AR 14575 (Archival Research)

Sun Jul 14 10:41:25 GMT 2024

Principal Investigator: Matthew McQuinn
PI Institution: University of Washington
Investigators (xml)

Title: A Flexible Cosmic Ultraviolet Background Model
Cycle: 24

HST studies of the IGM, of the CGM, and of reionization-era galaxies are all aided by ionizing background models, which are a critical input in modeling the ionization state of diffuse, ~10^4 K gas. The ionization state in turn enables the determination of densities and sizes of absorbing clouds and, when applied to the Ly-a forest, the global ionizing emissivity of sources. Unfortunately, studies that use these background models have no way of gauging the amount of uncertainty in the adopted model other than to recompute their results using previous background models with outdated observational inputs. As of yet there has been no systematic study of uncertainties in the background model and there unfortunately is no publicly available ultraviolet background code. A public code would enable users to update the calculation with the latest observational constraints, and it would allow users to experiment with varying the background model's assumptions regarding emissions and absorptions. We propose to develop a publicly available ionizing background code and, as an initial application, quantify the level of uncertainty in the ionizing background spectrum across cosmic time. As the background model improves, so does our understanding of (1) the sources that dominate ionizing emissions across cosmic time and (2) the properties of diffuse gas in the circumgalactic medium, the WHIM, and the Ly-a forest. HST is the primary telescope for studying both the highest redshift galaxies and low-redshift diffuse gas. The proposed program would benefit HST studies of the Universe at z~0 all the way up to z = 10, including of high-z galaxies observed in the HST Frontier Fields.