QSO Ly$\alpha$ Absorption Lines in Galaxy Superclusters and Voids
Session 72 -- High Z Line Measurements; Gravitational Microlensing: Distance Determination
Display presentation, Friday, January 14, 9:30-6:45, Salons I/II Room (Crystal Gateway)

## [72.04] QSO Ly$\alpha$ Absorption Lines in Galaxy Superclusters and Voids

J. T. Stocke, J. M. Shull, S. Penton, G. Burks (CASA) and M. Donahue (STScI)

We have used the Hubble Space Telescope (HST) Goddard High Resolution Spectrograph (GHRS) to search for Ly$\alpha$ absorption clouds in nearby galaxy voids ($cz \leq 10,000$ km~s$^{-1}$). Thus far, we have obtained GHRS spectra (G160M, 1225 -- 1255 \AA, 0.25 \AA\ resolution) of three very bright Active Galactic Nuclei, Mrk 501, I~Zw~I, and Mrk 335, at $V \leq 14.5$. We find 4 probable ($4.0 \sigma - 4.5 \sigma$) and 4 definite ($5 \sigma - 16 \sigma$) Ly$\alpha$ absorption lines, with equivalent widths $W_{\lambda} = 50 - 200$ m\AA\, corresponding to column densities N(H~I) = $10^{13}$ -- $10^{14}$ cm$^{-2}$, assuming a typical Doppler parameter of $b = 25$ km~s$^{-1}$.

Based on an updated version of the CfA redshift survey (Huchra and Clemens, private communication), most of these Ly$\alpha$ systems appear to be associated with supercluster - sized strings'' of galaxies similar to the Great Wall''. Toward Mrk 501, the nearest bright galaxy at the redshift of the strongest (200 m\AA) Ly$\alpha$ cloud lies $500 h_{75}^{-1}$~kpc off the line of sight. Models of H~I disks exposed to the intergalactic ionizing radiation field (Dove \& Shull 1994, ApJ, 423 , in press) show that the N(H~I) = $10^{13}$ cm$^{-2}$ contour in a typical spiral galaxy is reached at 100 kpc radial extent. Thus, the Ly$\alpha$ absorbers associated with galaxy-string systems may be the result of H~I in an extended halo, in dwarf satellite galaxies ($M_B > -15$), or in tidally-stripped gas. Most importantly for cosmological origins of baryons, one ($4.3 \sigma$) Ly$\alpha$ absorption line in the spectrum of Mrk 501 lies within the galaxy void in the foreground of the Great Wall''. The nearest bright galaxy, to a level $M_B \leq -18.5$ for $H_0 = 75$ km~s$^{-1}$~Mpc$^{-1}$, is more than 5 Mpc away. A pencil-beam survey of faint galaxies to $M_B = -16.0$ finds no galaxy within $100 h_{75}^{-1}$ kpc of the line of sight, at or near the absorber redshift.