JWST Insights into Narrow-line Little Red Dots

Abstract JWST has revealed a population of red and compact objects with unique “V-shaped” spectral energy distributions (SEDs) known as “Little Red Dots” (LRDs). Many LRDs exhibit broad Balmer lines and thus likely host active galactic nuclei (AGNs). Here we present a study of five LRDs without broad H α lines at z ≥ 5. They are selected from 32 LRDs that have NIRSpec high- or medium-resolution grating spectra covering H α , and their H α line widths are around 250 km s −1 . Compared to normal star-forming galaxies, narrow-line LRDs tend to have relatively higher H α line widths and luminosities. If they are dominated by galaxies, our SED modeling suggests that they are dusty, compact star-forming galaxies with high stellar masses and star formation rates. Alternatively, if their SEDs are produced by AGNs, the inferred black hole masses (<10 6 M ⊙ ) place them at the low-mass end of the AGN population. They may represent an early stage of super-Eddington growth, where the black holes have yet to accumulate significant masses. With large uncertainties, these black holes appear slightly overmassive relative to the local M BH – M * relation, but consistent or undermassive with respect to the M BH – σ stellar and M BH – M dyn relations. These narrow-line LRDs offer new insights into the diversity of LRDs. Finally, we find that ∼44% of high-redshift broad-line AGNs exhibit V-shaped SEDs. In addition, ∼20% of the previous LRD candidates are not real LRDs, and their V-shaped SEDs are caused by strong line emission.