arXiv subjects

J. Michael Shull

Publications and source records attributed to J. Michael Shull.

At least 91 records · Page 5Linked to original sources

arXiv2005

The Low-z Intergalactic Medium. II. LyB, OVI, and CIII Forest

We present the results of a large survey of HI, OVI, and CIII absorption lines in the low-redshift (z<0.3) intergalactic medium (IGM). We begin with 171 strong Lyalpha absorption lines (W>80 mA) in 31 AGN sight lines studied with the Hubble Space Telescope and measure corresponding absorption from higher-order Lyman lines with FUSE. Higher-order Lyman lines are used to determine N_HI and b_HI accurately through a curve-of-growth (COG) analysis. We find that the number of HI absorbers per column density bin is a power-law distribution, dN/dN_HI=N^-beta, with beta_HI=1.68+-0.11. We made 40 detections of OVI 1032,1038 and 30 detections of CIII 977 out of 129 and 148 potential absorbers, respectively. The column density distribution of CIII absorbers has beta_CIII=1.68+-0.04, similar to beta_HI but not as steep as beta_OVI=2.1+-0.1. From the absorption-line frequency, dN_CIII/dz=12^+3_-2 for W(CIII)>30 mA, we calculate a typical IGM absorber size r_0~400 kpc. The COG-derived b-values show that HI samples material with T<10^5 K, incompatible with a hot IGM phase. By calculating a grid of CLOUDY models of IGM absorbers with a range of collisional and photoionization parameters, we find it difficult to simultaneously account for the OVI and CIII observations with a single phase. The observations require a multiphase IGM in which HI and CIII arise in photoionized regions, while OVI is produced primarily through shocks. From the multiphase ratio N_HI/N_CIII, we infer the IGM metallicity Z_C=0.12 Z_sun, similar to our previous estimate of Z_O=0.09 Z_sun from OVI.

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arXiv2005

The Galaxy Environment of O VI Absorption Systems

We combine a FUSE sample of OVI absorbers (z < 0.15) with a database of 1.07 million galaxy redshifts to explore the relationship between absorbers and galaxy environments. All 37 absorbers with N(OVI) > 10^{13.2} cm^-2 lie within 800 h_70^-1 kpc of the nearest galaxy, with no compelling evidence for OVI absorbers in voids. The OVI absorbers often appear to be associated with environments of individual galaxies. Gas with 10 +/- 5% of solar metallicity (OVI and CIII) has a median spread in distance of 350-500 kpc around L* galaxies and 200-270 kpc around 0.1 L* galaxies (ranges reflect uncertain metallicities of gas undetected in Lya absorption). In order to match the OVI line frequency, dN/dz = 20 for N(OVI) > 10^{13.2} cm^-2, galaxies with L < 0.1 L* must contribute to the cross section. The Lya absorbers with N(HI) > 10^{13.2} cm^-2 cover ~50% of the surface area of typical galaxy filaments. Two-thirds of these show OVI and/or CIII absorption, corresponding to a 33-50% covering factor at 0.1 Z_sun and suggesting that metals are spread to a maximum distance of 800 kpc, within typical galaxy supercluster filaments. Approximately 50% of the OVI absorbers have associated Lya line pairs with separations Delta V = 50-200 km/s. These pairs could represent shocks at the speeds necessary to create copious OVI, located within 100 kpc of the nearest galaxy and accounting for much of the two-point correlation function of low-z Lya forest absorbers.

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arXiv2005

Critical Metallicity and Fine-Structure Emission of Primordial Gas Enriched by the First Stars

The influence of the first stars on the formation of second-generation objects at high redshift may be determined, in part, by their metal enrichment of surrounding gas. At a critical metallicity, Zcrit, primordial gas cools more efficiently by fine-structure lines of CII (157.74 um), OI (63.18 um, 145.5 um), SiII (34.8 um), and FeII (25.99 um, 35.35 um) than by HI or H2 emission. This cooling may alter the process of fragmentation into smaller units. We study the time-dependent cooling of primordial gas enriched by heavy elements from early massive stars, particularly O, Si, and Fe. We define Zcrit as the point when the total cooling rate by metals plus H2 equals the adiabatic compressional heating. We explore two metallicity scenarios: (1) a single metallicity for all heavy elements; (2) individual metallicities (Z_C, Z_O, Z_Si, Z_Fe) from theoretical supernova yields. For dense gas [n > 10^3 cm^(-3)] with metals in relative solar abundances, fragmentation occurs at Zcrit ~ 10^(-3.5) Z_sun. However, for lower density gas, [n = 1-100 cm^(-3)], particularly in halos enriched in Si, O, and Fe, we find Zcrit = 0.1-1% Zsun. The critical metallicity approaches a minimum value at high-n set by efficient LTE cooling, with thermalized level populations of fine-structure states and H2 rotational states (J = 2 and J = 3). Primordial clouds of 10^8 Msun at 200K are detectable in redshifted fine-structure lines, with far-infrared fluxes between 10^-22 and 10^-21 W/m^2. For metallicities Z_O = 10^(-3) and molecular fractions f(H2) = 10^(-3) the fine-structure emission lines of OI, SiII, and FeII could be 10^2 - 10^3 times stronger than the H2 rotational lines at 28.22 um (J = 2-0) and 17.03 um (J = 3-1).

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arXiv2005

A FUSE Survey of Interstellar Molecular Hydrogen toward High-Latitude AGN

We report results from a FUSE survey of interstellar molecular hydrogen (H2) along 45 sight lines to AGN at high Galactic latitudes (|b| > 20 degrees). Most (39 of 45) of the sight lines show detectable Galactic H2 absorption from Lyman and Werner bands between 1000 and 1126 A, with column densities ranging from N(H2) = 10^(14.17-19.82) cm^-2. In the northern Galactic hemisphere, we identify many regions of low column, N(H2) < 10^15 cm^-2, between longitude l = 60-180 degrees and at b > 54 degrees. These `"H2 holes" provide valuable, uncontaminated sight lines for extragalactic UV spectroscopy, and a few may be related to the "Northern Chimney" (low Na I absorption) and "Lockman Hole" with low N(HI). A comparison of high-latitude H2 with 139 OB-star sight lines surveyed in the Galactic disk suggests that high-latitude and disk H2 clouds may have different rates of heating, cooling, and UV excitation. For rotational states J = 0 and 1, the mean excitation temperature at high latitude, = 124 +/- 8 K, is somewhat above that in the Galactic disk, = 86 +/- 20 K. For J = 2-4, the = 498 +/- 28 K, and the column-density ratios, N(3)/N(1), N(4)/N(0), and N(4)/N(2), indicate a comparable degree of UV excitation in the disk and low halo for sight lines with N(H2) > 10^18. The distribution of molecular fractions at high latitude shows a transition at lower total hydrogen column density, log N_H = 20.38 +/- 0.13, than in the Galactic disk, log N_H(disk) = 20.7. If the FUV radiation fields are similar in disk and low halo, this suggests an enhanced (dust-catalyzed) H2 formation rate in higher-density, compressed clouds, which could be detectable as high-latitude, sheetlike infrared cirrus.

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arXiv2005

Molecular Hydrogen in Infrared Cirrus

We combine data from our recent FUSE survey of interstellar molecular hydrogen absorption toward 50 high-latitude AGN with COBE-corrected IRAS 100 micron emission maps to study the correlation of infrared cirrus with H2. A plot of the H2 column density vs. IR cirrus intensity shows the same transition in molecular fraction, f_H2, as seen with total hydrogen column density, N_H. This transition is usually attributed to H2 self-shielding, and it suggests that many diffuse cirrus clouds contain H2 in significant fractions, f_H2 = 1-30%. These clouds cover approximately 50% of the northern sky at latitudes b > 30 degrees, at temperature-corrected 100 micron intensities D_100 > 1.5 MJy/sr. The sheetlike cirrus clouds, with hydrogen densities n_H > 30 cm^-3, may be compressed by dynamical processes at the disk-halo interface, and they are conducive to H2 formation on grain surfaces. Exploiting the correlation between N(H2) and 100 micron intensity, we estimate that cirrus clouds at b > 30 contain approximately 3000 M_sun in H2. Extrapolated over the inner Milky Way, the cirrus may contain 10^7 M_sun of H2 and 10^8 M_sun in total gas mass. If elevated to 100 pc, their gravitational potential energy is ~10^53 erg.

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arXiv2005

An OVI Baryon Census of the Low-z Warm-Hot Intergalactic Medium

Intergalactic absorbers along lines of sight to distant quasars are a powerful diagnostic for the evolution and content of the intergalactic medium (IGM). In this study, we use the FUSE satellite to search 129 known lya absorption systems at z<0.15 toward 31 AGN for corresponding absorption from higher Lyman lines and the important metal ions OVI. We detect OVI in 40 systems over a smaller range of column density than seen in HI. The co-existence of OVI and HI suggests a multiphase IGM, with warm photoionized and hot ionized components. With improved OVI detection statistics, we find a steep distribution in OVI column density, dN(OVI)/dN_OVI N_OVI^(-2.2+-0.1), suggesting that numerous weak OVI absorbers contain baryonic mass comparable to the rare strong absorbers. The total cosmological mass fraction in this hot gas is at least Omega_WHIM=(0.0022+-0.0003. Gas in the WHIM at 10^5-6 K contributes at least 4.8+-0.9% of the total baryonic mass at z<0.15. We then combine empirical scaling relations for the observed "multiphase ratio", N_HI/N_OVI N_HI^(0.9+-0.1), and for hydrogen overdensity in cosmological simulations, N_HI delta_H^{0.7}, with the HI photoionization correction to derive the mean oxygen metallicity, Z_O (0.09 Z_sun)(f_OVI/0.2)^(-1) in the low-z multiphase gas. Given the spread in the empirical relations and in f_OVI, the baryon content in the OVI WHIM could be as large as 10%. Our survey is based on a large improvement in the number of OVI absorbers (40 vs. 10) and total redshift pathlength (dz 2.2 vs. dz 0.5) compared to earlier surveys.

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arXiv2004

The Hot Intergalactic Medium - Galaxy Connection: Two Strong Absorbers in the Sightline Toward PG 1211+143

We present HST/STIS and FUSE spectra of the QSO PG 1211+143 (z_em = 0.081) and a galaxy survey of the surrounding field. This sightline shows two strong intergalactic absorption systems at cz ~ 15,300 and 19,300. This sightline addresses the nature and origin of the OVI absorbers, and their connection to galaxies. We explore the relationship of these absorbers to the nearby galaxies and compare them to other OVI-bearing absorbers in diverse environments. At 15,300 km s^-1, we find four distinct HI components and associated CII, CIII, CIV, SiII, SiIII, SiIV, NV, and OVI, lying near a spiral-dominated galaxy group with a bright member galaxy 137 h_70^-1 kpc from the sightline. The observed ions of C, Si, and N are likely to be photoionized, but the OVI is more consistent with collisional ionization. The ion ratios in this absorber resemble the highly-ionized Galactic HVCs; it may also trace the hot intragroup medium gas or the unbound wind of an undiscovered dwarf galaxy. At 19,300 km s^-1, we find five HI components and associated CIII, SiIII, and collisionally-ionized OVI lying 146 h_70^-1 kpc from an isolated galaxy. The properties of the OVI-bearing gas are consistent with an origin in strong shocks between low-metallicity gas (>= 2 - 6 % solar) and one or more of the warm photoionized components. It is likely that these absorbers are related to the nearby galaxies, perhaps by outflows or gas stripped from unseen satellite galaxies by interactions. However, we cannot reject completely the hypothesis that they reside in the same large-scale structure in which the galaxies are embedded but are otherwise not directly related.

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arXiv2004

He II Reionization and Sources of Metagalactic Ionization

Intergalactic Ly-alpha opacity suggests that H I was reionized at z ~ 6, while He II reionization was delayed to z ~ 3. Both epochs are in conflict with inferences from CMB optical depth (WMAP) which suggest early reionization at z = 10-20. One of the major contributions of FUSE to cosmological studies has been the detection of He II Ly-alpha (Gunn-Peterson) absorption in the spectra of AGN at redshifts z > 2.03. Spectra of He II absorbers, taken in concert with corresponding H I (Ly-alpha) lines, allow us to fix the epoch of helium reionization at z(HeII) ~ 2.8 +/- 0.2. Here, I review FUSE observations of He II absorption, together with their implications for the sources and transport of ionizing radiation over 30-50 Mpc distances through the IGM. FUSE observations of He II absorption toward HE 2347-4342, combined with Keck and VLT observations of H I, are consistent with photoionization by QSOs, with a wide range of intrinsic spectral indices, and modified by filtering and reprocessing in the IGM. The He II/H I ratio (eta) exhibits variations over 1 Mpc distance scales (Delta z ~ 0.001). Intriguingly, this eta-ratio is also correlated with Ly-alpha filaments and voids. The ionizing radiation field appears to be softer (higher He II/H I) in the voids. These void regions may be ionized by local soft sources (dwarf starburst galaxies), or the QSO radiation may softened by escape from AGN cores and transport through denser regions in the cosmic web. The differences in ionizing spectra may explain the 1.4 Gyr lag between H I and He II reionization epochs.

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arXiv2004

The LyB and OVI Forest in the Local Universe

Intergalactic absorbers along lines of sight to distant quasars are a powerful diagnostic for the evolution and content of the IGM. In this study, we search known low-z Ly-alpha absorption systems for equivalent absorption in higher Lyman lines as well as the important metal ions OVI and CIII. OVI absorption is detected roughly half the time and shows a smaller range of column density than HI. This implies a multi-phase IGM with warm neutral and hot ionized components. We use our OVI detection statistics to determine that the fraction of intergalalctic mass at 10^5-10^6 K is at least 5% of the total baryonic mass in the local (z<0.15) universe.

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arXiv2004

The Baryon Content of the Local Intergalactic Medium

We describe our surveys of low column density Lyman-alpha absorbers [N(HI) = 10^(12.5-16.0) cm^-2], which show that the warm photoionized IGM contains 30% of all baryons at z < 0.1. This fraction is consistent with cosmological simulations, which also predict that an additional 20-40% of the baryons reside in hotter 10^(5-7) K gas, the warm-hot IGM (WHIM). The observed line density of Lya absorbers, dN/dz = 170 for N(HI) > 10^(12.8) cm^-2, is dominated by low-N(HI) systems that exhibit slower redshift evolution than those with N(HI) >10^(14). HST/FUSE surveys of OVI absorbers, together with recent detections of O VII with Chandra and XMM/Newton, suggest that 20-70% of all baryons could reside in the WHIM, depending on its assumed abundance (O/H = 10% solar). At the highest column densities, N(HI) > 10^(20.3), the damped Lya systems are often identified with gas-rich disks of galaxies over a large range in luminosities (0.03-1 L*) and morphologies. Lyman-limit systems [N(HI) = 10^(17.3-20.3)] appear to be associated with bound, bright (0.1-0.3 L*) galaxy halos. The Lya absorbers with N(HI) = 10^(13-16) cm^-2 are associated with filaments of large-scale structure in the galaxy distribution, although some may arise in unbound winds from dwarf galaxies. Our discovery that ~20% of low-z Lya absorbers reside in galaxy voids suggests that a substantial fraction of baryons may be entirely unrelated to galaxies. In the future, Hubble (and FUSE) can provide a census of the local baryons and the distribution of heavy elements in the IGM. These studies can be conducted quite efficiently if NASA can install the Cosmic Origins Spectrograph (COS) on Hubble, allowing an order-of-magnitude improvement in throughput and a comparable increase in our ability to study the IGM.

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arXiv2004

A Composite Extreme Ultraviolet QSO Spectrum from the Far Ultraviolet Spectroscopic Explorer

The Far Ultraviolet Spectroscopic Explorer (FUSE) has surveyed a large sample (> 100) of active galactic nuclei in the low-redshift universe (z < 1). Its response at short wavelengths makes it possible to measure directly the far ultraviolet spectral properties of quasistellar objects (QSOs) and Seyfert 1 galaxies at z < 0.3. Using archival FUSE spectra, we form a composite extreme ultraviolet (EUV) spectrum of QSOs at z < 0.67. After consideration of many possible sources of systematic error in our analysis, we find that the spectral slope of the FUSE composite spectrum, α= -0.56^+0.38_-0.28 for F_ν\propto ν^α, is significantly harder than the EUV (λ\lesssim 1200 A) portion of the composite spectrum of QSOs with z > 0.33 formed from archival Hubble Space Telescope spectra, α=-1.76 \pm 0.12. We identify several prominent emission lines in the \fuse composite and find that the high-ionization O VI and Ne VIII emission lines are enhanced relative to the HST composite. Power law continuum fits to the individual FUSE AGN spectra reveal a correlation between EUV spectral slope and AGN luminosity in the FUSE and FUSE + HST samples in the sense that lower luminosity AGNs show harder spectral slopes. We find an anticorrelation between the hardness of the EUV spectral slope and AGN black hole mass, using estimates of this quantity found in the literature. We interpret these results in the context of the well-known anticorrelation between AGN luminosity and emission line strength, the Baldwin effect, given that the median luminosity of the FUSE AGN sample is an order of magnitude lower than that of the HST sample.

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arXiv2004

Nucleosynthesis, Reionization, and the Mass Function of the First Stars

We critique the hypothesis that the first stars were very massive stars (VMS; M > 140 Msun). We review the two major lines of evidence for the existence of VMS: (1) that the relative metal abundances of extremely metal-poor Galactic halo stars show evidence of VMS enrichment, and (2) that the high electron-scattering optical depth (tau_e) to the CMB found by WMAP requires VMS for reionization in a concordance LambdaCDM cosmology. The supernova yield patterns of VMS are incompatible with the Fe-peak and r-process abundances in halo stars. Models including Type II supernovae and/or ``hypernovae'' from metal-free progenitors with 8 - 40 Msun can better explain the observed trends. With a simple metal-transport model, we estimate that halo enrichment curtails metal-free star formation after ~10^8 yr at z ~ 20. Because the ionizing photon efficiency of metal-free stars peaks at ~120 Msun and declines at higher mass, an IMF with a lower bound at M ~ 10 - 20 Msun and no VMS can maximize the ionizing photon budget and still be consistent with the nucleosynthetic evidence. An IMF devoid of low-mass stars is justified independently by models of the formation of primordial stars. Using a semi-analytic model for H I and He II reionization, we find that such an IMF can reproduce tau_e = 0.10 - 0.14, consistent with the range from WMAP. We conclude, on the basis of these results, that VMS are not necessary to meet the existing constraints commonly taken to motivate them.

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arXiv2004

A Composite Extreme Ultraviolet QSO Spectrum from FUSE

The Far Ultraviolet Spectroscopic Explorer (FUSE) has surveyed a large sample (> 100) of active galactic nuclei in the low redshift universe (z < 1). Its response at short wavelengths makes it possible to measure directly the EUV spectral shape of QSOs and Seyfert 1 galaxies at z < 0.3. Using archival FUSE spectra, we form a composite extreme ultraviolet (EUV) spectrum of QSOs at z < 1 and compare it to UV/optical composite spectra of QSOs at higher redshift, particularly the composite spectrum from archival Hubble Space Telescope spectra.

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arXiv2004

OVI, NV, and CIV in the Galactic Halo: I. Velocity-Dependent Ionization Models

We explore theoretical models of the ionization ratios of the Li-like absorbers NV, OVI, and CIV, in the Galactic halo. These ions are believed to form in nonequilibrium processes such as shocks, evaporative interfaces, or rapidly cooling gas, all of which trace the dynamics of the interstellar medium. As a useful new diagnostic, we focus on velocity-resolved signatures of several common physical structures: (1) a cooling Galactic fountain flow that rises, cools, and recombines as it returns to the disk; (2) shocks moving toward the observer; (3) a conductive interface with the observer located in the hotter gas. This last geometry occurs with the solar system inside a hot bubble, or when one looks out through the fragmenting top shell of our local bubble blown into the halo as part of the Galactic fountain. In Paper II, these models are compared to ionization-ratio data from FUSE and Hubble Space Telescope.

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arXiv2004

OVI, NV and CIV in the Galactic Halo: II. Velocity-Resolved Observations with Hubble and FUSE

We present a survey of NV and OVI (and where available CIV) in the Galactic halo, using data from the Far Ultraviolet Spectroscopic Explorer (FUSE) and the Hubble Space Telescope (HST) along 34 sightlines. These ions are usually produced in nonequilibrium processes such as shocks, evaporative interfaces, or rapidly cooling gas, and thus trace the dynamics of the interstellar medium. Searching for global trends in integrated and velocity-resolved column density ratios, we find large variations in most measures, with some evidence for a systematic trend of higher ionization (lower NV/OVI column density ratio) at larger positive line-of-sight velocities. The slopes of log[N(NV)/N(OVI)] per unit velocity range from -0.015 to +0.005, with a mean of -0.0032+/-0.0022(r)+/-0.0014(sys) dex/(km/s). We compare this dataset with models of velocity-resolved high-ion signatures of several common physical structures. The dispersion of the ratios, OVI/NV/CIV, supports the growing belief that no single model can account for hot halo gas, and in fact some models predict much stronger trends than are observed. It is important to understand the signatures of different physical structures to interpret specific lines of sight and future global surveys.

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arXiv2004

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, breaking to a flatter slope above 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.

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arXiv2003

The Fluctuating Intergalactic Radiation Field at Redshifts z = 2.3-2.9 from He II and H I Absorption towards HE 2347-4342

We provide an in-depth analysis of the He II and H I absorption in the intergalactic medium (IGM) at redshifts z = 2.3-2.9 toward HE 2347-4342, using spectra from the Far Ultraviolet Spectroscopic Explorer (FUSE) and the Ultraviolet-Visual Echelle Spectrograph (UVES) on the VLT telescope. Following up on our earlier study (Kriss et al. 2001, Science, 293, 1112), we focus here on two major topics: (1) small-scale variability (Delta z = 10^-3) in the ratio eta = N(He II)/N(H I); and (2) an observed correlation of high-eta absorbers (soft radiation fields) with voids in the (H I) Ly-alpha distribution. These effects may reflect fluctuations in the ionizing sources on scales of 1 Mpc, together with radiative transfer through a filamentary IGM whose opacity variations control the penetration of 1-5 ryd radiation over 30-40 Mpc distances. Owing to photon statistics and backgrounds, we can measure optical depths over the ranges 0.1 < tau(HeII) < 2.3 and 0.02 < tau(HI) < 3.9, and reliably determine values of eta = 4 tau(HeII)/tau(HI) over the range 0.1 to 460. Values of eta = 20-200 are consistent with models of photoionization by quasars with observed spectral indices alpha_s = 0-3. Values of eta > 200 may require additional contributions from starburst galaxies, heavily filtered quasar radiation, or density variations. Regions with eta < 30 may indicate the presence of local hard sources. We find that eta is higher in "void" regions, where H I is weak or undetected and 80% of the path length has eta > 100. These voids may be ionized by soft sources (dwarf starbursts) or by QSO radiation softened by escape from the AGN cores or transfer through the "cosmic web". The apparent differences in ionizing spectra may help to explain the 1.45 Gyr lag between the reionization epochs, z(HI) = 6.2 +/-0.2 and z(HeII) = 2.8 +/-0.2.

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arXiv2003

The Multiphase Intergalactic Medium towards PKS 2155-304

We study the cluster of H I and O VI absorption systems and the claimed detection of O VIII absorption from the intergalactic medium at z ~ 0.0567, associated with a group of galaxies toward the BL Lac object PKS 2155-304. As measured by spectrographs on the Hubble Space Telescope, Far Ultraviolet Spectroscopic Explorer, and Chandra, this system appears to contain gas at a variety of temperatures. We analyze this multi-phase gas in a clumpy-infall model. From the absence of C IV and Si III absorption in the Ly-alpha clouds, we infer metallicities less than 2.5-10% of solar values. The only metals are detected in two O VI absorption components, offset by +/- 400 km/s from the group barycenter (cz ~ 16,600 km/s). The O VI components may signify "nearside" and "backside" infall into the group potential well, which coincides with the claimed O VIII absorption. If the claimed O VIII detection is real, our analysis suggests that clusters of strong Ly-alpha and O VI absorbers, associated with groups of galaxies, may be the "signposts" of shock-heated, metal-enriched baryons. Through combined UV and X-ray spectra of H I and O VI, O VII, and O VIII, one may be able to clarify the heating mechanism of this multiphase gas.

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