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Reionization of Hydrogen in the intergalactic medium (IGM) was the landmark event of Cosmic Dawn. It was the first time that stars had a global impact on the universe, and the last time that most of the ordinary matter in the universe participated in a phase transition. Reionization is central to NASA’s goal to understand “when the first stars in the universe formed, and how they influenced the environments around them,” and is prioritized in both the 2010 and 2020 decadal surveys.
Timeline of our Universe spanning the Big Bang, Reionization, and present day
Image Credit: Robertson, B., Ellis, R., Dunlop, J. et al. Early star-forming galaxies and the reionization of the Universe. Nature 468, 49–55 (2010). https://doi.org/10.1038/nature09527
Purpose
Purpose
We need measures that probe local IGM, scalable to cover large areas, to measure the pace, inhomogeniety, and topology of reionization. Both Lyα galaxies and redshifted 21cm measurement are capable of this. Experiments to measure redshifted 21cm face the main challenge of Galactic foreground emission that exceeds the reionization signal by ∼ 4 orders of magnitude. Concurrence with Lyα surveys will yield robust results (Hutter et al. 2019). For Lyα the challenge lies in developing surveys at z > 7, which are both sensitive and cover multiple square degrees. This is only possible with Roman.
Lyman-α lines from high redshift galaxies offer uniquely powerful probes of reionization history. Lyα light is scattered in the presence of neutral hydrogen, and therefore provides a test of the IGM neutral fraction probing the central phase of reionization, when the IGM is 20% – 80% neutral. Moreover, this test is local and scalable. Deep slitless spectroscopy with Roman will be able to spectroscopically confirm galaxies with or without Lyα emission, and test the inhomogeneity and topology of reionization.
Mission Statement
Mission Statement
We will develop the required tools to simulate Roman slitless data. We will produce high fidelity simulations that include (a) the defocused higher diffractive orders in grism data, whose wavelength-dependent pattern cannot be simulated by existing packages; (b)the highly nonlinear prism dispersion; (c) position-dependent trace, dispersion solution, and passband edges, and (d) wavelength and position dependent point spread functions.
The results will ultimately benefit all applications of Roman slitless spectroscopy, reaching a wide user community. We will provide tools to inject simulated sources with user-provided spectra and spatial profiles into realistic background scenes, in order to test detection efficiency and measurement fidelity. This may be applicable to diverse general astrophysics survey programs, and also to galaxy redshift survey cosmology results from the high latitude wide area survey. We expect to work with the science centers to share results and algorithms, and also to offer data challenges and training for the general community. By developing these tools now, we will help the community to be ready for scientific applications of Roman spectroscopy, from galaxies to kilonovae, on day one of science operations.
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