Radio signals from early direct collapse black holes
  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.
