Discovery of a large-scale galaxy filament near a candidate
  intergalactic X-ray absorption system
  We present an analysis of the large-scale galaxy distribution around two
possible warm-hot intergalactic medium (WHIM) absorption systems reported along
the Markarian 421 sightline. Using the Sloan Digital Sky Survey, we find a
prominent galaxy filament at the redshift of the z=0.027 X-ray absorption line
system. The filament exhibits a width of 3.2 Mpc and length of at least 20 Mpc,
comparable to the size of WHIM filaments seen in cosmological simulations. No
individual galaxies fall within 350 projected kpc so it is unlikely that the
absorption is associated with gas in a galaxy halo or outflow. Another,
lower-significance X-ray absorption system was reported in the same Chandra
spectrum at z=0.011, but the large-scale structure in its vicinity is far
weaker and may be a spurious alignment. By searching for similar galaxy
structures in 140 random smoothed SDSS fields, we estimate a ~5-10% probability
of the z=0.027 absorber-filament alignment occurring by chance. If these two
systems are indeed physically associated, this would represent the first known
coincidence between large-scale galaxy structure and a blind X-ray WHIM
detection.
