Discovery of a Lyα Cloud in a Cosmic Void

Abstract

In the course of a Hubble Space Telescope (HST) Goddard High Resolution Spectrograph (GHRS) program to discover local Lyα absorption lines in the UV spectra of bright (V < 14.5) AGN situated behind nearby galaxy voids, we have discovered one definite and two possible Lyα absorption lines located in voids. Despite the fact that our observations probe nearly equal pathlengths through local superclusters and voids, only one of the eight definite Lyα detections is within a void. Although the majority of local Lyα lines appear associated with supercluster structures, none of our detections is closer than 450 kpc to a bright galaxy. Thus there is no strong evidence for associating these clouds with individual galaxies. This is unlike the situation for higher equivalent width (EW) lines (W_λ >300mÅ) where Lanzetta et al. (1995; L95) find most Lyα clouds closely associated with bright galaxies. Our results agree with those of Morris et al. (1993; M93) who found only a loose association of lower EW Lyα clouds with galaxies. These results suggest that at least some local Lyα clouds are truly intergalactic in origin.