0 Assigned ID: 5495 ----- Attempting Submission 1 (Tue Mar 26 17:09:22 GMT 2024) ----- HST Phase I Proposal 5495 successfully submitted. Receipt: # 5495-1 ----- Attempting Submission 2 (Tue Mar 26 22:41:53 GMT 2024) ----- HST Phase I Proposal 5495 successfully submitted. Receipt: # 5495-2 ----- Attempting Submission 3 (Tue Mar 26 22:48:48 GMT 2024) ----- Understanding the properties of gas in the intergalactic and circumgalactic medium is essential for gaining insights into galaxy formation and evolution. However, accurately determining the characteristics of this gas is challenging due to the uncertain shape of the extragalactic ultraviolet background (UVB), which plays a vital role in the ionization and thermal state of the gas. In this proposal, we present a novel method to constrain the spectral shape of the UVB using absorption line systems with multiple metal lines. We will analyze a set of seven metal absorption systems featuring 6-10 species of metal ions and spanning a redshift range from z=0 to 0.62, as identified in the archival spectra observed with the Cosmic Origin Spectrograph on board the Hubble Space Telescope. To constrain the shape of the UVB, we will apply Voigt profile fitting techniques and utilize a comprehensive suite of Cloudy models. Our approach will involve considering a minimum of 25 sets of UVB models, comprising 14 new models specifically generated for this analysis. Through Bayesian inference analysis, we aim to determine the spectral shape of the UVB and its redshift evolution up to z=0.62. The results of this study will not only improve the accuracy of UVB synthesis models but also provide valuable insights into the quasar spectral energy distribution in extreme UV wavelength ranges, currently beyond the reach of existing instruments. The proposal team is composed of a combination of researchers with extensive experience in (1) synthesizing the extragalactic UV background, (2) high-resolution ultraviolet and optical spectroscopy of QSO absorption systems, and (3) theoretical studies of the Lyman-alpha forest, the intergalactic and circumgalactic medium (IGM and CGM). PI Dr. Khaire is an expert in synthesizing extragalactic UV background as well as theoretical studies of IGM and CGM, analyzing HST/COS data, and cosmological hydrodynamical simulations. In his recent paper (Khaire & Srianand 2019) he published a new model of UV background which has been used by more than 110 individual studies so far. In the past, he has worked on the effect of uncertain UV background on the properties of IGM CGM (see Acharya & Khaire 2022, Hussain et al. 2017). He has experience in analyzing metal absorption lines and cloudy modeling are suitable for the proposed work. He has a total of 28 refereed publication including 9 as a lead author. Co-PI Ms. Surya Ramchandran is a graduate student working under the guidance of PI Dr. Khaire. She is an expert in Voigt profile fitting and metal identifications. She has been trained for a year to perform these tasks. The Voight profile fitting presented in the proposal was performed by her. Co-I Dr. Abhisek Mohapatra is an expert in analyzing metal absorptions from moderate to high-resolution UV and optical quasar spectra. He has extensive technical skills in Voigt profile fitting routines (VPFIT, VPGUESS, MCMC code rbvfit), and CLOUDY photoionization modeling. His Ph.D. thesis was based on analyzing the optically thin intervening absorbers using physically viable photoionization models to obtain various physical parameters such as density, overdensity, metallicity, and line of sight thickness (Mohapatra et al. 2019, 2021). Co-I Mr. Anshuman Acharya worked with Dr. Khaire and wrote a paper on estimating systematic uncertainty on the density and metallicity of CGM because of uncertain UV background models (Acharya and Khaire 2022). The proposed method is based on the follow-up of the same paper - Acharya and Khaire 2022. Co-I Prof. Hennawi is a very experienced QSO absorption-line researcher with established track records and important publications on a variety of specific topics in this general area. Co-I Mr. Teng Hu is a graduate student working with PI Khaire and Co-I Henaawi. He is also working on analyzing Lyman alpha forest using HST COS to study the ionization and thermal state of the IGM. He is an expert in hydro simulations of the IGM, Lyman alpha forest data analysis, and automated Voigt profile fitting. Intergalactic Medium and the Circumgalactic Medium 0 false false false false false false false SUPPORTED