The Goddard High Resolution Spectrograph aboard the Hubble Space Telescope was used with the G160M grating to obtain high-resolution (0.2 A) spectra of three very bright active galactic nuclei located behind voids in the nearby distribution of bright galaxies (i.e., CfA and Arecibo redshift survey regions). A total of eight definite (>4 o) Ly_ absorption lines were discovered ranging in equivalent width from 26 to 240 mA at Galactocentric velocities 1740-7740 km s-1. Of these eight systems, we locate seven in superciuster structures and one, in the sight line of Mrk 501 at 7740 km s-1, in a void. In addition, one of two tentative (3-4 a) Ly_ absorption lines are found in voids. Thus, the voids are not entirely devoid of matter, and not all Lye, clouds are associated with galaxies. Also, since the path lengths through voids and superclusters probed by our observations thus far are nearly equal, there is some statistical evidence that the Ly_ clouds avoid the voids. The nearest galaxy neighbors to these absorbing clouds are 0.45-5.9 Mpc away, too far to be physically associated by most models. The lower equivalent width absorption lines (W_ < 100 mA) are consistent with random locations with respect to galaxies and may be truly intergalactic, similar to the bulk of the Ly_ forest seen at high 2. These results on local Ly_ clouds are in full agreement with those found by Morris et al. (1993) for the 3C 273 sight line but are different from the results for higher equivalem width systems where closer cloud-galaxy associations were found by Lanzetta et al. (1994). Pencil-beam optical and 21 cm radio line observations of the area of sky surrounding Mrk 501 fail to find faint galaxies near the velocities of the Ly_ clouds in that sight line. Specifically, for the "void absorption" system at 7740 km s -1, we find no galaxy at comparable redshift to the absorber within 100 h_ kpc (Ho = 75 h_5 km s -I Mpc -1) with an absolute magnitude of B _ -16 and no object with H I mass >7 × 108 h;_ M o within 500 h;_ kpc. Thus, neither a faint optical galaxy nor a gas-rich, optically dim or low surface brightness galaxy is present close to this absorber. Subject headings: large-scale structure of universe --quasars: absorption lines --intergalactic medium obtained at the Space Telescope Science Insitute, which is operated by AURA, Inc., under NASA contract NAS 5-26555. 2 Observations at the Palomar Observatory were made as part of a collaborative agrecmem between the California Institute of Technology and the Carnegie Institution of Washington.

We provide new post-COSTAR data on one sightline (Mrk 421) and updated data from another (I Zw 1) from our Hubble Space Telescope (HST) survey of intergalactic Lyα clouds located along sightlines to four bright quasars passing through well-mapped galaxy voids (16,000 km s −1 pathlength) and superclusters (18,000 km s −1 ). We report two more definite detections of low-redshift Lyα clouds in voids: one at 3047 km s −1 (heliocentric) toward Mrk 421 and a second just beyond the Local Supercluster at 2861 km s −1 toward I Zw 1, confirming our earlier discovery of Lyα absorption clouds in voids . We have now identified 10 definite and 1 probable low-redshift neutral hydrogen absorption clouds toward four targets, a frequency of approximately one absorber every 3400 km s −1 above 10 12.7 cm −2 column density. Of these 10 absorption systems, 3 lie within voids; the probable absorber also lies in a void. Thus, the tendency of Lyα absorbers to "avoid the voids" is not as clear as we found previously. If the Lyα clouds are approximated as homogeneous spheres of 100 kpc radius, their masses are ∼ 10 9 M ⊙ (about 0.01 times that of bright L * galaxies) and they are 40 times more numerous, comparable to the density of dwarf galaxies and of low-mass halos in numerical CDM simulations. The Lyα clouds contribute a fraction Ω cl ≈ 0.003h −1 75 to the closure density of the universe, comparable to that of luminous matter. These clouds probably require a substantial amount of non-baryonic dark matter for gravitational binding. They may represent extended haloes of low-mass protogalaxies which have not experienced significant star formation or low-mass dwarf galaxies whose star formation ceased long ago, but blew out significant gaseous material.

In May 1994 the BL Lac object PKS 2155-304 was observed continuously for ∼10 days with IUE and EUVE and for 2 days with ASCA, as well as with ROSAT and with ground-based radio, infrared, and optical telescopes. The light curves show a well-defined X-ray flare followed by a broader, lower amplitude extreme ultraviolet (EUV) flare ∼1 day later and a broad, low-amplitude UV flare ∼2 days later. X-ray fluxes obtained at three well separated times the preceding week indicate at least one previous flare of comparable amplitude or perhaps ongoing stochastic X-ray variations, and additional rapid variability was seen at the beginning of the IUE observation, when extremely sharp changes in UV flux occurred. The X-ray flux observed with ASCA flared by a factor of ∼ 2