UV spectra of little red dots: Broad-line N/C, gas recycling and black hole masses
Radiative-transfer models of supermassive black holes accreting inside dense gas cocoons reproduce the X-ray-to-near-infrared spectral energy distribution of "little red dots" (LRDs) using the code Sirocco. In the ultraviolet, these models also predict weak broad emission lines and a series of metal absorption lines, which deep spectroscopy from the JWST SPURS survey now detects in several LRDs. Here we use the models to interpret these features. We find that N and C ions of equal charge (3+ or 2+) occupy largely the same gas in the simulations, such that the combination of the broad N IV]/C IV and N III]/C III] flux ratios simultaneously constrains both the electron density and the N/C abundance ratio of the gas in the LRD proper. The nitrogen and carbon emission of SPURS-GN-2, an LRD at $z=7.19$, requires an order of magnitude nitrogen excess compared to the solar value, N/C $= 12^{+3}_{-2}$ (N/C)$_\odot$, in the broad-line gas with electron density $\log n_e = 9.7^{+0.3}_{-0.5}$ cm$^{-3}$. Thus, around half the broad-line CNO nuclei have passed through hydrogen burning. Because the ionised gas is replaced on a century timescale, sustaining this enrichment favours gas recycling. UV absorption lines test this independently. In two of the four LRDs, line blueshift velocities clearly increase with the photon energy required to ionise the absorber gas. In the models the absorption lines are spatially stratified, indicating that the gas is launched near the escape speed and slowed by gravity. In GN-2, where the measurement can be made, the inferred deceleration requires radiation pressure close to the Eddington limit of a $10^{6-7}$ M$_\odot$ black hole. The lines are also typically strongly saturated, making inferences of direct abundance ratios misleading.