Publication Date:
2015-11-27
Description:
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 quasi-stellar object (QSO) sightline that passes within d = 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 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 ~2 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.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
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