[Paper Review] UNCOVER spectroscopy confirms a surprising ubiquity of AGN in red galaxies at $z>5$

Jenny E. Greene, Ivo Labbé+32 more|arXiv (Cornell University)|Sep 11, 2023

Galaxies: Formation, Evolution, Phenomena24 citations

TL;DR

JWST UNCOVER spectroscopy reveals that a large fraction of compact red z>5 sources are dust-reddened AGN, with broad H-alpha indicating active black holes and high implied number densities.

ABSTRACT

JWST is revealing a new population of dust-reddened broad-line active galactic nuclei (AGN) at redshifts $z\gtrsim5$. Here we present deep NIRSpec/Prism spectroscopy from the Cycle 1 Treasury program UNCOVER of 15 AGN candidates selected to be compact, with red continua in the rest-frame optical but with blue slopes in the UV. From NIRCam photometry alone, they could have been dominated by dusty star formation or AGN. Here we show that the majority of the compact red sources in UNCOVER are dust-reddened AGN: $60\%$ show definitive evidence for broad-line H$α$ with FWHM$\, >2000$ km/s, for $20\%$ current data are inconclusive, and $20\%$ are brown dwarf stars. We propose an updated photometric criterion to select red $z>5$ AGN that excludes brown dwarfs and is expected to yield $>80\%$ AGN. Remarkably, among all $z_{ m phot}>5$ galaxies with F277W$-$F444W$>1$ in UNCOVER at least $33\%$ are AGN regardless of compactness, climbing to at least $80\%$ AGN for sources with F277W$-$F444W$>1.6$. The confirmed AGN have black hole masses of $10^7-10^9$ M$_{\odot}$. While their UV-luminosities ($-16>M_{ m UV}>-20$ AB mag) are low compared to UV-selected AGN at these epochs, consistent with percent-level scattered AGN light or low levels of unobscured star formation, the inferred bolometric luminosities are typical of $10^7-10^9$ M$_{\odot}$ black holes radiating at $\sim 10-40\%$ of Eddington. The number densities are surprisingly high at $\sim10^{-5}$ Mpc$^{-3}$ mag$^{-1}$, 100 times more common than the faintest UV-selected quasars, while accounting for $\sim1\%$ of the UV-selected galaxies. While their UV-faintness suggest they may not contribute strongly to reionization, their ubiquity poses challenges to models of black hole growth.

Motivation & Objective

• Motivate the study of black hole growth and galaxy co-evolution at z>5 using JWST’s infrared capabilities.
• Assess whether compact red z>5 sources dominant in photometric samples are AGN or dusty star-forming galaxies.
• Quantify the AGN fraction among red, compact z>5 sources and derive black hole masses and accretion properties.
• Evaluate the effectiveness of photometric color cuts in selecting z>5 AGN and compare with UV-selected populations.

Proposed method

• Utilize UNCOVER JWST imaging (NIRCam) to select compact red sources with specific color criteria favoring red optical continua and blue UV slopes.
• Obtain NIRSpec/PRISM spectroscopy for 17 AGN candidates to search for broad Balmer lines (H-alpha) and other AGN indicators.
• Fit emission-line complexes (H-beta, [O III], H-alpha, [N II]) with narrow and broad components to identify broad H-alpha (FWHM > 2000 km/s).
• Estimate black hole masses from H-alpha widths and luminosities, dereddened and demagnified using lensing models, and compute bolometric luminosities.
• Assess contamination (e.g., brown dwarfs) and use Balmer decrement and continuum slopes to constrain extinction.
• Explore photometric AGN selection efficiency and derive AGN fractions as a function of color cuts (e.g., F277W−F444W).

Figure 1: Left: The primary color-color selection used to select the compact red sources. We show the full sample of 40 compact red sources (open circles), the 17 clean targets (yellow) and the spectroscopically targeted sources (black dots are those observed with the MSA). Comparison of the photome

Experimental results

Research questions

• RQ1 What fraction of compact red z>5 sources are broad-line AGN as revealed by NIRSpec/PRISM spectroscopy?
• RQ2 What are the black hole masses and accretion properties (bolometric luminosities, Eddington ratios) of these AGN?
• RQ3 How effective are JWST color and morphology cuts at selecting AGN at z>5, and how do AGN fractions vary with color thresholds?
• RQ4 How do the AGN identified in red z>5 sources compare to UV-selected AGN populations in terms of luminosity and number density?

Key findings

• 60% of the targeted compact red sources show definitive broad-line H-alpha emission (FWHM > 2000 km/s).
• 20% of targets yield inconclusive results, and 20% are brown dwarfs.
• Among zphot>5 galaxies with F277W−F444W >1, at least 33% are AGN, rising to at least 80% for F277W−F444W >1.6.
• Confirmed AGN have black hole masses in the range 10^7–10^9 solar masses.
• UV luminosities are relatively low (-16 > M_UV > -20 AB mag), but bolometric luminosities imply accretion at ~10–40% of Eddington.
• AGN number densities are ~10^-5 Mpc^-3 mag^-1, about 100x fainter UV-selected quasars, and represent ~1% of UV-selected galaxies.
• The ubiquity of these red z>5 AGN poses challenges to models of early black hole growth and co-evolution with galaxies.

Figure 2: Left: NIRSpec/PRISM spectra of the nine confirmed broad-line AGN in the sample. Spectra are plotted in the rest wavelength and have been normalized at 2500Å. Vertical lines indicate rest wavelengths of hydrogen lines (Ly $\alpha$ , H $\beta$ , H $\alpha$ ) in dashed, broad permitted metal