Towards the statistical detection of the warm–hot intergalactic medium in intercluster filaments of the cosmic web

Modern analyses of structure formation predict a universe tangled in a ‘cosmic web’ of dark matter and diffuse baryons. These theories further predict that at low z, a significant fraction of the baryons will be shock-heated to T ∼ 105–107 K yielding a warm–hot intergalactic medium (WHIM), but whose actual existence has eluded a firm observational confirmation. We present a novel experiment to detect the WHIM, by targeting the putative filaments connecting galaxy clusters. We use HST/COS to observe a remarkable quasi-stellar object (QSO) sightline that passes within Δd = 3 Mpc from the seven intercluster axes connecting seven independent cluster pairs at redshifts 0.1 ≤ z ≤ 0.5. We find tentative excesses of total H i, narrow H i (NLA; Doppler parameters b < 50 km s−1), broad H i (BLA; b ≥ 50 km s−1) and O vi absorption lines within rest-frame velocities of Δv ≲ 1000 km s−1 from the cluster-pairs redshifts, corresponding to ∼2, ∼1.7, ∼6 and ∼4 times their field expectations, respectively. Although the excess of O vi likely comes from gas close to individual galaxies, we conclude that most of the excesses of NLAs and BLAs are truly intergalactic. We find the covering fractions, f c, of BLAs close to cluster pairs are ∼4–7 times higher than the random expectation (at the ∼2σ c.l.), whereas the f c of NLAs and O vi are not significantly enhanced. We argue that a larger relative excess of BLAs compared to those of NLAs close to cluster pairs may be a signature of the WHIM in intercluster filaments. By extending this analysis to tens of sightlines, our experiment offers a promising route to detect the WHIM.