\)) with high quality NIRSpec/PRISM spectra, we demonstrate that the host-subtracted median stack displays a Balmer break \(>2\times\) stronger than massive quiescent galaxies, with the rest-optical continuum resembling a blackbody-like SED (\(T_{\rm{eff}}\approx4050\) K, \(\log(L_{\rm{bol}})\approx43.9\) erg s\({\mathstrut}^{-1}\), \(R_{\rm{eff}}\approx1300\) au). We measure a steep Balmer decrement (H\(\alpha\)/H\(\beta>10\)) and numerous density-sensitive features (e.g., FeII, HeI, OI). These are hallmark signatures of dense gas envelopes, providing population-level evidence that BH*s indeed power LRDs. In the median LRD, BH*s account for \(\sim20\%\) of the UV emission, \(\sim50\%\) at the Balmer break, and \(\sim90\%\) at wavelengths longer than H\(\alpha\) with the remainder arising from the host. BH*s preferentially reside in low-mass galaxies (\(M_{\rm{\star}}\approx 10^{8}\,{\rm M}_{\rm{\odot}}\)) undergoing recent starbursts, as evidenced by extreme emission line EWs (e.g., [OIII]5008Å\(\approx1100\)Å, CIII]\(\approx12\)Å), thereby favoring BH* origins linked to star-formation. We show V-shaped LRD selections are biased to high BH*/host fractions (\(\gtrsim60\%\) at 5500Å) – less dominant BH*s may be powering JWST’s blue broad-line AGN. We find BH*s are so commonplace and transient (duty cycle \(\sim1\%\), lifetime \(\sim10\) Myrs) that every massive black hole may have once shone as a BH*.