Tag Archives: theory

First NenuFAR Cosmic Dawn paper

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The first Nenufar CD KSP paper, Accurate modelling of the Lyman-α coupling for the 21-cm signal, observability with NenuFAR, and SKA (Semelin et al 2023), has been published in A&A.
The full text can be found here, and here is the abstract :

The measurement of the 21 cm signal from the Cosmic Dawn is a major goal for several existing and upcoming radio interferometers such as NenuFAR and SKA. During this era before the beginning of the Epoch of Reionisation, the signal is more difficult to observe due to brighter foregrounds, but it reveals additional information on the underlying astrophysical processes encoded in the spatial fluctuations of the spin temperature of hydrogen. To interpret future measurements, controlling the level of accuracy of the Lyman-α flux modelling is mandatory. In this work, we evaluate the impact of various approximations that exist in the main fast modelling approach compared to the results of a costly full radiative transfer simulation. The fast SPINTER code, presented in this work, computes the Lyman-α flux including the effect of wing scatterings for an inhomogeneous emissivity field, but assuming an otherwise homogeneous expanding universe. The LICORICE code computes the full radiative transfer in the Lyman-α line without any substantial approximation. We find that the difference between homogeneous and inhomogeneous gas density and temperature is very small for the computed flux. On the contrary, neglecting the effect of gas velocities produces a significant change in the computed flux. We identify the causes (mainly Doppler shifts due to velocity gradients) and quantify the magnitude of the effect in both an idealised setup and a realistic cosmological situation. We find that the amplitude of the effect, up to a factor of ∼2 on the 21 cm signal power spectrum on some scales (depending on both other model parameters and the redshift), can be easily discriminated with an SKA-like survey and can already be approached, particularly for exotic signals, by the ongoing NenuFAR Cosmic Dawn Key Science Program.

Image of the month : Cosmic Dawn III, a fully coupled radiative hydrodynamic simulation of the first billion years (May 2022)

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8 cMpc/h sub-region of the simulation, at z=7.3. White : gas density. Blue : ionizing photon density. Red : hot gas, heated by supernovae and shocks.

Cosmic Dawn III (CoDa III) is the latest iteration of the cosmological simulations of the EoR developed at the Observatoire astronomique de Strasbourg. It was run on Summit at the Oak ridge Leadership Computing Facility, the most powerful computing center in the world until June 2020. It required 131072 CPUs, 24576 GPUs, and produced roughly 20 PB of data !

Alongside gravity, hydrodynamics, star formation, and radiative transfer, the ~94 cMpc simulation includes processes such as supernovae feedback and chemical enrichment. The mass resolution in CoDa III is 8 times finer than in CoDa II (2020) which allows for a better description of galactic haloes and absorbers in the intergalactic medium. Coda III features improved models for star formation, ionizing emissivity, and dust, with respect to CoDa II.

Movies of the evolution of the simulation on different scales are available on Pierre Ocvirk’s Youtube Channel.

Ref : Lewis, J. S., Ocvirk, P., et al (2022). The short ionizing photon mean free path at z= 6 in Cosmic Dawn III, a new fully-coupled radiation-hydrodynamical simulation of the Epoch of Reionization. arXiv preprint arXiv:2202.05869.

Credits : P. Ocvirk, D. Aubert, J. Lewis, N. Gillet, J. Chardin

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