JWST Insights into Narrow-line Little Red Dots

Zijian Zhang, Linhua Jiang, Weiyang Liu, Luis C. Ho, Kohei Inayoshi

First listed 2025-06-04 | Last updated 2026-01-05

Abstract

James Webb Space Telescope (JWST) has revealed a population of red and compact objects with a unique V-shape SED at z >= 4 known as Little Red Dots (LRDs). Most of the LRDs with existing spectral observations exhibit broad Balmer lines and are thus likely to host active galactic nuclei (AGNs). Here we present a study of LRDs with no broad H-alpha component. Our sample consists of five LRDs at z~5 with H-alpha line widths of about 250 km/s. They are selected from 32 LRDs that have NIRSpec high- or medium-resolution grating spectra covering H-alpha. During our construction of the sample, we find that approximately 20 percent of the LRD candidates previously selected do not show red continuum emission but resemble the V-shape spectra due to strong line emission. Compared to normal star-forming galaxies, narrow-line LRDs tend to have relatively higher H-alpha line widths and luminosities. If these LRDs are dominated by galaxies, our SED modeling suggests that they are dusty, compact star-forming galaxies with high stellar masses and star formation rates (SFRs). Alternatively, if their SEDs are produced by AGNs, the inferred central black hole masses (MBH) are in the range of 10^5 to 10^6 solar masses, placing 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 measurement uncertainties, these black holes appear slightly overmassive relative to the local MBH-Mstar relation, but consistent or undermassive with respect to the MBH-sigma and MBH-Mdyn relations. We further find that nearly half of the high-redshift broad-line AGNs exhibit V-shape SEDs. (abridged)

Short digest

Using NIRSpec medium/high-resolution data and line-free photometry, the authors isolate five narrow-line LRDs at z≈5 (H-alpha FWHM ≈250 km/s; one ≈475 km/s) from 32 LRDs with H-alpha coverage, and show that about 20% of color-selected LRD candidates fake a V-shape continuum via strong lines. Relative to normal star-forming galaxies, these objects have higher H-alpha widths and luminosities but smaller equivalent widths. SED fits allow either dusty, compact, high-M* and high-SFR galaxies or low-mass AGN with MBH≈10^5–10^6 Msun, plausibly in an early, super-Eddington phase; the BHs are slightly overmassive vs MBH–M* yet consistent/undermassive vs MBH–sigma and MBH–Mdyn within large errors. Nearly half of high‑z broad-line AGN show V‑shape SEDs, underscoring that LRD-like photometry does not uniquely imply broad-line activity.

Key figures to inspect

• Figure 1: Compare ‘fake’ versus ‘real’ V‑shape SEDs after subtracting Hα+[O III]+Hβ; verify how line contamination can mimic LRD colors and how the authors construct line-free photometry.
• Figure 2: UV and optical slopes from line-free photometry and the F444W size–magnitude plane; check where the five narrow-line LRDs land relative to popular LRD color/size cuts and the stellar locus for compactness tests.
• Figure 3: Redshift–magnitude distribution with the five targets highlighted; see that the narrow-line LRDs cluster around z≈5 and how they compare in brightness to reference star-forming samples.
• Figure 4: NIRCam cutouts and NIRSpec grating fits around Hα; inspect single-Gaussian fits, reported FWHM (~250 km/s; one ~475 km/s), and goodness-of-fit metrics (AIC/BIC) that support the ‘narrow-line’ classification.