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".