Publication Date:
2015-12-13
Description:
We explore the possibility of detecting hydrogen radio recombination lines from 0 〈 z 〈 10 quasars. We compute the expected H n α flux densities as a function of absolute magnitude and redshift by considering (i) the range of observed active galactic nucleus spectral indices from UV to X-ray bands, (ii) secondary ionizations from X-ray photons, and (iii) stimulated emission due to non-thermal radiation. All these effects are important to determine the line fluxes. We find that the combination of slopes: α X,hard = –1.11, α X,soft = –0.7, α EUV = –1.3, α UV = –1.7, maximizes the expected flux, f H n α 10 μJy for z ~ 7 quasars with M AB = –27 in the n ~ 50 lines; allowed spectral energy distribution variations produce variations by a factor of 3 around this value. Secondaries boost the line intensity by a factor of 2–4 , while stimulated emission in high- z quasars with M AB –26 provides an extra boost to radio recombination line flux observed at ~ 1 GHz if recombinations arise in H ii regions with T e 10 3 – 5 K, n e 10 3 – 5 cm –3 . We compute the sensitivity required for a 5 detection of H n α lines using the Square Kilometre Array (SKA), finding that the SKA-MID could detect sources with M AB –27 ( M AB –26) at z 8 ( z 3) in less than 100 h of observing time. These observations could open new paths to searches for obscured SMBH progenitors, complementing X-ray, optical/IR and sub-mm surveys.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
