Radio signals from early direct collapse black holes

TL;DR

This study predicts the radio detectability of early direct collapse black holes using upcoming telescopes like SKA and ngVLA, suggesting that jetted DCBHs with certain properties could be observable and distinguishable from star-forming galaxies.

Contribution

It introduces a jet-based model to estimate the radio signals from high-redshift DCBHs and assesses their detectability with future radio telescopes.

Findings

Jetted DCBHs with mass >10^5 M_sun are detectable by SKA/ngVLA.

Approximately 100 DCBHs per square degree at z=10 could be observed.

SKA-low and SKA-mid observations can help distinguish DCBHs from star-forming galaxies.

Abstract

We explore the possibility to detect the continuum radio signal from direct collapse black holes (DCBHs) by upcoming radio telescopes such as the SKA and ngVLA, assuming that after formation they can launch and sustain powerful jets at the accretion stage. We assume that the high-$z$ DCBHs have similar jet properties as the observed radio-loud AGNs, then use a jet model to predict their radio flux detectability. If the jet power $P_{\rm jet}\gtrsim10^{42-43}$ erg s$^{-1}$, it can be detectable by SKA/ngVLA, depending on the jet inclination angle. Considering the relation between jet power and black hole mass and spin, generally, jetted DCBHs with mass $\gtrsim10^5~M_\odot$ can be detected. For a total jetted DCBH number density of $\sim2.5\times10^{-3}$ Mpc$^{-3}$ at $z=10$, about 100 deg$^{-2}z^{-1}$ DCBHs are expected to be above the detection threshold of SKA1-mid (100 hours integration). If the jet "blob" emitting most of the radio signal is dense and highly relativistic, then the DCBH would only feebly emit in the SKA-low band, because of self-synchrotron absorption (SSA) and blueshift. Moreover, the free-free absorption in the DCBH envelope may further reduce the signal in the SKA-low band. Thus, combining SKA-low and SKA-mid observations might provide a potential tool to distinguish a DCBH from a normal star-forming galaxy.