0 Assigned ID: 5583 ----- Attempting Submission 1 (Tue Mar 26 19:34:51 GMT 2024) ----- HST Phase I Proposal 5583 successfully submitted. Receipt: # 5583-1 ----- Attempting Submission 2 (Tue Mar 26 20:08:58 GMT 2024) ----- HST Phase I Proposal 5583 successfully submitted. Receipt: # 5583-2 ----- Attempting Submission 3 (Tue Mar 26 20:18:43 GMT 2024) ----- HST Phase I Proposal 5583 successfully submitted. Receipt: # 5583-3 ----- Attempting Submission 4 (Tue Mar 26 21:20:10 GMT 2024) ----- One of the remarkable achievements of the current cosmology lies in the percent-level concordance between theory and observations of the intergalactic medium (IGM) at z > 1.7. Yet, the IGM at lower redshift (z < 1.7), which can only be studied via HST UV spectra, has unveiled puzzling discrepancies. Notably, the Doppler b-parameters of Lyman-alpha absorption lines are on average ~10 km/s wider than any existing simulation of the IGM. Furthermore, recent analyses at z~1 reveal an unexpected absence of IGM cooling. This contradicts the fundamental prediction that post-He II reionization (z < 2.5), the IGM should cool down owing to the Hubble expansion. Addressing these discrepancies, the wider b-parameters and the IGM temperature at z~1, require considering either non-standard heating mechanisms, exotic dark matter, or missing turbulence. To tackle this challenge, we aim to assess the thermal and ionization states of the IGM using 144 archival HST COS and STIS quasar spectra that probe the Lyman-alpha forest at 0 < z < 1.7. Our strategy includes a detailed measurement of the 2D b and H I column density distribution and the absorber density dN/dz, across ten redshift intervals through an automated Voigt profile analysis of all spectra. By comparing these observations with a large suite of hydrodynamical simulations via advanced machine learning-based inference, we intend to accurately measure the IGM's thermal and ionization history over the last 10 billion years. Our proposal will conclusively determine whether the low-z IGM thermal state conforms to theoretical expectations or if there is a need to explore the potential emergence of new physics at these cosmic times. Our project group brings together a diverse range of researchers, each with considerable expertise in specific fields. These include (1) high-definition ultraviolet and optical spectroscopy applied to QSO absorption systems, and (2) theoretical studies of the Ly-alpha forest, the intergalactic medium (IGM), and cosmological hydrodynamical simulations. Furthermore, our team has access to the resources of the National Energy Research Scientific Computing Center (NERSC) as well as other supercomputing resources. Principal Investigator Teng Hu is an experienced reshearcher in the field of low-z IGM, Lyman-alpha forest, statistical inference and machine learning. Hu has experience in analysing large datasets of Lyman-alpha forest from both observations and various hydrodynamical simulations, including investigations on the physics properties of the related Lyman-alpha absorbers (Hu et. al 2023a). He (Hu et. al. 2022) adopted a new inference method to measure the thermal and ionization stste of the low-z IGM based on the Lyman-alpha forest deposition with the hdelp of machine learning technique. Observational spectra are used in the study to propogate the noise and instrumental effects to the simulated spectra. Such method makes it feasible to conduct robust measurements of the IGM thermal and ionization state with very high precision under realistic condition. It is one of the key compnoent of this proposal. In Hu et. al. (2023b) the aforementioned method is applied to 12 HST STIS spectra, which indicates that there are no evidence for the predicted cooling down of the IGM at low-z, suggesting a need for further investigation. Co-principal investigator Khaire, co-investigators prof. Hennawi, and prof. Tripp are very experienced QSO absorption-line researchers with established track records and important publications on a variety of topics in this general area. Prof. Tripp is also an expert in dealing with HST COS/STIS QSO spectra. He will be leading the efforts of continuum fitting of QSO spectra and metal line identification. Prof. Hennawi (along with his former Ph.D. student Dr. Hector Hiss) has developed automated Voigt profile fitting codes (Hiss et al. 2019) and measured z > 2 thermal states of the IGM (also used in Walther et al.2019). We adopt such automated Voigt profile fitting code, and modified it to work on HST COS STIS spectra with different LSFs. The Code has been used in many studies (Hu et al 2022,2023a,b, Khaire et. al. 2019, 2023). PI Teng Hu, co-PI Dr. Khaire and Prof. Hennawi have all the tools to analyze different hydrodynamical simulations, create forward model spectra and perform sophisticated parameter estimations as demonstrated by many published works. Co-investigator Dr. Zarija Lukic and Dr. Onorbe are experts in the hydrodynamical simulations of the intergalactic medium. Dr. Lukic pioneered the massively parallel grid simulation Nyx which is extremely fast and efficient. Nyx simulations have been used in many IGM studies, including PI Teng Hu's new approach for measuring low-redshift IGM thermal and ionization state and CO-PI Khaire's power-spectrum analysis. Dr. Lukic has recently implemented AGN feedback models into the Nyx code. We will be using Nyx to create a suite of simulations by tuning the heating parameters as well as the strength of AGN feedback. Prof. Lukic has direct access to the NERSC supercomputer and ample resources to perform these simulations. Intergalactic Medium and the Circumgalactic Medium Large Scale Structure of the Universe 0 false false false false false false false SUPPORTED