[Paper Review] Little Red Dots: an abundant population of faint AGN at z~5 revealed by the EIGER and FRESCO JWST surveys

Jorryt Matthee, Rohan P. Naidu+26 more|arXiv (Cornell University)|Jun 8, 2023

Galaxies: Formation, Evolution, Phenomena37 citations

TL;DR

The authors search for broad H-alpha emitters at z~4–6 using JWST EIGER and FRESCO data, identifying 20 broad-line H-alpha emitters and inferring faint AGN with SMBH masses ~10^7–10^8 Msun and a higher-than-expected number density at z~5.

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$α$ 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$α$ 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$_{ m UV}=-21$ 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$α$ 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.

Motivation & Objective

• Quantify the prevalence of faint AGN at z~5 through broad H-alpha emission.
• Characterize the properties of broad-line H-alpha emitters and their SMBHs.
• Assess whether faint AGN contribute significantly to cosmic reionization.
• Compare observed AGN number densities with quasar luminosity functions and simulations.

Proposed method

• Combine JWST NIRCam imaging with wide-field slitless spectroscopy from EIGER and FRESCO to build a flux-limited H-alpha sample over z=4.0–6.0.
• Identify broad H-alpha components by fitting narrow+broad Gaussian profiles to grism spectra with S/N>5 for the broad component.
• Perform spatially resolved extraction and contamination cleaning to distinguish central galaxies from nearby companions.
• Fit multi-component models to extract broad and narrow H-alpha fluxes and derive FWHM for the broad component (typical ~2000 km/s).
• Estimate H-alpha luminosities and line ratios (including [NII]) to argue for an AGN origin despite X-ray non-detections.

Experimental results

Research questions

• RQ1 What is the space density of faint AGN with broad H-alpha at z~4–6?
• RQ2 What are the SMBH masses and accretion properties implied by broad H-alpha emission at high redshift?
• RQ3 How does the faint AGN population at z~5 compare with extrapolations of quasar UV luminosity functions and with simulations?
• RQ4 Do faint AGN significantly contribute to reionization, or are they largely obscured?

Key findings

• Identified 20 broad-line H-alpha emitters at z=4.2–5.5 with broad components contributing ~30–90% of the total H-alpha flux.
• Broad H-alpha line widths span ~1200–3700 km/s; inferred SMBH masses are ~10^7–10^8 Msun.
• In the UV-luminosity range M_UV,AGN+host ≈ -21 to -18, the number densities are ~1e-5 cMpc^-3, an order of magnitude higher than simple quasar LF extrapolations but these AGN reside in <1% of star-forming galaxies at z~5.
• The SMBH mass function from these sources agrees with large cosmological simulations.
• A strong correlation is found between reddening and the AGN-fraction of total galaxy luminosity, implying that early SMBH growth is heavily obscured and faint AGN contribute modestly to reionization.
• Evidence of complex H-alpha profiles in two objects suggests absorption signatures from dense gas and possible early AGN feedback.