What is my research about?
Epoch of Reionization and Cosmic Dawn, 21-cm cosmology, Ly\(\alpha\) forest,
cosmological simulations, intergalactic medium, supermassive black holes
21-cm forest in the late reionization models
The 21-cm forest signal is a series of absorption features in the spectra of distant radio-loud sources, such as quasars, arising from neutral hydrogen structures between the observer and the source (the bottom panel in the animation above). As the light emitted from such sources travels through the universe, it redshifts (its wavelength becomes longer). Some photons redshift to the wavelength of \(\sim21.11\,\rm cm\) (blue vertical stripe) and might encouter cold and neutral hydrogen gas which absorbs some of these 21-cm photons. This leaves an absorption feature in the spectrum of the background source. The rest of the light travels further and different set of photons redshifts to the 21-cm wavelength and gets absorbed by the neutral hydrogen gas. The amount of the photons that gets absorbed depends on various things, including the density (the top panel) of the Intergalactic medium (IGM) and its ionization (the second panel) and thermal (the third panel) state.
The reionization, is the last phase transition of our universe, when the light emitted by the first stars and galaxies turned the universe from neutral to ionized. The whole processes was initiated \(\sim13.5\) billion years ago and lasted for \(\sim1\) billion years. It can be described with a creation of ionization bubbles around the galaxies which then expanded and overlapped until they filled in the whole universe. Given that the 21-cm forest is sensitive to how neutral and how warm the IGM is, the 21-cm forest can be used to explore how the reionization has progressed. Towards the end of reionization the 21-cm signal is expected to be diminished because the neutral hydrogen, which is needed for this signal, is depleted. Fortunately, recent observations seem to suggest that the reionization was completed significantly later. This will make the observations of the 21-cm forest signal easier in the future. In my research I model the 21-cm forest signal in this late-reionization scenario using cosmological simulations like the Sherwood-relics simulation suite or 21cmFAST to see how plausible the detection of this signal is with both already operational and future telescopes and explore the potential of such observations to constrain the IGM properties during the reionization.
21-cm forest in close proximity to quasar
I also study how the radiation from the host quasar affects its surroundings and the 21-cm forest close to it (blue curve in the bottom panel of the above animation). Quasars are extremely bright nuclei of galaxies whose engines are accreting supermassive black holes. Their effect has been researched extensively for an analogous signal, Ly\(\alpha\) forest (green curve in the bottom panel) which is also a series of absorption features in the spectra of quasar arising from neutral hydrogen, but due to a different transition in the neutral hydrogen atom. Hence, instead of being observed in radio, it appears in the UV, optical and infrared spectrum. In the top panel, which shows the neutral hydrogen fraction of the IGM, one can see how UV photons sourced by the quasar (which is located on the left) ionize the IGM close to the quasar. The ionization front propagates throught the IGM and this this is mimicked by the Ly\(\alpha\) forest. In the middle panel the temperature of the gas is shown. The temperature follows the ionization front too, but there is also a tail of higher temperature beyond the ionisation front. This is driven by the X-ray photons. This tail of higher temperature suppresses the 21-cm forest signal. Given these differences between the 21-cm and Ly\(\alpha\) forest, particularly if their extent is dictated by X-rays or UV photons, they can be complimentary in studies of the accreting supermassive black holes.