The Low-Redshift Intergalactic Medium

The low-redshift Ly-alpha forest of absorption lines provides a probe of large-scale baryonic structures in the intergalactic medium, some of which may be remnants of physical conditions set up during the epoch of galaxy formation. We discuss our recent Hubble Space Telescope (HST) observations and interpretation of low-z Ly-alpha clouds toward nearby Seyferts and QSOs, including their frequency, space density, estimated mass, association with galaxies, and contribution to Omega-baryon. Our HST/GHRS detections of 70 Ly-alpha absorbers with N_HI > 10^12.6 cm-2 along 11 sightlines covering pathlength Delta(cz) = 114,000 km/s show f(>N_HI) ~ N_HI^{-0.63 +- 0.04} and a line frequency dN/dz = 200 +- 40 for N_HI > 10^12.6 cm-2 (one every 1500 km/s of redshift). A group of strong absorbers toward PKS 2155-304 may be associated with gas (400-800) h_75^-1 kpc from 4 large galaxies, with low metallicity (< 0.003 solar) and D/H < 2 x 10^-4. At low-z, we derive a metagalactic ionizing radiation field from AGN of J_0 = 1.3^{+0.8 -0.5} x 10^-23 ergs/cm2/s/Hz/sr and a Ly-alpha-forest baryon density Omega-baryon = (0.008 +- 0.004) h_75^-1 [J_-23 N_14 b_100]^{1/2} For clouds of characteristic size b = (100 kpc)b_100.Comment: 5 figure

CO/H2 Abundance Ratio ~ 10^{-4} in a Protoplanetary Disk

The relative abundances of atomic and molecular species in planet-forming disks around young stars provide important constraints on photochemical disk models and provide a baseline for calculating disk masses from measurements of trace species. A knowledge of absolute abundances, those relative to molecular hydrogen (H2), are challenging because of the weak rovibrational transition ladder of H$_{2}$ and the inability to spatially resolve different emission components within the circumstellar environment. To address both of these issues, we present new contemporaneous measurements of CO and H2 absorption through the "warm molecular layer" of the protoplanetary disk around the Classical T Tauri Star RW Aurigae A. We use a newly commissioned observing mode of the Hubble Space Telescope-Cosmic Origins Spectrograph to detect warm H2 absorption in this region for the first time. An analysis of the emission and absorption spectrum of RW Aur shows components from the accretion region near the stellar photosphere, the molecular disk, and several outflow components. The warm H2 and CO absorption lines are consistent with a disk origin. We model the 1092-1117A spectrum of RW Aur to derive log10 N(H2)~=~19.90$^{+0.33}_{-0.22}$ at T$_{rot}$(H2) ~=~440~+/-~39 K. The CO $A$~--~$X$ bands observed from 1410-1520A are best fit by log10 N(CO)~=~16.1~$^{+0.3}_{-0.5}$ at T$_{rot}$(CO) ~=~200$^{+650}_{-125}$ K. Combining direct measurements of the HI, H2, and CO column densities, we find a molecular fraction in the warm disk surface of $f_{H2}$~>=~0.47 and derive a molecular abundance ratio of CO/H2~=~1.6$^{+4.7}_{-1.3}$~x~10$^{-4}$, both consistent with canonical interstellar dense cloud values.Comment: ApJ - accepted. 13 pages, 8 figure

The Local Ly-alpha Forest IV: STIS G140M Spectra and Results on the Distribution and Baryon Content of HI Absorbers

We present HST STIS/G140M spectra of 15 extragalactic targets, which we combine with GHRS/G160M data to examine the statistical properties of the low-z Ly-alpha forest. We evaluate the physical properties of these Ly-alpha absorbers and compare them to their high-z counterparts. We determine that the warm, photoionized IGM contains 29+/-4% of the total baryon inventory at z = 0. We derive the distribution in column density, N_HI^(1.65+/-0.07) for 12.5 < log [N_HI] 14.5. The slowing of the number density evolution of high-W Ly-alpha clouds is not as great as previously measured, and the break to slower evolution may occur later than previously suggested (z~1.0 rather than 1.6). We find a 7.2sigma excess in the two-point correlation function (TPCF) of Ly-alpha absorbers for velocity separations less than 260 km/s, which is exclusively due to the higher column density clouds. From our previous result that higher column density Ly-alpha clouds cluster more strongly with galaxies, this TPCF suggests a physical difference between the higher and lower column density clouds in our sample.Comment: 71 pages, 6 tables, 26 EPS figures, to appear in ApJ Supplemen

The Local Lyman-Alpha Forest: Absorbers in Galaxy Voids

We have conducted pointed redshift surveys for galaxies in the direction of bright AGN whose HST far-UV spectra contain nearby (cz <~ 30,000 kms), low column density (12.5 <= log N_{HI} (cm s^{-2}) <= 14.5) Ly-alpha forest absorption systems. Here we present results for four lines-of-sight which contain nearby (cz <~ 3000 kms) Ly-alpha absorbers in galaxy voids. Although our data go quite deep (-13 <= M_{B}(limit) <= -14) out to impact parameters of 100-250 h_{70}^{-1} kpc, these absorbers remain isolated and thus appear to be truly intergalactic, rather than part of galaxies or their halos. Since we and others have discovered no galaxies in voids, the only baryons detected in the voids are in the Ly-alpha ``clouds''. Using a photoionization model for these clouds, the total baryonic content of the voids is 4.5% +/- 1.5% of the mean baryon density.Comment: 5 pages, 1 figure, accepted for publication in Astrophysical Journal Letter

The Metallicity of Intergalactic Gas in Cosmic Voids

We have used the Hubble/STIS and FUSE archives of ultraviolet spectra of bright AGN to identify intergalactic Lya absorbers in nearby (z < 0.1) voids. From a parent sample of 651 Lya absorbers, we identified 61 void absorbers located more than 1.4/h_70 Mpc from the nearest L* or brighter galaxy. Searching for metal absorption in high-quality (S/N > 10) spectra at the location of three diagnostic metal lines (O VI 1032, C IV 1548, Si III 1206), we detected no metal lines in any individual absorber, or in any group of absorbers using pixel co-addition techniques. The best limits on metal-line absorption in voids were set using four strong Lya absorbers with N(H I) > 10^{14} cm^-2, with 3-sigma equivalent-width limits ranging from 8 mA (O VI), 7-15 mA (C IV), and 4-10 mA (Si III). Photoionization modeling yields metallicity limits Z < 10^{-1.8+/-0.4} Z_sun, from non-detections of C IV and O VI, some 6 times lower than those seen in Lya and OVI absorbers at z < 0.1. Although the void Lya absorbers could be pristine material, considerably deeper spectra are required to rule out a universal metallicity floor produced by bursts of early star formation, with no subsequent star formation in the voids. The most consistent conclusion derived from these low-z results, and similar searches at z = 3-5, is that galaxy filaments have increased their mean IGM metallicity by factors of 30-100 since z = 3.Comment: Accepted for ApJ, 8 pages including Fig 1a,