Probing the Size of Low‐Redshift Lyα Absorbers

The 3C 273 and RX J1230.8+0115 sightlines probe the outskirts of the VirgoCluster at physical separations between the sightlines of 200-500 h_70 kpc. Wepresent an analysis of HST STIS echelle and FUSE UV spectroscopy of RXJ1230.8+0115 in which we detect five Lyman-alpha absorbers at Virgo distances.One of these absorbers is a blend of two strong metal line absorbers coincidentin velocity with the highest neutral hydrogen column density absorber in the 3C273 sightline ~350 h_70 kpc away. The consistency of the metal line columndensity ratios in the RX J1230.8+0115 sightline allows us to determine theionization mechanism (photoionization) for these absorbers. While the lowsignal-to-noise ratio of the FUSE spectrum limits our ability to model theneutral hydrogen column density of these absorbers, we are able to constrainthem to be in the range 10^{16-17} cm^-2. The properties of these absorbers aresimilar to those of the 3C 273 absorber studied by Tripp et al. However, theinferred line-of-sight size for the 3C 273 absorber is only 70 pc, much smallerthan those inferred in RX J1230.8+0115, which are 10-30 h_70 kpc. The smallsizes of all three absorbers are at odds with the >~350 h_70 kpc minimumtransverse size implied by an application of the standard QSO line pairsanalysis. On the basis of absorber associations between these two sight lineswe conclude that a large-scale structure filament produces a correlated, notcontiguous, gaseous structure in this region of the Virgo Supercluster. Thesedata may indicate that we are detecting overdensities in the large scalestructure filaments in this region. Alternatively, the presence of a galaxy 71h_70 kpc from a 3C 273 absorber may indicate that we have probed outflowing,starburst driven shells of gas associated with nearby galaxies.Comment: 13 pages, accepted Ap