Implications of Broad [OIII]4364 and UV Line Emission in Two Little Red Dots at z ~ 7 - 8

We present deep, NIRSpec G140M and G395M spectroscopy of Little Red Dots (LRDs) at z = 6.68 and z = 8.35. Both LRDs show broad Balmer and [O III]λ4364 emission. The broad [O III]λ4364 lines have FWHM ∼ 1000 km/s, about 1/3 that of the Hβ lines. Assuming gas temperatures T ∼ 15,000 - 25,000 K, the [O III]λ4364/[O III]λ5008 ratios of the broad lines yield high gas densities, log n/cm⁻³ = 6.3 to 7.9, 3-10× higher than those in broad-line regions of low-redshift quasars. If the broad-lines trace virial motions, it is evidence for metal-enhanced gas clouds, ∼1-10 pc from the LRD engine. Both LRDs show narrow [C III]λ1907 + C III]λ1909, and O III]λλ1661,1666. The C III] ratios yield narrow-line gas densities, log n/cm⁻³ = 4.2-5.2, similar to those in other star-forming galaxies. The line equivalent widths, EW(O III]), EW(C III]), are at, or exceed, limits expected for stellar populations, likely requiring an additional ionizing source. The LRDs also have [O III]λ4364/Hγ ratios that favor ionization from an accretion disk, possibly combined with stars. Both LRDs show nitrogen enhancement based on detections of N III]λ1746 or N IV]λ1486, which may imply rapid, recent star-formation. These results favor a scenario where the LRD gas envelopes are highly stratified, having high-density clouds with a non-unit covering factors and a complex geometry, such that ionizing radiation from the LRD accretion disk, combined with that from star-forming regions, produce the nebular emission features.

CC-BY-4.0