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 HI, narrow H_I (NLA; Doppler parameters b < 50 km s^-1), broad H_I (BLA; b ≥ 50 km s^-1) and OVI 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 OVI 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 OVI 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.