Evidence of violation of case B recombination in little red dots

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Abstract
Context. Little Red Dots (LRDs) are a new class of compact extragalactic objects with a v-shaped optical spectral energy distribution breaking close to the Balmer break wavelength and broad, typically exponentially shaped lines. They are believed to be supermassive black holes surrounded by very dense ionised gas, leading us to search for any departures from case B recombination in these systems. Aims. For this paper we examined the ratios of multiple hydrogen Balmer lines (H α , H β , H γ , and H δ ) to determine whether there is indication of deviation from simple case B recombination processes. Methods. We analysed a dozen high-S/N LRDs with JWST/NIRSpec and measured Balmer ratios in the seven objects with coverage of at least three lines. We decomposed the line ratios into their respective broad and narrow components. Results. Broad-line ratios are consistent with case B plus severe dust extinction in all objects but one, RUBIES-EGS-4233_49140 at z = 6.68; in this case the ratios deviate by more than 5 σ and are inconsistent with all known dust curves. The narrow components are consistent with minimal dust attenuation, while two objects exhibit narrow H α /H β ≈ 1.8. Conclusions. Such low decrements are observed in highly ionised density bounded nebulae, associated with starburst environments. Nevertheless, the two flat decrement cases can be reconciled assuming an unresolved absorption feature. RUBIES EGS-49140 shows a high broad H α /H β , but the H γ /H α and H δ /H α ratios are lower than expected for extinction-modified case B, hinting at an unphysically steep dust law. These line ratios may be due to increased optical depth in the Balmer lines as a direct effect of high-density gas (log n e > 9) surrounding the black hole. If case B recombination does hold in most LRDs, they must be moderately to heavily dust obscured ( A V ≃ 1−8), while the host-galaxy should be dust-free, suggesting that the extinction in the broad lines is local to the LRD and is not due to the general interstellar medium of the host galaxy. Nevertheless, this dust-based scenario poses several caveats. The apparent general consistency of LRDs with case B may be an artefact of H γ and H δ non-detections.
Funding Information
  • Danmarks Grundforskningsfond (140)
  • Staatssekretariat für Bildung, Forschung und Innovation
  • European Research Council (101071865)
  • European Research Council (788113)

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