Little Red Dots: an abundant population of faint AGN at z~5 revealed by the EIGER and FRESCO JWST surveys (2306.05448v3)

Abstract: Characterising the prevalence and properties of faint active galactic nuclei (AGN) 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 H$\alpha$ emitters at $z\approx4-6$ using deep JWST/NIRCam imaging and wide field slitless spectroscopy from the EIGER and FRESCO surveys. We identify 20 H$\alpha$ lines at $z=4.2-5.5$ that have broad components with line widths from $\sim1200-3700$ km s$^{-1}$, contributing $\sim30-90$ % of the total line flux. We interpret these broad components as being powered by accretion onto SMBHs with implied masses $\sim10^{7-8}$ M${\odot}$. In the UV luminosity range M${\rm UV}=-21$ to $-18$, we measure number densities of $\approx10^{-5}$ cMpc$^{-3}$. This is an order of magnitude higher than expected from extrapolating quasar UV luminosity functions. Yet, such AGN are found in only $<1$ % of star-forming galaxies at $z\sim5$. The SMBH mass function agrees with large cosmological simulations. In two objects we detect narrow red- and blue-shifted H$\alpha$ absorption indicative, respectively, of 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.

• The paper identifies 20 broad Hα emitters at z~5 with velocities of 1200–3700 km/s, indicating SMBH masses from 10^7 to 10^8 solar masses.
• It measures an unexpectedly high AGN density of ~10⁻⁵ cMpc⁻³, exceeding predictions from conventional quasar UV luminosity functions.
• The study informs early black hole growth models, highlighting minimal reionization contributions due to significant obscuration and early AGN feedback.

Overview of "Little Red Dots: An Abundant Population of Faint AGN at z~5 Revealed by the EIGER and FRESCO JWST Surveys"

The paper conducted by Matthee et al. utilizes the cutting-edge capabilities of the James Webb Space Telescope (JWST) to investigate faint Active Galactic Nuclei (AGN) in the early universe, specifically targeting broad H$\alpha$ emitters in the redshift range of approximately 4.2 to 5.5. By leveraging data from the EIGER and FRESCO slitless spectroscopic surveys, the research aims to augment our understanding of supermassive black holes (SMBHs) and their role during cosmic reionization.

Key Findings

• Identification of AGN: Through deep JWST/NIRCam imaging and slitless spectroscopy, 20 broad H$\alpha$ emission lines at redshifts between 4.2 and 5.5 were identified. These AGN were discovered based on their broad line components, which exhibited velocities ranging from ~1200 to 3700 km s$^{-1}$, signifying accretion onto SMBHs with mass estimates ranging from 10^7 to 10^8 solar masses.
• Number Density: The paper measured a number density of these faint AGN at ~10^-5 cMpc^-3 within a UV luminosity range of M$_{\rm UV, AGN+host} = -21$ to $-18$. This is notably an order of magnitude higher than expected from conventional quasar UV luminosity functions derived from brighter AGN observations.
• Redshift Distribution Insight: A redshift distribution concentrated around z~5 aligns with the peak volume and sensitivity of the JWST data in the relevant wavelength range.

Implications

• Black Hole Growth and AGN Feedback: Two objects exhibited complex H$\alpha$ profiles, speculatively interpreted as absorption processes within dense gas environments around SMBHs. This may represent an early feedback phase that affects dust paths, suggesting that these AGN's observed features might precede more massive, dust-cleared blue quasars.
• Reionization Contributions: Though faint AGN proved more numerous than previously cataloged, their contribution to cosmic reionization was deemed minimal due to high obscuration, suggesting that while they contain ionizing potential, actual photon escape fractions are likely low.
• Theoretical Models: The inferred SMBH mass function aligns well with cosmological simulations like EAGLE, indicating that such faint AGN represent a previously underappreciated population. These findings can have significant implications for models predicting early black hole seeding and growth mechanisms.

Future Directions

These observations drive us towards several areas of future exploration:

1. Enhanced Spectroscopic Surveys: Continued and deeper spectroscopic follow-up will be necessary for a more comprehensive understanding of the faint-end AGN population and the verification of the absorption features in the H$\alpha$ line profiles.
2. AGN's Role in the Early Universe: Further analysis into dusty AGN scenarios is needed to clarify how these SMBHs fit into the broader narrative of galaxy formation and evolution, particularly during the epoch of reionization.
3. JWST's Further Utilization: As JWST data continues to provide unprecedented detail in high-redshift studies, it will be pivotal in refining our understanding of SMBH formation, their growth, and their influence on their host galaxies and surroundings.

In sum, this research capitalizes on JWST’s capabilities to significantly advance our understanding of faint AGN demographics and their role in the early universe, challenging existing paradigms and offering fresh avenues for theoretical and observational advancements.