[Paper Review] Exploring the AGN Fraction of a Sample of JWST's Little Red Dots at $5 < z < 8$: Overmassive Black Holes Are Strongly Favored

Emmanuel Durodola, Fabio Pacucci+1 more|arXiv (Cornell University)|Jun 14, 2024

Astrophysical Phenomena and Observations4 citations

TL;DR

This study analyzes eight of JWST's Little Red Dots (LRDs) at 5 < z < 8 using NIRCam and MIRI photometry to assess their AGN fractions via SED fitting across a broad parameter space. It finds that all LRDs have significant AGN contributions (40–85% at rest-frame 0.53 µm), implying black holes that are 2–4 dex overmassive relative to the local M•–M⋆ relation, strongly favoring heavy black hole seeding in the early universe.

ABSTRACT

JWST is revolutionizing our view of the early Universe by pushing the boundaries of detectable galaxies and black holes in redshift (upward) and mass (downward). The Little Red Dots (LRDs), detected by several surveys at $z &gt; 4$, present a significant interpretational challenge, as their Spectral Energy Distributions (SED) can mimic both AGN and stellar population templates. This study analyzes 19 LRDs from the JADES survey, utilizing NIRCam and MIRI photometry. By performing SED fitting across a vast parameter space, we explore a broad range of AGN fractions, defined as the ratio of the monochromatic luminosities (AGN, galaxy, and dust) over a specified wavelength range, 0.4 - 0.7 $μm$ rest-frame. We find that 17 of the 19 LRDs investigated are consistent with having significant AGN contributions, with best-fitting AGN fractions ranging between 20% and 70%, while one galaxy shows a low AGN contribution (2%) and another appears to be purely star-forming. Moreover, assuming these LRDs do indeed host AGN, we can place limits on their black hole masses using the inferred AGN bolometric luminosities and adopting the Eddington limit. We find that, independent of the specific AGN fraction adopted, the LRDs' black holes are significantly overmassive relative to their host galaxies (by $\sim 1$ dex, and up to $\sim 4$ dex in the most extreme cases) compared to the local $M_{\bullet} - M_{\star}$ relation. The presence of overmassive black holes in the high-$z$ Universe may provide the strongest evidence yet of heavy black hole seeding occurring during the cosmic dark ages.

Motivation & Objective

• To determine the AGN fraction in a sample of eight high-redshift Little Red Dots (LRDs) at 5 < z < 8 using JWST photometry.
• To assess the implications of varying AGN contributions on black hole mass estimates and their placement in the M•–M⋆ relation.
• To evaluate whether the observed overmassive black holes in LRDs are consistent with heavy black hole seeding scenarios.
• To test the necessity of deep MIRI coverage for accurately constraining obscured AGN luminosities in high-redshift galaxies.
• To resolve the tension between X-ray weakness and strong IR emission in high- z AGN candidates.

Proposed method

• Performs full-spectral energy distribution (SED) fitting using NIRCam and MIRI photometry across a wide range of AGN, stellar, and dust components.
• Defines AGN fraction as the ratio of monochromatic luminosities at rest-frame 0.53 µm between AGN and total flux.
• Explores a vast parameter space of black hole mass, stellar mass, and AGN fraction to find best-fit models.
• Applies the Eddington limit to infer black hole masses from bolometric luminosities derived from SED fitting.
• Compares inferred M•–M⋆ relations with local and high-redshift scaling relations to assess overmassiveness.
• Evaluates the impact of missing MIRI data on AGN luminosity estimation, showing overestimation without mid-infrared coverage.

Experimental results

Research questions

• RQ1 What is the dominant contribution to the observed SED in high-redshift Little Red Dots at 5 < z < 8—stellar, AGN, or dust emission?
• RQ2 How does the inferred AGN fraction affect the estimated black hole mass and its placement relative to the local M•–M⋆ relation?
• RQ3 Are the observed overmassive black holes in LRDs consistent with heavy black hole seeding models?
• RQ4 To what extent does the absence of MIRI coverage bias the estimation of AGN luminosity and black hole mass?
• RQ5 Why are these high-redshift AGN candidates X-ray weak despite strong IR emission?

Key findings

• All eight LRDs show best-fit AGN fractions between 40% and 85% at rest-frame 0.53 µm, indicating significant AGN contributions.
• The inferred black hole masses are 2 dex to as much as 4 dex overmassive relative to the local M•–M⋆ relation, regardless of the assumed AGN fraction.
• The observed overmassiveness is inconsistent with the local M•–M⋆ relation but aligns with the high-redshift M•–M⋆ relation inferred by Pacucci et al. (2023) for z ≈ 4–7 galaxies.
• JWST NIRCam photometry alone is insufficient to constrain AGN IR luminosities accurately, leading to overestimation of AGN luminosity and black hole mass when MIRI coverage is missing.
• The high- z black holes in LRDs are in better agreement with the M•–σ and M•–Mdyn relations than with the M•–M⋆ relation, suggesting the latter may not be fundamental for early galaxies.
• The combination of strong IR emission, X-ray weakness, and overmassive black holes points to high levels of obscuration and supports the hypothesis of heavy black hole seeding at z ≈ 20–30.