Intergalactic Hydrogen Clouds OT Low Redshift: Connections to Voids and Dwarf Galaxies

THE ASTRONOMICAL JOURNAL VOLUME 111, NUMBER 1 ,_ _ _A/J-}_u)_OJANUARy" 1996INTERGALACTIC HYDROGEN CLOUDS AT LOW REDSHIFT:CONNECTIONS TO VOIDS AND DWARF GALAXIES lJ. MICHAEL SHULL,2 JOHN T. STOCKE, AND STEVE PENTONCenter for Astrophysics and Space Astronomy, Department of Astrophysical, Planetary, and Atmospheric Sciences, University ofColorado, CB-389,Boulder, Colorado 80309"eived1995 June 30; revised 1995 September 29NASA-CR-204891ABSTRACTWe provide new post-COSTAR data on one sightline (Mrk 421) and updated data from another (I Zw 1) fromour Hubble Space Telescope (HST) survey of intergalactic Lyo_ clouds located along sightlines to four brightquasars passing through well-mapped galaxy voids (16 000 km s -1 pathlength) and superclusters (18 000km s-l). We report two more definite detections of low-redshift Lya clouds in voids: one at 3047 km s-l(heliocentric) toward Mrk 42 l and a second just beyond the Local Supercluster at 2861 km s-t toward I Zw1, confirming our earlier discovery of Lya absorption clouds in voids [Stocke et HI. , ApJ, 451, 24 (1995)]. We have now identified ten definite and one probable Iow-redshift neutral hydrogen absorption cloudstoward four targets, a frequency of approximately one absorber every 3400 km s-1 above 1012'7 cm -2column density. Of these ten absorption systems, three lie within voids; the probable absorber also lies ina void. Thus, the tendency of Lyot absorbers to "avoid the voids" is not as clear as we found previously. Ifthe Lyo_ clouds are approximated as homogeneous spheres of 100 kpc radius, their masses are -109 . F/_®(about 0.01 times that of bright L* galaxies) and they are 40 times more numerous, comparable to thedensity of dwarf galaxies and of low-mass halos in numerical CDM simulations. The Lyot clouds contributea fraction llct_0. 003h_-s t to the closure density of the universe, comparable to that of luminous matter.These clouds probably require a substantial amount of nonbaryonic dark matter for gravitational binding.They may represent extended haloes of low-mass protogalaxies which have not experienced significant starformation or low-mass dwarf galaxies whose star formation ceased long ago, but blew out significantgaseous material. © 1996 American Astronomical Society.1. INTRODUCTIONOver the past two decades, redshift surveys of the nearbyuniverse (Geller & Huchra 1989; Fairall et HI. 1990) havemapped out a highly inhomogeneous galaxy distribution,with large "voids" bounded by sheetlike superclusters. Agoal of studies of the content of these voids (Sanduleak P Szomuru etal. 1994; Weinberg etal. 1991;Strauss & Huchra 1988) is to understand how galaxy forma-tion and subsequent interactions fit into a cosmologicalframework. With the Hubble Space Telescope (HST), wehave undertaken a spectral search for low-redshift neutralhydrogen (Lytr) absorption clouds along sightlines to brightquasars behind well-mapped voids and superclusters. In ourearlier work from Cycle 2 (Stocke et al. 1995, hereafter re-ferred to as Paper I), on the first three sightlines, we detectedeight definite Lyo_ absorption lines, ranging in equivalentwidth from 26 to 240 m/_. Seven of these absorbers werelocated in supercluster galaxy structures. One absorber, inthe sightline toward Mrk 501, was located in a void, morethan 5.9 Mpc from the nearest bright galaxy.IBased on observations with the NASA/ESA Hubble Space Telescope, ob-tainedat the Space Telescope ScienceInstitute, which is operatedby AURA,Inc., under NASA Contract No. NAS5-26555.2also at JILA, University of Colorado and National Institute of StandardsandTechnology.This discovery demonstrated that the voids are not en-tirely empty of gaseous matter, but the statistics were low. Infact, a number of H 1 galaxies, 1RAS galaxies, and emission-line galaxies have been found within the boundaries of theBootes void (Tifft et al. 1986; Weistrop & Downes 1988;Dey etal. 1990; Szomoru et al. 1993), the Pegasus void(Fairall et al. 1990), and the Pisces-Perseus void (Henning1992). However, the filamentary structure of galaxies insidethe Bootes and Pegasus voids is similar to that found in thevoids between the Local Supercluster and the "Great Wall"in the CfA survey (Huchra et HI. 1993; see also our Fig. 3). Because the total extent of the voids is somewhat uncertainat the distance of Bootes, we chose our targets toward morelocal voids, with better-mapped galaxy distributions. For ex-ample, one Lya cloud in the direction of Mrk 501 reported inPaper I is found within a filamentary structure of galaxiesseparating two large voids whose total size is comparable tothe Bootes void.In this paper, we report the results of additional HSTCycle 4 observations of I Zw 1 and Mrk 421. Toward I Zw 1,we are now able to upgrade a "probable" void absorber to"definite" status, and we have detected another Lya absorberin a void toward the fourth target, Mrk 421. We update thedistribution of Lyo_ clouds toward all four sightlines, analyzethe physical parameters of these Lya clouds, and make anestimate of their contribution to the baryonic mass density ofthe Universe. We conclude with a discussion of a possible72 Astron. J. 111 (1), January 1996 0004-6256/96/111(1)/72/6/$6.00 © 1996 Am. Astron. Soc. 72