We search the Planck data for a thermal Sunyaev-Zeldovich (tSZ) signal due to gas filaments between pairs of Luminous Red Galaxies (LRGs) taken from the Sloan Digital Sky Survey Data Release 12 (SDSS/DR12). We identify $sim$260,000 LRG pairs in the DR12 catalog that lie within 6-10 $h^{-1} mathrm{Mpc}$ of each other in tangential direction and within 6 $h^{-1} mathrm{Mpc}$ in radial direction. We stack pairs by rotating and scaling the angular positions of each LRG so they lie on a common reference frame, then we subtract a circularly symmetric halo from each member of the pair to search for a residual signal between the pair members. We find a statistically significant (5.3$sigma$) signal between LRG pairs in the stacked data with a magnitude $Delta y = (1.31 pm 0.25) times 10^{-8}$. The uncertainty is estimated from two Monte Carlo null tests which also establish the reliability of our analysis. Assuming a simple, isothermal, cylindrical filament model of electron over-density with a radial density profile proportional to $r_c/r$ (as determined from simulations), where $r$ is the perpendicular distance from the cylinder axis and $r_c$ is the core radius of the density profile, we constrain the product of over-density and filament temperature to be $delta_c times (T_{rm e}/10^7 , {rm K}) times (r_c/0.5h^{-1} , {rm Mpc}) = 2.7 pm 0.5$. To our knowledge, this is the first detection of filamentary gas at over-densities typical of cosmological large-scale structure. We compare our result to the BAHAMAS suite of cosmological hydrodynamic simulations (McCarthy et al. 2017) and find a slightly lower, but marginally consistent Comptonization excess, $Delta y = (0.84 pm 0.24) times 10^{-8}$.
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