Semi-analytic forecasts for Roman – the beginning of a new era of deep-wide galaxy surveys

The Nancy Grace Roman Space Telescope, NASA’s next flagship observatory, will redefine deep-field galaxy survey with a field of view two orders of magnitude larger than Hubble and an angular resolution of matching quality. These future deep-wide galaxy surveys necessitate new simulations to forecast their scientific output and to optimise survey strategies. In this work, we present five realizations of 2-deg2 lightcones, containing a total of & 25 million simulated galaxies with −16 & MUV & −25 spanning z ∼ 0 to 10. This dataset enables a new set of experiments with the impacts of survey size on the derived galaxy formation and cosmological constraints. The intrinsic and observable galaxy properties are predicted using a well-established, physics-based semi-analytic modelling approach. We provide forecasts for number density, cosmic SFR, field-to-field variance, and angular two-point correlation functions, and demonstrate how the future wide-field surveys will be able to improve these measurements relative to current generation surveys. We also present a comparison between these lightcones and others that have been constructed with empirical models

The Late Reionization of Filaments

We study the topology of reionization using accurate three-dimensional radiative transfer calculations post-processed on outputs from cosmological hydrodynamic simulations. In our simulations, reionization begins in overdense regions and then "leaks" directly into voids, with filaments reionizing last owing to their combination of high recombination rate and low emissivity. This result depends on the uniquely-biased emissivity field predicted by our prescriptions for star formation and feedback, which have previously been shown to account for a wide array of measurements of the post-reionization Universe. It is qualitatively robust to our choice of simulation volume, ionizing escape fraction, and spatial resolution (in fact it grows stronger at higher spatial resolution) even though the exact overlap redshift is sensitive to each of these. However, it weakens slightly as the escape fraction is increased owing to the reduced density contrast at higher redshift. We also explore whether our results are sensitive to commonly-employed approximations such as using optically-thin Eddington tensors or substantially altering the speed of light. Such approximations do not qualitatively change the topology of reionization. However, they can systematically shift the overlap redshift by up to $\Delta z\sim 0.5$, indicating that accurate radiative transfer is essential for computing reionization. Our model cannot simultaneously reproduce the observed optical depth to Thomson scattering and ionization rate per hydrogen atom at $z=6$, which could owe to numerical effects and/or missing early sources of ionization.Comment: 16 pages, 9 figures, accepted to MNRA

Imprints of temperature fluctuations on the z 3c 5 Lyman-\u3b1 forest: a view from radiation-hydrodynamic simulations of reionization

Reionization leads to large spatial fluctuations in the intergalactic temperature that can persist well after its completion. We study the imprints of such fluctuations on the z similar to 5 Ly alpha forest flux power spectrum using a set of radiation-hydrodynamic simulations that model different reionization scenarios. We find that large-scale coherent temperature fluctuations bring similar to 20-60 per cent extra power at k similar to 0.002 s km(-1), with the largest enhancements in the models where reionization is extended or ends the latest. On smaller scales (k greater than or similar to 0.1 s km(-1)), we find that temperature fluctuations suppress power by less than or similar to 10 per cent. We find that the shape of the power spectrum is mostly sensitive to the reionization mid-point rather than temperature fluctuations from reionization's patchiness. However, for all of our models with reionization mid-points of z <= 8 (z <= 12), the shape differences are <= 20 per cent (<= 40 per cent) because of a surprisingly well-matched cancellation between thermal broadening and pressure smoothing that occurs for realistic thermal histories. We also consider fluctuations in the ultraviolet background, finding their impact on the power spectrum to be much smaller than temperature fluctuations at k >= 0.01 s km(-1). Furthermore, we compare our models to power spectrum measurements, finding that none of our models with reionization mid-points of z < 8 is strongly preferred over another and that all of our models with mid-points of z >= 8 are excluded at 2.5 sigma. Future measurements may be able to distinguish between viable reionization models if they can be performed at lower k or, alternatively, if the error bars on the high-k power can be reduced by a factor of 1.5