Little Red Dots at z~2 in EIGER reveal a gentle decline with respect to their peak number density at z~5

Shrriya Kapoor, Jorryt Matthee, Alberto Torralba, Ivan G. Kramarenko, Rongmon Bordoloi, Jenny E. Greene, Edoardo Iani, Daichi Kashino, Zhaoran Liu, Ruari Mackenzie, Sara Mascia, Rohan P. Naidu, Rob Simcoe

First listed 2026-07-02 | Last updated 2026-06-30

Abstract

We report the discovery of a sample of little red dots (LRDs) at $z \approx 2$ identified from deep JWST/NIRCam imaging and wide-field slitless spectroscopy over $140$ arcmin$^2$ from the EIGER survey. With an improved blind broad-line identification algorithm, we select 19 sources at spectroscopic redshifts $z = 1.55-3.18$ identified via rest-frame near-infrared lines (Paschen-$β$, HeI+Pa$γ$ and OI). Based on a range of spectro-photometric criteria, we classify five of these sources as LRDs and the other 14 as classical active galactic nuclei (AGNs). This classification is corroborated by some X-ray detections among the AGNs. Classical AGNs dominate the number counts above optical luminosities M$_{5100}<-22.5$, whereas the LRD fraction among broad-line sources reaches 100 % at M$_{5100}\approx-20$. The LRDs span the range in Balmer break strengths seen in the higher redshift populations. Blue-shifted HeI absorption is detected in the two reddest sources. The HeI/Pa$γ$ ratio cleanly separates LRDs from classical AGNs and seems to anti-correlate with Balmer break strength, likely tracing HeI self-absorption at higher gas column densities. Our LRD sample has a similar optical luminosity range as their high-redshift counterparts, corresponding to black hole masses of $\sim10^{6}$ M$_{\odot}$ at the Eddington luminosity. We measure LRD number densities of $\approx 7\times10^{-6}$ cMpc$^{-3}$ at $z = 1.9-2.5$, which indicates that LRDs represent $\lesssim 3$ % of the AGN population at these epochs. Our results confirm the previously reported decline in the LRD number density with respect to $z \approx 5$ based on photometric surveys, although we find the decline to be more gentle than earlier emphasized.

Short digest

Using 140 arcmin^2 of deep EIGER JWST/NIRCam imaging plus F356W slitless spectroscopy, this paper applies an improved blind broad-line search to find 19 broad-line sources at z=1.55-3.18 from rest-frame near-infrared lines, and classifies 5 as little red dots (LRDs) and 14 as classical AGNs. The main discriminator is a combined spectro-photometric picture in which LRDs dominate only at faint optical luminosities around M5100≈-20, span a range of Balmer-break strengths, and are cleanly separated from classical AGNs by the He I/Paγ ratio, which also appears to anti-correlate with Balmer-break strength; the two reddest LRDs additionally show blueshifted He I absorption. The z~2 LRDs occupy a similar optical-luminosity range to higher-redshift LRDs, consistent with ~10^6 M⊙ black holes radiating near Eddington. Their inferred number density of about 7×10^-6 cMpc^-3 at z=1.9-2.5 shows that LRDs are at most a few percent of the AGN population by this epoch and that the drop from the z~5 peak is real but gentler than some earlier claims suggested.

Key figures to inspect

• Figure 2. Use this as the core discovery figure: it shows the actual 2D NIRCam WFSS detections for the five LRDs, including the continuum-subtracted emission-line maps used by the blind search. It makes the sample definition concrete and lets readers see how the broad-line signatures are identified in practice.
• Figure 5. This figure is the cleanest overview of how the authors separate LRDs from the rest of the broad-line sample in observed parameter space. The optical-luminosity versus size plane, together with flags for X-ray detections, He I outflows, template fits, and high He I/Pa ratios, directly supports the claim that classical AGNs dominate at higher luminosity while LRDs take over at the faint end.
• Figure 6. Recommend this for the paper’s strongest object-level spectroscopic evidence. The fitted He I+Pa profiles show that two of the reddest LRDs have blueshifted He I absorption, a result highlighted in the abstract and important for the dense-gas, self-absorption interpretation.
• Figure 7. This is the most important physical-diagnostic figure because it shows that the integrated He I 1.083 μm to Paγ ratio cleanly separates LRDs from non-LRD broad-line sources and appears to anti-correlate with Balmer-break strength. It ties the line physics directly to the SED differences that motivate the LRD classification.
• Figure 11. Choose this as the bottom-line synthesis figure. It places the new z~2 measurement into the redshift evolution of LRD number density and visually communicates the paper’s main conclusion that the decline from the z~5 peak is confirmed but is more gradual than previously emphasized.