Evolutionary Tracks and Spectral Properties of Quasi-stars and Their Correlation with Little Red Dots

TL;DR

This paper models quasi-stars using evolutionary simulations to connect their properties with observed high-redshift objects called Little Red Dots, suggesting they could be progenitors of supermassive black holes.

Contribution

It introduces new evolutionary models of quasi-stars with predictions of their observable properties, linking theoretical models to recent JWST observations of LRDs.

Findings

Models follow a Hayashi track during late stages.

Scaling relations for quasi-star mass and luminosity.

Quasi-stars could explain the origin of supermassive black holes.

Abstract

JWST has revealed a population of red, compact, high-redshift (${z\sim3-10}$) objects referred to as ``Little Red Dots'' (LRDs). These objects exhibit unusual spectral features reminiscent of stellar spectra with blackbody-like SEDs, large hydrogen Balmer breaks, Balmer line absorption, and classical stellar absorption features such as calcium H&K and the calcium triplet. Following the recent suggestion that these may be actively accreting direct-collapse black holes in the process of assembly, i.e. quasi-stars, we present evolutionary models of quasi-stars using our recently released, publicly available MESA-QUEST modeling framework. We compute a grid of models spanning a range of black hole masses and predict the luminosities, temperatures, surface gravities, and lifetimes of these objects. We find that these models lie along a Hayashi track once they hit their ``late-stage'' which constitutes the majority of their lives ($\sim 20$~Myr). We present scaling relations for estimating the mass of a quasi-star as a function of the bolometric luminosity, as well as the bolometric luminosity as a function of the effective temperature for the Hayashi track. The short lifetimes in tandem with the observed number density of LRDs imply the possibility that every supermassive black hole was once a quasi-star. We compare synthetic spectra of our quasi-star models to observations of LRDs, and show that these models are broadly capable of reproducing the continuum spectra of observed LRDs. These results indicate that quasi-stars are promising candidates for the origin of supermassive black holes via direct collapse in the early universe.