No Luminous Little Red Dots: A Sharp Cutoff in Their Luminosity Function

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Abstract
One of the most surprising results of early James Webb Space Telescope (JWST) observations is the discovery of an abundance of red, compact, broad-line objects dubbed “little red dots” (LRDs) at z > 4. Their spatial density (∼10 −4 –10 −5 cMpc −3 ) is 100 times more abundant than UV-selected quasars at that redshift if one extrapolates the quasar luminosity function (QLF) down to the LRD regime. However, whether LRDs dominate black hole accretion at quasar-like luminosities ( L bol ≳ 10 45−46 erg s −1 ) remains unanswered, as probing the bright end of the LRD luminosity function requires a much larger area than that which is able to be surveyed by JWST. In this work, we present our search for the brightest LRDs ( K < 23.7) at 4.5 < z < 4.9 using wide-area multiwavelength imaging surveys from the near-UV to the infrared bands. With over 15 deg 2 of sky coverage, we only identify one single LRD candidate at z phot ≈ 4.6, which translates into a spatial density of n ( M 5100 < −23.5) ≈ 10 −8 cMpc −3 —this is nearly 10 times less abundant than the UV-selected quasars at similar optical luminosity. When combined with the LRD sample identified by JWST at the same redshift range, we find a sharp cutoff in the optical luminosity function at λL 5100 ≈ 2.2 × 10 44 erg s −1 , while the quasar luminosity function (QLF) turnover occurs at ≳20 times higher luminosity. We therefore confirm the exclusively low-luminosity nature of LRDs, ruling out that LRDs are the counterparts of quasars. Furthermore, we speculate that, if the shape of the luminosity function holds up, it points to LRDs being powered by low-mass black holes with a narrow range of Eddington-level accretion rates.
Funding Information
  • National Aeronautics and Space Administration (RSA 1628138)
  • National Aeronautics and Space Administration (80NSSC21K0630)

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