Speaker
Description
The Epoch of Reionization (EoR) is one of the least understood periods in cosmic history. Observations from different probes, including quasar absorption spectra, the cosmic microwave background (CMB) polarization, Lyman-$\alpha$ emission from high redshift galaxies, suggest that the hydrogen neutral fraction in the intergalactic medium (IGM) was likely less than $5\%$ at $z < 6$ and more than $70\%$ at $z \sim 10$, with a mid-point of reionization occurring near $z \sim 7.5$.
A crucial probe of cosmic reionization is the redshifted 21 cm hydrogen hyperfine transition, as it directly probes the ionization state of the IGM, unlike most other observables. Most of the current interferometric observations aim to observe the signal as a contrast against CMB. For instance, the HERA Collaboration placed strict upper limits on the spatial and redshift evolution of the 21 cm power spectrum in the range $0.23-1.9\;h\,\text{Mpc}^{-1}$ at $z=7.9$ and $z=10.4$. Over the last few years, probing with the absorption against a bright radio source has attracted considerable attention due to the increasing number of discovered quasars at $z > 6$, a fraction of which are also radio-loud.
In this talk, I will present the first observation with the Giant Metrewave Radio Telescope (GMRT) of J352.4034-15.3373, which is a radio-loud quasar near the end of EoR, $z=5.84$, covering the $200-225$ MHz frequency range. We achieved a $6$ mJy rms noise per 390 kHz channel and no absorption detection, yielding a $95\%$ upper limit on the 21 cm optical depth $\tau_{21} < 10^{-2}$. Assuming that the IGM was heated up to $500$ K, this upper limit translates into $95\%$ upper limits on the IGM neutral fraction $x_{\rm HI} < 0.3$ and $0.45$ at $z=5.25$ and $5.84$, respectively. I will conclude my talk by outlining the role of SKAO in constraining reionization and the thermal properties of the IGM via observations of 21 cm absorption against high-redshift quasars.
| Topics | Epoch of Reionization and Cosmic Dawn |
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