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Register a new userThe impact of Lyman-$alpha$ emission line heating and cooling on the cosmic dawn 21-cm signal
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Avery Meiksin
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Allowing for enhanced Ly$alpha$ photon line emission from Population III dominated stellar systems in the first forming galaxies, we show the 21-cm cosmic dawn signal at $10<z<30$ may substantially differ from standard scenarios. Energy transfer by Ly$alpha$ photons emerging from galaxies may heat intergalactic gas if HII regions within galaxies are recombination bound, or cool the gas faster than by adiabatic expansion if reddened by winds internal to the haloes. In some cases, differential 21-cm antenna temperatures near $-500$ mK may be achieved at $15<z<25$, similar to the signature detected by the EDGES 21-cm cosmic dawn experiment.
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The 21-cm signal from the Cosmic Dawn (CD) is likely to contain large fluctuations, with the most extreme astrophysical models on the verge of being ruled out by observations from radio interferometers. It is therefore vital that we understand not only the astrophysical processes governing this signal, but also other inherent processes impacting the signal itself, and in particular line-of-sight effects. Using our suite of fully numerical radiative transfer simulations, we investigate the impact on the redshifted 21-cm from the CD from one of these processes, namely the redshift-space distortions (RSDs). When RSDs are added, the resulting boost to the power spectra makes the signal more detectable for our models at all redshifts, further strengthening hopes that a power spectra measurement of the CD will be possible. RSDs lead to anisotropy in the signal at the beginning and end of the CD, but not while X-ray heating is underway. The inclusion of RSDs, however, decreases detectability of the non-Gaussianity of fluctuations from inhomogeneous X-ray heating measured by the skewness and kurtosis. On the other hand, mock observations created from all our simulations that include telescope noise corresponding to 1000 h observation with the Square Kilometre Array telescope show that we may be able image the CD for all heating models considered and suggest RSDs dramatically boost fluctuations coming from the inhomogeneous Ly-$alpha$ background.
We present an analytic formalism to compute the fluctuating component of the ion{H}{1} signal and extend it to take into account the effects of partial Lyman-$alpha$ coupling during the era of cosmic dawn. We use excursion set formalism to calculate the size distribution of randomly distributed self-ionized regions. These ionization bubbles are surrounded by partially heated and Lyman-$alpha$ coupled regions, which create spin temperature $T_S$ fluctuations. We use the ratio of number of Lyman-$alpha$ to ionizing photon ($f_L$) and number of X-ray photons emitted per stellar baryons ($N_{rm heat}$) as modeling parameters. Using our formalism, we compute the global ion{H}{1} signal, its auto-correlation and power spectrum in the redshift range $10 le z le 30$ for the $Lambda$CDM model. We check the validity of this formalism for various limits and simplified cases. Our results agree reasonably well with existing results from N-body simulations, in spite of following a different approach and requiring orders of magnitude less computation power and time. We further apply our formalism to study the fluctuating component corresponding to the recent EDGES observation that shows an unexpectedly deep absorption trough in global ion{H}{1} signal in the redshift range $15 <z< 19$. We show that, generically, the EDGES observation predicts larger signal in this redshift range but smaller signal at higher redshifts. We also explore the possibility of negative real-space auto-correlation of spin temperature and show it can be achieved for partial Lyman-$alpha$ coupling in many cases corresponding to simplified models and complete model without density perturbations.
The 21-cm signal of neutral hydrogen is a sensitive probe of the Epoch of Reionization (EoR) and Cosmic Dawn. Currently operating radio telescopes have ushered in a data-driven era of 21-cm cosmology, providing the first constraints on the astrophysical properties of sources that drive this signal. However, extracting astrophysical information from the data is highly non-trivial and requires the rapid generation of theoretical templates over a wide range of astrophysical parameters. To this end emulators are often employed, with previous efforts focused on predicting the power spectrum. In this work we introduce 21cmGEM - the first emulator of the global 21-cm signal from Cosmic Dawn and the EoR. The smoothness of the output signal is guaranteed by design. We train neural networks to predict the cosmological signal using a database of ~30,000 simulated signals which were created by varying seven astrophysical parameters: the star formation efficiency and the minimal mass of star-forming halos; the efficiency of the first X-ray sources and their spectrum parameterized by spectral index and the low energy cutoff; the mean free path of ionizing photons and the CMB optical depth. We test the performance with a set of ~2,000 simulated signals, showing that the relative error in the prediction has an r.m.s. of 0.0159. The algorithm is efficient, with a running time per parameter set of 0.16 sec. Finally, we use the database of models to check the robustness of relations between the features of the global signal and the astrophysical parameters that we previously reported.
The global 21-cm signal from the cosmic dawn is affected by a variety of heating and cooling processes. We investigate the impact of heating due to Lyman-$alpha$ (Ly~$alpha$) photons on the global 21-cm signal at cosmic dawn using an analytical expression of the spectrum around the Ly~$alpha$ resonance based on the so-called `wing approximation. We derive a new expression for the scattering correction and for the first time give a simple close-form expression for the cooling due to injected Ly~$alpha$ photons. We perform a short parameter study by varying the Ly~$alpha$ background intensity by four orders of magnitude and establish that a strong Ly~$alpha$ background is necessary, although not sufficient, in order to reproduce the recently detected stronger-than-expected 21-cm signal by the EDGES Collaboration. We show that the magnitude of this Ly~$alpha$ heating is smaller than previously estimated in the literature by two orders of magnitude or more. As a result, even a strong Ly~$alpha$ background is consistent with the EDGES measurement. We also provide a detailed discussion on different expressions of the Ly~$alpha$ heating rate used in the literature.
Studying the cosmic dawn and the epoch of reionization through the redshifted 21 cm line are among the major science goals of the SKA1. Their significance lies in the fact that they are closely related to the very first stars in the universe. Interpreting the upcoming data would require detailed modelling of the relevant physical processes. In this article, we focus on the theoretical models of reionization that have been worked out by various groups working in India with the upcoming SKA in mind. These models include purely analytical and semi-numerical calculations as well as fully numerical radiative transfer simulations. The predictions of the 21 cm signal from these models would be useful in constraining the properties of the early galaxies using the SKA data.
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