A Critical Evaluation of the Physical Nature of the Little Red Dots

TL;DR

This paper reviews the nature of Little Red Dots, a new class of early active galactic nuclei, discussing their properties, possible origins, and how future observations can clarify their role in black hole formation.

Contribution

It provides a comprehensive evaluation of the physical characteristics, origins, and evolutionary status of LRDs, proposing that they are a transient phase in early black hole growth.

Findings

LRDs host black holes of about 10^6-7 solar masses.

Purely stellar models are unlikely to explain LRDs' properties.

LRDs are likely a transient phase in early black hole accretion.

Abstract

Little Red Dots (LRDs) are a newly identified class of active galactic nuclei (AGNs) uncovered by JWST deep surveys. Their enigmatic properties challenge the canonical AGN paradigm and have stimulated ideas on early massive black hole (BH) formation. In this review, we summarize how early BHs shape the characteristic features of LRDs, how their nuclear environments differ from those of normal AGNs, and how future observations can distinguish between competing scenarios. Our main conclusions are as follows: (1) LRDs show broad-line emission consistent with mass accretion onto BHs with $M_{\rm BH}\simeq 10^{6-7}~M_\odot$, suggesting that AGN activity is a plausible origin of their dominant red optical emission. (2) Stellar components can reproduce the continuum energetics through dusty star formation. However, the required stellar mass would be too large to remain consistent with other LRD properties. Therefore, a purely stellar origin is unlikely to be the dominant power source, although star formation may still contribute to the UV emission. (3) The coexistence of broad-line emission with Balmer absorption and break features on LRD spectra, neither of which can be explained by stellar populations, suggests that nuclear BHs are enshrouded by dense gas with a high covering fraction. (4) Gas-enshrouded AGNs can produce red optical spectra without requiring dust reddening through a combination of gas attenuation and thermal self-emission with an effective temperature of $T_{\rm eff}\simeq 5000~{\rm K}$, which also accounts for the flat infrared continuum. (5) From the spectral features and redshift evolution, LRDs are likely a transient phase in early BH growth, possibly the first accretion episodes of newborn BHs. (6) Testing models for LRD spectra and origins through time variability, ionizing sources, post-LRD objects, and low-redshift analogs is particularly promising.