SPURS: An Ultra-deep View Inside the Compact, Nitrogen-Enriched Nuclei of Little Red Dots

Abstract: We present the first ultra-deep rest-UV spectroscopy of four UV-bright Little Red Dots (LRDs), obtained from the SPURS Cycle 4 Large Program. The spectra reveal broad CIV (FWHM $\approx2700-2800$ km s$<sup>{-1}$) in two LRDs, alongside narrow-line densities elevated above star-forming galaxies ($n_e\sim10<sup>4-10<sup>5$ cm$<sup>{-3}$, reaching $10^6$ cm$<sup>{-3}$ in the most extreme source) and nitrogen-enhancements in all four LRDs. We detect broad HeII emission (FWHM $\approx930$ km s$<sup>{-1}$) in one LRD, and two others with fast P-Cygni absorption ($\gtrsim2200$ km s$<sup>{-1}$). Strong interstellar absorption lines and Ly$α$ damping wings reveal the UV continuum is deeply embedded in neutral gas ($N_{\rm HI}\gtrsim10<sup>{22}$ cm$<sup>{-2}$) in all four LRDs. Detections of fluorescent FeII and OI emission and fine-structure absorption indicate this gas lies close to the UV-emitting region. In archival $z&gt;4$ samples, we find nitrogen and strong CIII] emission are significantly more common in LRDs than in the galaxy population. The transmission of broad CIV, tracing the broad-line region or cocoon, depends on rest-optical color within our sample, consistent with an orientation-dependent picture in which bluer, less obscured sightlines offer a more direct, polar view of the central engine and its outflows. We find several potential signatures of very massive stars, whose winds may contribute to nitrogen enhancement. We investigate other abundance patterns expected from supermassive stars but our results are inconclusive. Our results place the UV-emitting region within $\lesssim8$ pc of the LRD nucleus, consistent with an actively assembling nuclear star cluster. Dynamical interactions in this extremely dense environment, including tidal disruption of stars, may explain the high incidence of nitrogen enhancements in LRDs.