Radio signals from early direct collapse black holes

Abstract

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 Pjet ≳ 1042–1043 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 $\gtrsim 10^5\, {\rm M}_\odot$ can be detected. For a total jetted DCBH number density of ∼2.5 × 10−3 Mpc−3 at z = 10, about 100 deg−2z−1 DCBHs are expected to be above the detection threshold of SKA1-mid (100-h 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.