0 Assigned ID: 420 ----- Attempting Submission 1 (Fri Apr 05 12:58:05 GMT 2019) ----- HST Phase I Proposal 420 (whim27.apt) successfully submitted. Receipt: # 420-1 ----- Attempting Submission 2 (Fri Apr 05 15:37:47 GMT 2019) ----- HST Phase I Proposal 420 (whim27.apt) successfully submitted. Receipt: # 420-2 ----- Attempting Submission 3 (Fri Apr 05 15:44:00 GMT 2019) ----- Resubmission for reobservations (HOPR 91807 for HST program 15835) 30652115 ----- Attempting Submission 1 (Mon Jul 22 16:37:52 GMT 2019) ----- ----- Attempting Submission 2 (Thu Jul 30 14:37:32 GMT 2020) ----- Near-Ultraviolet Follow-up of the X-ray-detected Warm-Hot Intergalactic Medium Toward 1ES 1553+113 One of the strengths of modern cosmology is that the cosmic baryon density predicted by Big Bang nucleosynthesis precisely matches the measured value from the cosmic microwave background. At high redshift the vast majority of these baryons are found in the photoionized intergalactic medium. However, cosmological simulations of structure formation predict that a substantial portion of these baryons will be shock heated into the elusive warm-hot intergalactic medium (WHIM) as the universe evolves. Confirmation of these predictions through observations of the WHIM and measurements of its physical state provide a sensitive test of our understanding of the growth of structure in the universe. Recently, Nicastro et al. (2018) reported the first detections of two intervening OVII absorption systems in the X-ray spectrum of the blazar 1ES 1553+113. These systems are thought to be arising from the WHIM with a temperature of ~10^6 K. Nevertheless, the redshift of the blazar is not known with adequate precision, which makes the origin(s) of the OVII bearing gas and its contribution to the cosmic baryon density highly uncertain. Here we propose to obtain NUV spectra of the blazar using the COS G185M grating. The proposed spectra will enable us to detect intervening Lyman alpha absorbers with log N(HI)/cm-2 > 13.0 from z = 0.45 - 0.7. The stringent lower (upper) limit on the blazar's redshift to be obtained from the presence (absence) of any such absorber will be crucial to validate or rule out the claim of potentially solving the so called "missing baryons problem". In order to place stringent constraints on the blazar (1ES1553+113) redshift and determine whether the O VII absorber at z = 0.4339 is in the blazar's host environment, we will obtain NUV COS spectra to search for intervening IGM HI Lya lines at z > 0.41. Contiguous spectral coverage in the NUV from 1767 to 2068 Angstrom with a S/N per resolution element of 12 will enable us to detect intervening IGM Lya absorption systems with log N(HI) > 13 from z = 0.45 to 0.7. Based on the expected number of such HI systems per unit redshift from Danforth et al. (2016), we expect to detect 10 Lya lines per dz = 0.1 interval. Non-detection of Lya at z > 0.45 in combination with our deep and highly complete galaxy redshift survey in the field would indicate that the blazar is in the same environment as the WHIM candidate at z=0.4339. Based on the COS ETC, obtaining the desired spectral coverage and S/N will require observations with COS G185M gratings with central wavelength settings of 1882, 1913, 1941, and 1953, each observed for one orbit for a total of four orbits of COS NUV spectroscopy. We will split each central wavelength setting into four FP-POS positions as recommended to minimize flat fielding errors. Moreover, as per the instructions after the gyro failure, we will complete the total allocated 4 orbits in two visits. Since the co-ordinates of the target are obtain from GAIA with an accuracy better than 0.02", we will directly use PEAKXD followed by PEAKD for acquisition. For the flux observed in the existing COS spectrum (F(1300) = 1.5e-14), ACQ/IMAGE is a viable option. However, it exceeds the bright limit if the flux is increased by a factor of 2.5 (1 mag) owing to unexpected flare. Therefore, we adopt the spectroscopic acquisition for instruments' safety. We used the existing archival COS spectrum of the blazar for calculating the exposure time for spectroscopic acquisition (COS.sa.1366483). Impact of reduced-gyro operations: There are no significant impact of the reduced-gyro mode (RGM) operations except for an increased target acquisition time of 2 minutes. Prof. Jane Charlton is an expert in quasar absorption line (QAL) spectroscopy and ionization modeling of quasar absorbers. She has been the PI (Co-I) on 9 (over 20) HST programs. She will contribute to the analysis of the absorption line data. Prof. Hsiao-Wen Chen is an expert in the field of circumgalactic medium studies and has led many previous HST observing programs (5 PI and over 20 Co-I) with over 20 papers published based on HST observations. She has extensive experience in processing and analyzing absorption spectra obtained using COS. Dr. Sean Johnson is an expert in QAL spectroscopy and galaxy redshift surveys using multi-object spectroscopy (MOS). He has been the PI/Co-I of several HST programs. He will contribute to the analysis of the data to be obtained from COS and already obtained from MOS observations of the blazar field using Magellan. Prof. John Mulchaey is an expert in galaxy evolution. His research focuses on X-ray observations of groups of galaxies. He will contribute to the data interpretation and to follow-up galaxy spectroscopy using Magellan. Dr. Sowgat Muzahid is an expert in QAL spectroscopy. He has conducted several absorption line surveys, mostly using archival COS spectra, which led to over 10 publications. He will contribute to the COS data reduction, analysis, and data interpretation. Prof. Anand Narayanan is an expert in UV spectroscopy using COS. He has worked closely with the COS GTO team. He will contribute to the absorption line analysis. Prof. Joop Schaye is an expert in galaxy formation, hydrodynamical simulations, and QAL spectroscopy. He is the PI of the EAGLE cosmological simulations. He will contribute to the interpretation of observed data. Ms. Nastasha Wijers is a graduate student working on the properties of UV and X-ray absorbers in cosmological simulations of galaxy formation (EAGLE, c-EAGLE). She will produce mock COS observations using the EAGLE simulations. In summary, we have a team with all the required skills to bring the program to a successful completion. 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