THE H-I ENVIRONMENT OF NEARBY LYMAN-ALPHA ABSORBERS

We present the results of a VLA and WSRT search for H I emission from the vicinity of seven nearby clouds, which were observed in Ly alpha a bsorption with HST toward Mrk 335, Mrk 501, and PKS 2155-304. Around t he absorbers, we searched a volume of 40' x 40' x 1000 km s(-1); for o ne of the absorbers we probed a velocity range of only 600 km s(-1). T he H I mass sensitivity (5 sigma) very close to the lines of sight var ies from 5 x 10(6) M. at best to 5 x 10(8) M. at worst. We detected H I emission in the vicinity of four out of seven absorbers. The closest galaxy we find to the absorbers is a small dwarf galaxy at a projecte d distance of 68h(-1) kpc from the sight line toward Mrk 335. This opt ically uncataloged galaxy has the same velocity (V = 1970 km s(-1)) as one of the absorbers, is fainter than the SMC, and has an H I mass of only 4 x 10(7) M.. We found a somewhat more luminous galaxy at exactl y the velocity (V = 5100 km s(-1)) of one of the absorbers toward PKS 2155-304 at a projected distance of 230h(-1) kpc from the sight line, Two other, stronger absorbers toward PKS 2155-304 at V approximate to 17 000 km s(-1) appear to be associated with a loose group of three br ight spiral galaxies, at projected distances of 300 to 600h(-1) kpc. T hese results support the conclusions emerging from optical searches th at most nearby Ly alpha forest clouds trace the large-scale structures outlined by the optically luminous galaxies, although this is still b ased on small-number statistics. We do not find any evidence from the H I distribution or kinematics that there is a physical association be tween an absorber and its closest galaxy. While the absorbing clouds a re at the systemic velocity of the galaxies, the H I extent of the gal axies is fairly typical, and at least an order of magnitude smaller th an the projected distance to the sight line at which the absorbers are seen. On the other hand, we also do not find evidence against such a connection. In total, we detected H I emission from five galaxies, of which two were previously uncataloged and one did not have a known red shift. No H I emission was detected from the vicinity of the two absor bers, which are located in a void and a region of very low galaxy dens ity; but the limits are somewhat less stringent than for the other sig ht lines. These results are similar to what has been found in opticall y unbiased H I surveys. Thus, the presence of Ly alpha absorbers does not significantly alter the H I detection rate in their environment. ( C) 1996 American Astronomical Society.