Thesis:

Characterizing the intergalactic medium in the cosmic web

Currently, at redshift z ≤ 1, the ΛCDM paradigm predicts that a significant fraction of baryons is located in the warm-hot intergalactic medium (WHIM), which is most likely found in cosmic web filaments. The WHIM is a low-density (nH ∼ 10−6–10−4 cm−3) plasma with temperatures of (log(T/K) ∼ 5–7) and mild overdensities of ∼ 10–30 times the mean density of the Universe. Characterizing the physical state of this dominant baryonic matter and its distribution is crucial, because it allows us to complete the physical picture of baryon evolution in the low-redshift Universe. In the current ΛCDM cosmological framework, matter in the Universe is organized into a complex network known as the cosmic web, composed of galaxy clusters, filaments, sheets, and large underdense voids. Galaxy clusters occupy the densest nodes of this network, while filaments form the interconnected structures along which matter flows under gravity. Owing to its low density and high ionization state, this gas is most effectively probed through far-ultraviolet (FUV) absorption-line spectroscopy of background quasars, where residual neutral hydrogen and highly ionized metals produce detectable absorption features. Far-ultraviolet absorption-line spectroscopy with the Cosmic Origin Spectrograph instrument in the Hubble Space Telescope (HST/COS) is particularly effective for tracing the warm–hot intergalactic medium (WHIM) at low redshift (z < 0.5), where a significant fraction of the cosmic baryons is predicted to reside. In particular, absorption lines of HI Broad Lyα absorbers (BLAs; Doppler parameter b ≥ 40 km s−1) and OVI have been used as WHIM tracers. This work focuses on probing the signatures of the WHIM by analyzing the incidence of its tracers, where cosmic web filaments connecting massive galaxy clusters may exist. We conducted a study of FUV absorption spectroscopy acquired with the HST/COS along QSO sightlines. For this experiment, a sample of ten QSO sightlines at z ≤ 1 was selected based on their proximity to cluster-pairs, targeting potential cosmic web filaments that may connect massive galaxy clusters. Nine QSO sightlines were acquired from the observational proposal ID15293 (PI Tejos), designed for our study. The tenth sightline comes(...).