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

Abstract

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 ∼ 10 5 − 10 7 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 QSO sightline that passes within ∆d = 3 Mpc from the 7 inter-cluster axes connecting 7 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 ∆v 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 inter-cluster filaments. By extending the present analysis to tens of sightlines our experiment offers a promising route to detect the WHIM.

Detection of low-metallicity warm plasma in a galaxy overdensity environment at z ∼ 0.2

“…The results of the sampling, shown in Figure 9, indicates a less than 1% probability of finding 6 or more luminous galaxies in a similar region of space by random coincidence. The absorber thus seems to be residing in a galaxy overdensity region, which is consistent with its physical properties, as warm H i gas is known to be strongly correlated with the spatial distribution of galaxies and largescale filaments (Stocke et al 2014;Nevalainen et al 2015;Wakker et al 2015;Pachat et al 2016;Tejos et al 2016).…”

Evolution of Mass and Velocity Field in the Cosmic Web: Comparison Between Baryonic and Dark Matter

“…Cosmological hydrodynamic simulations predicted that the WHIM resides in moderate overdense filaments and possibly walls, and host ∼ 50% of the total baryons, which is still being searched by intensively observations(e.g. Cen & Ostriker 1999;Dave et al 2001;Bregman 2007;Fang et al 2010;Shull et al 2012;Tejos et al 2016).…”

COS Observations of the Cosmic Web: A Search for the Cooler Components of a Hot, X-Ray Identified Filament

“…The chosen integration window is twice the spectral FWHM of our COS data. This equivalent width limit corresponds to a column density upper limit of log( N H I /cm −2 ) < 13.7 for a single-component Voigt profile with Doppler width of b = 20 km s −1 , which is typical of H I absorbers in cluster environment (e.g., Tejos et al 2016;Burchett et al 2018;Pradeep et al 2019). Likewise, we do not detect any significant absorption from low-, intermediate-, or high-ionization metals within the search window.…”

“…chance superpositions of bright background sources. Enabled by the tremendous power of the Hubble Space Telescope (HST), diffuse gas has been observed at scales of the circumgalactic medium (CGM) around individual galaxies (see review by Tumlinson et al 2017) out to intercluster filaments (Tejos et al 2016). In particular, cool gas (T ∼ 10 4 − 10 5 K) is best traced through narrow neutral hydrogen (H I) absorption (e.g., Zahedy et al 2019).…”