The Age of Discovery with the James Webb: Excavating the Spectral Signatures of the First Massive Black Holes

TL;DR

This paper models the spectral signatures of early supermassive black hole seeds detectable by JWST, highlighting unique color criteria and emission lines for identifying rapidly growing black holes at high redshift.

Contribution

It introduces a detailed spectral energy distribution model for super-Eddington accreting black holes at high redshift, aiding their detection with JWST.

Findings

Strong Hα emission significantly influences broad-band colors.

Specific color criteria can robustly identify seed black holes.

NIRSpec can test accretion disk properties of these black holes.

Abstract

The James Webb Space Telescope (JWST) will open a new window of the most distant universe and unveil the early growth of supermassive black holes (BHs) in the first galaxies. In preparation for deep JWST imaging surveys, it is crucial to understand the color selection of high-redshift accreting seed BHs. We model the spectral energy distribution of super-Eddington accreting BHs with millions of solar masses in metal-poor galaxies at $z\gtrsim 8$, applying post-process line transfer calculations to radiation hydrodynamical simulation results. Ten kilosecond exposures with the NIRCam and MIRI broad-band filters are sufficient to detect the radiation flux from the seed BHs with bolometric luminosities of $L_{\rm bol}\simeq 10^{45}~{\rm erg~s}^{-1}$. While the continuum colors are similar to those of typical low-$z$ quasars, strong H$\alpha$ line emission with a rest-frame equivalent width ${\rm EW}_{\rm rest}\simeq 1300~\r{A}$ is so prominent that the line flux affects the broad-band colors significantly. The unique colors, for instance F356W$-$F560W $\gtrsim 1$ at $7<z<8$ and F444W$-$F770W $\gtrsim 1$ at $9<z<12$, provide robust criteria for photometric selection of the rapidly growing seed BHs. Moreover, NIRSpec observations of low-ionization emission lines can test whether the BH is fed via a dense accretion disk at super-Eddington rates.