Simulations of the formation of large-scale structure predict that dark matter , low density highly ionized gas , and galaxies form 10–40 Mpc scale filaments .
These structures are easily recognized in the distribution of galaxies .
Here we use Ly \alpha absorption lines to study the gas in 30x6 Mpc filament at cz \sim 3500 km s ^ { -1 } , defined using a new catalogue of nearby ( cz < 10,000 km s ^ { -1 } ) galaxies , which is complete down to a luminosity of about 0.05 L _ { * } for the region of space analyzed here .
With HST spectra of 24 AGN we sample the gas in this filament .
All of our sightlines pass outside the virial radius of any known filament galaxy .
Within 500 kpc of the filament axis the detection rate is \sim 80 % , but no detections are seen more than 2.1 Mpc from the filament axis .
The width of the Ly \alpha lines correlates with filament impact parameter and the four BLAs in our sample all occur within 400 kpc of the filament axis , indicating increased temperature and/or turbulence .
Comparing to simulations , we find that the recent Haardt & Madau ( 2012 ) extragalactic ionizing background predicts a factor 3–5 too few ionizing photons .
Using a more intense radiation field matches the hydrogen density profile within 2.1 Mpc of the filament axis , but the simulations still overpredict the detection rate between 2.1 and 5 Mpc from the axis .
The baryonic mass inside filament galaxies is 1.4 \times 10 ^ { 13 } M _ { \odot } , while the mass of filament gas outside galaxy halos is found to be 5.2 \times 10 ^ { 13 } M _ { \odot } .