Diffuse far-infrared and ultraviolet emission in the NGC4435/4438
  system: tidal stream or Galactic cirrus?
  We report the discovery of diffuse far-infrared and far-ultraviolet emission
projected near the interacting pair NGC4435/4438, in the Virgo cluster. This
feature spatially coincides with a well known low surface-brightness optical
plume, usually interpreted as tidal debris. If extragalactic, this stream would
represent not only one of the clearest examples of intracluster dust, but also
a rare case of intracluster molecular hydrogen and large-scale intracluster
star formation. However, the ultraviolet, far-infrared, HI and CO emission as
well as the dynamics of this feature are extremely unusual for tidal streams
but are typical of Galactic cirrus clouds. In support to the cirrus scenario,
we show that a strong spatial correlation between far-infrared and
far-ultraviolet cirrus emission is observed across the center of the Virgo
cluster, over a scale of several degrees. This study demonstrates how dramatic
Galactic cirrus contamination can be, even at optical and ultraviolet
wavelengths and at high galactic latitudes. If ignored, the presence of diffuse
light scattered by Galactic dust clouds could significantly bias our
interpretation of low surface-brightness features and diffuse light observed
around galaxies and in clusters of galaxies.
