Wide-Field Science – Large
James Rhoads / NASA – Goddard Space Flight Center, PI
Reionization of intergalactic hydrogen was the first time that stars and galaxies had a global impact on the universe around them, and the last phase transition for ordinary matter in the universe. The Nancy Grace Roman Space Telescope will enable critically needed wide field surveys for galaxies in the epoch of reionization (EoR). In particular, Roman’s wide field spectroscopic capabilities will allow direct observation of the Lyman alpha line from EoR sources, providing direct, local tests for neutral intergalactic gas, over scales that are large enough to study this inherently inhomogeneous process and that are unachievable with other facilities.
This proposal builds upon the work of the previous Cosmic Dawn Science Investigation Team to prepare for WFI investigations of Cosmic Dawn through deep imaging and spectroscopy. For this we will:
(1) Simulate cosmic dawn galaxies, by combining large semi-analytic simulations with high-resolution hydrodynamic simulations and radiative transfer of Lyman-alpha at z > 7. These simulations will be informed and constrained by deep observations from JWST.
(2) Develop high fidelity scene simulations, custom built for Roman Grism and Prism, starting with observed deep and wide field imaging from JWST.
(3) Optimize selection methods for high redshift line emitters, quasars, and Lyman Break Galaxies, incorporating spectroscopic information that will increase the robustness and efficiency compared to photometric searches.
(4) Explore quantitative metrics to measure reionization using robust measures such as clustering of Lyman alpha sources, topology of Lyman alpha emitter distribution, and cross-correlation with 21cm measures.
Applying wide field slitless spectroscopy to studies of cosmic dawn requires optimized extraction of spectral lines 10 times fainter than those sought in the High Latitude Wide Area Survey. Such observations will have more significant crowding than shallower surveys. This requires new strategies for both observations and source detection algorithms, and data simulations with unprecedented detail that go beyond the tools and methods currently planned.
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.
Using these tools, we will examine how depth, area, number of distinct roll angles, and other observational parameters affect the ability to recover input structures. We will also develop figures of merit that will help quantitatively evaluate future tradeoffs such as depth vs area. We will define strategies to study the ionizing photon budget as a function of redshift, and to study the dependence of epoch-of-reionization galaxy properties on their local environments.
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.