EIGER Survey Finds Little Red Dots Persist at z≈2 with Number Density Declining Gently from z≈5

The EIGER team applied an improved blind broad-line algorithm to 140 arcmin² of NIRCam slitless spectroscopy, isolating 19 broad-line emitters via Paschen-β, He I + Pa γ and O I. Five sources met LRD criteria through spectro-photometric cuts including red continua and compact morphology while fourteen were classified as classical AGNs, with X-ray detections supporting the latter. LRDs dominate at M_{5100} ≈ −20 and exhibit Balmer-break strengths and He I absorption patterns matching higher-redshift samples, implying black-hole masses near 10^6 M_⊙ at Eddington accretion. This spectroscopic confirmation at lower redshift shows the LRD population is not confined to the reionization era.

Standard galaxy-evolution models calibrated on optical surveys predicted rapid disappearance of such compact, reddened systems below z≈4 because of dust-clearing and merger-driven growth. The EIGER result instead indicates LRDs represent ≲3 % of the AGN population at z≈2 yet maintain similar luminosities and column densities, suggesting that optical selection misses a persistent obscured phase of early black-hole assembly. The He I/Pa γ ratio anti-correlating with Balmer-break strength points to self-absorption in high-column-density gas, a signature optical surveys cannot access.

Future wide-field grism programs and deeper X-ray stacking will test whether the gentle decline continues to z<1.5 or steepens once AGN feedback clears the obscuring envelopes. If the space density remains above 3×10^{-6} cMpc^{-3} at z=1.2, models must incorporate longer-lived dusty accretion episodes rather than rapid quenching.