Little Red Dots: The Assembly of Early Supermassive Black Holes in the JWST Era

TL;DR

The paper reviews recent JWST discoveries of high-redshift objects called 'little red dots', exploring their nature as either active galactic nuclei or star-forming regions, and discusses their implications for early supermassive black hole formation.

Contribution

It synthesizes observational evidence on LRD objects and discusses their potential role in understanding early SMBH growth in the universe.

Findings

LRD objects may be SMBH or star-forming regions.

High Eddington ratios suggest rapid SMBH growth.

Potential link to formation of massive early SMBHs.

Abstract

Since the launch of James Webb Space Telescope (JWST) in late 2021, our understanding of high-redshift objects has faced several upheavals. JWST has discovered much more massive galaxies and supermassive black holes (SMBH) than cosmological models had expected. Furthermore, JWST observations have revealed an entirely novel population of high-redshift objects. Characterized by a dominant red rest-frame component and point-like morphology, these ``little red dots'' (LRD) have set off a flurry of observational and theoretical follow-up. The current identity of LRD is highly debated, yet falling into two main scenarios: active galactic nuclei (i.e., SMBH) or compact star-forming regions. If star-forming, LRD would represent the highest stellar densities ever observed. If SMBH, their high Eddington fractions, and already high masses, help elucidate the growth of the most massive SMBH found by JWST in the early Universe ($z \gtrsim4)$. In this mini-review, we present the observational evidence accumulated to date, including sub-millimeter probes of LRD dust masses, constraints on radio and X-ray emission from stacking, and rest-frame ultraviolet \& optical measurements provided by JWST. Furthermore, we highlight how identifying additional LRD that are truly primarily SMBH-driven may help to shed light on the formation of `overly massive' SMBH discovered by JWST within the first billion years since the Big Bang.