The Age of Discovery with the James Webb: Excavating the Spectral Signatures of the First Massive 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 $L_{\mathrm{bol}}\simeq 10^{45}{\mathrm{erg}\mathrm{s}}^{-1}$. While the continuum colors are similar to those of typical low-$z$ quasars, strong H$\alpha$ line emission with a rest-frame equivalent width ${\mathrm{EW}}_{\mathrm{rest}}\simeq 1300\mathring{A}$ 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.

10 pages, 5 figures, 1 table, accepted for publication in ApJL

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• RUBIES: JWST/NIRSpec Confirmation of an Infrared-luminous, Broad-line Little Red Dot with an Ionized Outflow
• Seeking the growth of the first black hole seeds with JWST
• Supercritical growth pathway to overmassive black holes at cosmic dawn: coevolution with massive quasar hosts
• Probing the $z\gtrsim 6$ quasars in a universe with IllustrisTNG physics: Impact of gas-based black hole seeding models
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• How long do high-redshift massive black hole seeds remain outliers in black hole vs. host galaxy relations?
• Little Red Dots as Direct-collapse Black Hole Nurseries
• Bulgeless disks, dark galaxies, inverted color gradients, and other expected phenomena at higher z. The chromatic surface brightness modulation (CMOD) effect
• An empirical approach to selecting the first growing black hole seeds with JWST/NIRCam