Little Red Dots: An Abundant Population of Faint Active Galactic Nuclei at z ∼ 5 Revealed by the EIGER and FRESCO JWST Surveys

Abstract

Characterizing the prevalence and properties of faint active galactic nuclei (AGNs) in the early Universe is key for understanding the formation of supermassive black holes (SMBHs) and determining their role in cosmic reionization. We perform a spectroscopic search for broad Hα emitters at z ≈ 4–6 using deep JWST/NIRCam imaging and wide field slitless spectroscopy from the EIGER and FRESCO surveys. We identify 20 Hα lines at z = 4.2–5.5 that have broad components with line widths from ∼1200–3700 km s−1, contributing ∼30%–90% of the total line flux. We interpret these broad components as being powered by accretion onto SMBHs with implied masses ∼107–8 M ⊙. In the UV luminosity range M UV,AGN+host = −21 to −18, we measure number densities of ≈10−5 cMpc−3. This is an order of magnitude higher than expected from extrapolating quasar UV luminosity functions (LFs). Yet, such AGN are found in only <1% of star-forming galaxies at z ∼ 5. The number density discrepancy is much lower when compared to the broad Hα LF. The SMBH mass function agrees with large cosmological simulations. In two objects, we detect complex Hα profiles that we tentatively interpret as caused by absorption signatures from dense gas fueling SMBH growth and outflows. We may be witnessing early AGN feedback that will clear dust-free pathways through which more massive blue quasars are seen. We uncover a strong correlation between reddening and the fraction of total galaxy luminosity arising from faint AGN. This implies that early SMBH growth is highly obscured and that faint AGN are only minor contributors to cosmic reionization.

Exploring Low-mass Black Holes through Tidal Disruption Events in the Early Universe: Perspectives in the Era of the JWST, Roman Space Telescope, and LSST Surveys

“…The BH masses for those JWST-detected faint AGNs are M • ∼ 10 6−8 M e , which is 1-2 orders of magnitude lower than those of previously known quasars at z > 4. Intriguingly, some of the sources have extremely red colors in the rest-frame optical bands, while the rest-frame UV color seems fairly consistent with those of known Lyman-break galaxies at similar redshifts (e.g., Harikane et al 2023;Kocevski et al 2023;Labbe et al 2023;Barro et al 2024;Greene et al 2024;Matthee et al 2024). The so-called "little red dot" sources are interpreted to be in a transition stage from a dust-obscured starburst to an unobscured luminous quasar due to the expelling of gas and dust (Fujimoto et al 2022).…”

“…2. An intriguing subset consists of the dust-reddened AGNs (referred to as "little red dots"), characterized by moderate obscuration in the SED and compact morphology (Harikane et al 2023;Kocevski et al 2023;Labbe et al 2023;Barro et al 2024;Matthee et al 2024). This class of AGNs shows a distinctive spectral shape, which can be attributed to a dust-reddened AGN continuum coupled with excess emission in the rest-frame UV produced either by the unobscured stellar continuum of the host galaxy or scattered light from the buried AGN (e.g., Zakamska et al 2005;Polletta et al 2006;Noboriguchi et al 2023).…”

“…Recently, the unprecedented sensitivity of the James Webb Space Telescope (JWST) has opened a new window to explore the low-luminosity active galactic nuclei (AGNs) at z ∼ 4-7 (Onoue et al 2023). Spectroscopic observations have confirmed the broad components of emission lines and the presence of AGNs (e.g., Harikane et al 2023;Kocevski et al 2023;Larson et al 2023;Maiolino et al 2023;Matthee et al 2024). The BH masses for those JWST-detected faint AGNs are M • ∼ 10 6−8 M e , which is 1-2 orders of magnitude lower than those of previously known quasars at z > 4.…”

Discovery of Merging Twin Quasars at z = 6.05

“…Its extremely high IR sensitivity has revealed a weak broad component in Balmer emission lines of many high-z galaxies, signaling the presence of low-luminosity active galactic nuclei (AGNs) out to z ∼ 11 (e.g., Greene et al 2024;Maiolino et al 2023). The number density of such AGNs exceeds the extrapolation of the classical QLF by several orders of magnitude (e.g., Harikane et al 2023;Matthee et al 2024), changing the paradigm of SMBH activity happening in the EoR.…”

Constraints on the Evolution of the Ionizing Background and Ionizing Photon Mean Free Path at the End of Reionization

“…Chardin et al (2015Chardin et al ( , 2017 showed that the strong fluctuations in the ionizing background would be a natural consequence of a bright and rare source population, i.e., luminous quasars (see also Meiksin 2020), but the initial suggestions of an overabundant faint quasar population at z > 5 (Giallongo et al 2015) are now generally disfavored (e.g., D' Aloisio et al 2017;Matsuoka et al 2018;Parsa et al 2018). While interest in such quasar-dominated models has begun to resurface in light of the discovery of a substantial population of faint AGNs at z > 5 by JWST surveys (e.g., Harikane et al 2023;Kocevski et al 2023;Labbe et al 2023;Maiolino et al 2023;Matthee et al 2024), due to their typically highly reddened nature, it is still unknown whether these objects contribute substantially to the ionizing photon budget. explored the possibility that galaxysourced ionizing background fluctuations could be amplified by a coupling to the mean free path of ionizing photons (see also D'Aloisio et al 2018) following analytic arguments from McQuinn et al (2011), but in order to match the broad distribution of Lyα forest opacity, the average mean free path would have to be a factor of a few shorter than the extrapolation from lower-redshift measurements (Worseck et al 2014).…”