B. Yue & A. Ferrara explored the possibility of detection of the continuum radio signal from direct collapse black holes (DCBHs) by upcoming radio telescopes such as the SKA and ngVLA. They showed that, if the jet power 𝑃jet ≳ 1042¯43 erg s¯1, it can be detectable by SKA/ngVLA, depending on the jet inclination angle. Their study recently appeared in the Journal Monthly Notices of the Royal Astronomical Society.
Direct collapse black holes (DCBHs) are high-mass black hole seeds, putatively formed within the redshift range (15 < z < 30), when the Universe was about 100-250 million years old. Unlike seeds formed from the first population of stars (also known as Population III stars), direct collapse black hole seeds are formed by a direct, general relativistic instability. They are very massive, with a typical mass at formation of ~ 105 M. DCBHs have not been detected until now, either because they are too faint and/or too rare. There are many promising detections techniques have been proposed for their detection such as the cosmic infrared background, multi-color sampling of the Spectral Energy Distribution (SED) combined with X-ray surveys, a specific Ly𝛼 signature, and the neutral hydrogen 𝜆 = 3 cm maser line.
Now, by assuming that the DCBHs can launch and sustain powerful jets at the accretion stage after formation, B. Yue & A. Ferrara explored the possibility of detection of the continuum radio signal from direct collapse black holes (DCBHs) by upcoming radio telescopes such as the SKA and ngVLA.
“DCBHs may also produce radio signal during the collapse stage. However this stage is shorter therefore the detection probability is smaller. Our model supposes that DCBHs can launch powerful jets similar to those observed in the radio-loud AGNs.”— they said.
They applied the jet properties of the observed blazars to the jetted high-𝑧 DCBHs, and used the publicly available GT09 jet model to predict the radio signal from the jet.
They showed that, if the jet power 𝑃jet ≳ 1042¯43 erg s¯1, it can be detectable by SKA/ngVLA with 100 integration hours, depending on the jet inclination angle. However, as the jet power depends on both black hole mass and spin, the DCBHs with mass ≳ 105 𝑀 can be detected. But, less than this value (i.e. 105 𝑀) it can be hard to detect DCBHs with SKA.
Additionally, by considering the spin distribution they showed that, about 10¯3 of DCBHs with MBH = 106 M, if they are all jetted, are detectable. Moreover, if all DCBHs are jetted, for the most optimal case ~ 100 deg¯2𝑧¯1 at z = 10 would be detected by SKA1-mid with 100 hours integration time.
Finally, it has been suggested that, if the jet “blob” emitting most of the radio signal is dense (𝑟blob ∼ 50 𝑟s) and highly relativistic (i.e. has large bulk motion velocity (ν ∼ 10)), 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.
However, their study contains some uncertainties such as it is not clear how many of DCBHs can launch jets, number density of the DCBHs and their mass function etc.
“Future theoretical investigations, for example the numerical simulations, would help to improve our predictions. However, the uncertainties could only be reduced by observations themselves. For example, if the radio signal from DCBHs is much lower than our predictions, then it is likely that most DCBH cannot launch strong jet. This will force us to investigate the difference between DCBHs and low-𝑧 black holes, for example on the magnetic field, the density of the envelope, or the spin.”— they concluded.
Featured image: Observed radio flux from a jetted DCBH, for different jet powers, Lorentz factors and inclination angles, as indicated in the label. Sensitivities of different telescopes are given. For all sensitivities the integration time is 100 hours, and bandwidth is given in the text © B. Yue & A. Ferrara
Reference: B Yue, A Ferrara, Radio signals from early direct collapse black holes, Monthly Notices of the Royal Astronomical Society, 2021;, stab2121, https://doi.org/10.1093/mnras/stab2121
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