Abundant Dust Found in Intergalactic Space

Galactic dust constitutes approximately half of the elements more massive than helium produced in stellar nucleosynthesis. Notwithstanding the formation of dust grains in the dense, cool atmospheres of late-type stars, there still remain huge uncertainties concerning the origin and fate of galactic stardust. In this Letter, we identify the intergalactic medium (i.e., the region between gravitationally bound galaxies) as a major sink for galactic dust. We discover a systematic shift in the color of background galaxies viewed through the intergalactic medium of the nearby M81 group. This reddening coincides with atomic, neutral gas previously detected between the group members. The dust-to-H I mass ratio is high (1/20) compared to that of the solar neighborhood (1/120), suggesting that the dust originates from the center of one or more of the galaxies in the group. Indeed, M82, which is known to be ejecting dust and gas in a starburst-driven superwind, is cited as the probable main source.