An estimate of the electron density in filaments of galaxies at z~0.1

Most of the baryons in the Universe are thought to be contained within filaments of galaxies, but as yet, no single study has published the observed properties of a large sample of known filaments to determine typical physical characteristics such as temperature and electron density. This paper presents a comprehensive large-scale search conducted for X-ray emission from a population of 41 bona fide filaments of galaxies to determine their X-ray flux and electron density. The sample is generated from Pimbblet et al.'s (2004) filament catalogue, which is in turn sourced from the 2 degree Field Galaxy Redshift Survey (2dFGRS). Since the filaments are expected to be very faint and of very low density, we used stacked ROSAT All-Sky Survey data. We detect a net surface brightness from our sample of filaments of (1.6 +/- 0.1) x 10^{-14} erg cm^{-2} s^{-1} arcmin^{-2} in the 0.9-1.3 keV energy band for 1 keV plasma, which implies an electron density of n_{e} = (4.7 +/- 0.2) x 10^{-4} h_{100}^{1/2} cm^{-3}. Finally, we examine if a filament's membership to a supercluster leads to an enhanced electron density as reported by Kull & Bohringer (1999). We suggest it remains unclear if supercluster membership causes such an enhancement.