The 21-cm signature of X-ray heated halos around galaxies during cosmic dawn

Abstract: X-rays emitted by high mass X-ray binaries (HMXBs) and supernovae-driven winds in the first galaxies during Cosmic Dawn are expected to warm the intergalactic medium prior to its reionization. While most of the heating will be uniform on measurable scales, exceptionally bright sources will produce a warm ring around them with a distinctive 21-cm signature. The detection of such systems would confirm X-rays are a source of IGM heating during Cosmic Dawn and provide a test of models predicting higher X-ray luminosities per star formation rate compared with present-day galaxies. We illustrate the effect for a star-forming galaxy in a $10^{11}\, M_\odot$ halo at $z=12$, treating the photoionizing radiation and X-rays using a novel fully time-dependent 3D ray-tracing radiative transfer code. We consider a range in possible spectra for the HMXBs and star formation efficiencies, as well as the possible effect of an extended halo around the galaxy. We find detection of the signal would require integration times of a few thousand hours using SKA1-Low except for a bright galaxy like a starburst, but only a thousand hours for the expected noise levels of SKA2-Low. Depending on the surrounding gas density profile, the 21-cm signature of X-ray heating may still require an exceptionally high star formation rate, either intrinsic to the source or provided by other systems clustered near it, to avoid dominance of the signal by absorption from the surrounding gas.