Using the Cosmic Origins Spectrograph (COS) aboard the Hubble Space Telescope with both far-UV (FUV) and near-UV (NUV) gratings, we measure the ionizing spectra of two bright, intermediate-redshift quasars in their rest-frame extreme ultraviolet (EUV). The availability of both NUV and FUV spectra allows us to define the quasar continuum and correct for strong Lyman-limit systems (LLS) that fall in the gap between the FUV and optical. Each AGN has a prominent LLS, but the flux recovery shortward of their Lyman edges allows us to fit and restore the true AGN continuum. In the EUV (450-912 A) these AGN have flux distributions, F_ν ∝ ν^-α_ν, with spectral indices α_ν = 1.11±0.22 (SBS 1010+535, z_ AGN = 1.5086) and α_ν = 0.98±0.22 (HS 0747+4259, z_ AGN = 1.9006), both considerably harder than the mean index, α_ν = 1.41±0.15, in a COS composite spectrum of 159 UV-bright AGN. These two AGN are outliers in the index distribution, perhaps resulting from their extremely high UV luminosity (10^48 erg s^-1), estimated black-hole masses (0.5-1)×10^10 M_⊙, and effects on the inner accretion disk and Comptonized winds.

Using the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope with both far-UV (FUV) and near-UV (NUV) gratings, we measure the ionizing spectra of two bright, intermediate-redshift quasars in their rest-frame extreme ultraviolet (EUV). The availability of both NUV and FUV spectra allows us to define the quasar continuum and correct for strong Lyman-limit systems (LLSs) that fall in the gap between the FUV and optical. Each active galactic nucleus (AGN) has a prominent LLS, but the flux recovery shortward of their Lyman edges allows us to fit and restore the true AGN continuum. In the EUV (450-912 & Aring;), these AGN have flux distributions, F nu proportional to nu-alpha nu , with spectral indices alpha nu = 1.11 +/- 0.22 (SBS 1010+535, zAGN = 1.5086) and alpha nu = 0.98 +/- 0.22 (HS 0747+4259, zAGN = 1.9006), both considerably harder than the mean index, alpha nu = 1.41 +/- 0.15, in a COS composite spectrum of 159 UV-bright AGN. These two AGN are outliers in the index distribution, perhaps resulting from their extremely high UV luminosity (0.4-0.9) x 1048 erg s-1), estimated black-hole masses (0.5-1) x 1010 M circle dot, and effects on the inner accretion disk and Comptonized winds.

We report results from a Far Ultraviolet Spectrographic Explorer (FUSE) survey of interstellar molecular hydrogen (H-2) in the Galactic disk toward 139 O-type and early B-type stars at Galactic latitudes divide b divide <= 10 degrees, with updated photometric and parallax distances. H-2 absorption is measured using the far-UV Lyman and Werner bands, including strong R(0), R(1), and P(1) lines from rotational levels J = 0 and J = 1 and excited states up to J = 5 (sometimes J = 6 and 7). For each sight line, we report column densities N (H2), N (HI), N(J), and N (H) = N (HI) + 2N (H2) and the molecular fraction f (H2) = 2N (H2)/N (H). Our survey extends the 1977 Copernicus H-2 survey up to N (H) approximate to 5 x 10(21) cm(-2). The lowest rotational states have excitation temperatures and rms dispersions T (01) = 88 +/- 20 K and T (02) = 77 +/- 18 K, suggesting that J = 0, 1, 2 are coupled to the gas kinetic temperature. Populations of higher-J states exhibit mean excitation temperatures T (24) = 237 +/- 91 K and T (35) = 304 +/- 108 K, produced primarily by UV radiative pumping. Correlations of f (H2) with E(B - V) and N (H) show a transition to f (H2) >= 0.1 at N (H) greater than or similar to 10(21) cm(-2) and E(B - V) greater than or similar to 0.2, interpreted with an analytic model of H-2 formation-dissociation equilibrium and attenuation of the far-UV radiation field by self-shielding and dust opacity. Results of this disk survey are compared to previous FUSE studies of H-2 in translucent clouds, at high Galactic latitudes, and in the Magellanic Clouds. Using updated distances to the target stars, we find average sight-line values f (H2) = 0.20 and N (H)/E(B - V) = 6.07 x 10(21) cm(-2) mag(-1).

Using offset-corrected Gaia-EDR3 parallax measurements and spectrophotometric methods, we have determined distances for 69 massive stars in the Carina OB1 association and associated clusters: Trumpler 16 (21 stars), Trumpler 14 (20 stars), Trumpler 15 (3 stars), Bochum 11 (5 stars), and South Pillars region (20 stars). Past distance estimates to the Carina Nebula range from 2.2 to 3.6 kpc, with uncertainties arising from photometry and anomalous dust extinction. The EDR3 parallax solutions show considerable improvement over DR2, with typical errors $\sigma_{\varpi}/\varpi \approx$~3-5%. The O-type stars in the Great Carina Nebula lie at essentially the same distance ($2.35\pm0.08$ kpc), quoting mean and rms variance. The clusters have distances of $2.32\pm0.12$ kpc (Tr 16), $2.37\pm0.15$ kpc (Tr 14), $2.36\pm0.09$ kpc (Tr 15), and $2.33\pm0.12$ kpc (Bochum 11) in good agreement with the $\eta$ Car distance of around 2.3 kpc. O-star proper motions suggest internal (2D) velocity dispersions $\sim4$ km/s for Tr 14 and Tr 16. Reliable distances allow estimates of cluster sizes, stellar dynamics, luminosities, and fluxes of photoionizing radiation incident on photodissociation regions in the region. We estimate that Tr 14 and Tr 16 have half-mass radii $r_h = 1.5-1.8$ pc, stellar crossing times $t_{\rm cr} = r_h/v_m \approx 0.7-0.8$ Myr, and two-body relaxation times $t_{rh} \approx 40-80$ Myr. The underlying velocity dispersion for Tr 14, if a bound cluster, would be $v_m \approx 2.1^{+0.7}_{-0.4}$ km/s for $N = 7600^{+5800}_{-2600}$ stars. With the higher dispersions of the O-stars, mass segregation might occur slowly, on times scales of 3-6~Myr.

Quasars and other active galactic nuclei (AGN) are significant contributors to the metagalactic ionizing background at redshifts z < 3. Recent Hubble Space Telescope (HST)/Cosmic Origins Spectrograph (COS) composite spectra of AGN find a harder flux distribution in the Lyman continuum, (αs = 1.41 ± 0.15), compared to previous studies. This index appears to be inconsistent with observed He ii/H i absorption ratios (η) in the Lyα forest. We explore the effects of internal AGN absorption in the He ii (4 ryd) continuum using an analytic source-function model of the ionizing background in which the emissivity (jν) arises from quasars, reprocessed by the opacity (κν) of the intervening Lyα forest and distinct AGN escape fractions and at 1 ryd and 4 ryd, respectively. We also examine H i and He ii photoelectric heating from the reprocessed background, whose spectral index (αb > αs) depends on αs, the H i column density slope β, and the ratio . We compare the model to Lyα absorption lines of He ii and H i toward the quasar HE 2347−4342. Internal AGN absorption with but would increase the index by Δαb ≈ 0.3–1.0, corresponding to η = 60–200 for β ≈ 1.5–1.6, in agreement with HST/COS observations at z ≈ 2.5–2.9. The observed range of ratios, η < 200, constrains αb < 3.4 and . Individual AGN with softer spectra, αs > 1.7, and more internal He ii absorption could produce a few absorbers with η > 300, in proximity to AGN transverse to the sight line.

For application to surveys of interstellar matter and Galactic structure, we compute new spectrophotometric distances to 139 OB stars frequently used as background targets for UV spectroscopy. Many of these stars have updated spectral types and digital photometry with reddening corrections from the Galactic O-Star (GOS) spectroscopic survey. We compare our new photometric distances to values used in previous IUE and FUSE surveys and to parallax distances derived from Gaia-DR2, after applying a standard (0.03 mas) offset from the quasar celestial reference frame. We find substantial differences between photometric and parallax distances (at d > 1.5 kpc) with increasing dispersion when parallax errors exceed 8%. Differences from previous surveys arise from new GOS stellar classifications, especially luminosity classes, and from reddening corrections. We apply our methods to two OB associations. For Perseus OB1 (nine O-stars) we find mean distances of $2.47\pm0.57$ kpc (Gaia parallax) and $2.99\pm0.14$ kpc (photometric) using a standard grid of absolute magnitudes (Bowen et al. 2008). For 29 O-stars in Car OB1 associated with Trumpler-16, Trumpler-14, Trumpler-15, and Collinder-228 star clusters, we find $2.87\pm0.73$ kpc (Gaia parallax) and $2.60\pm0.28$ kpc (photometric). Using an alternative grid of O-star absolute magnitudes (Martins et al. 2005) shifts these photometric distances 7% closer. Improving the distances to OB-stars will require attention to spectral types, photometry, reddening, binarity, and the grid of absolute magnitudes. We anticipate that future measurements in Gaia-DR3 will improve the precision of distances to massive star-forming regions in the Milky Way.

A small survey of the UV-absorbing gas in 12 low- z galaxy groups has been conducted using the Cosmic Origins Spectrograph on board the Hubble Space Telescope . Targets were selected from a large, homogeneously selected sample of groups found in the Sloan Digital Sky Survey. A critical selection criterion excluded sight lines that pass close (<1.5 virial radii) to a group galaxy, to ensure absorber association with the group as a whole. Deeper galaxy redshift observations are used both to search for closer galaxies and also to characterize these 10 13.5 –10 14.5 M ⊙ groups, the most massive of which are highly virialized with numerous early-type galaxies (ETGs). This sample also includes two spiral-rich groups, not yet fully virialized. At group-centric impact parameters of 0.3–2 Mpc, these signal-to-noise ratios = 15–30 spectra detected H i absorption in 7 of 12 groups; high (O vi ) and low (Si iii ) ion metal lines are present in two-thirds of the absorption components. None of the three most highly virialized, ETG-dominated groups are detected in absorption. Covering fractions ≳50% are seen at all impact parameters probed, but do not require large filling factors despite an enormous extent. Unlike halo clouds in individual galaxies, group absorbers have radial velocities that are too low to escape the group potential well without doubt. This suggests that these groups are “closed boxes” for galactic evolution in the current epoch. Evidence is presented that the cool and warm group absorbers are not a pervasive intra-group medium (IGrM), requiring a hotter ( T ∼ 10 6 –10 7 K) IGrM to be present to close the baryon accounting.

We investigate the association between galaxies and metal-enriched and metal-deficient absorbers in the local universe (z < 0.16) using a large compilation of far-ultraviolet spectra of bright active galactic nuclei targets observed with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope. In this homogeneous sample of 18 O VI detections at N-O (VI) >= 13.5 cm(-2) and 18 nondetections at N-O (VI) < 13.5 cm(-2) using Ly alpha absorbers with N-H (I) >= 10(14) cm(-2) I, the maximum distance O VI extends from galaxies of various luminosities is similar to 0.6. Mpc, or similar to 5 virial radii, confirming and refining earlier results. This is an important value that must be matched by numerical simulations, which input the strength of galactic winds at the sub- grid level. We present evidence that the primary contributors to the spread of metals into the circum- and intergalactic media are sub-L* galaxies (0.25L* < L < L*). The maximum distances that metals are transported from these galaxies is comparable to, or less than, the size of a group of galaxies. These results suggest that, where groups are present, the metals produced by the group galaxies do not leave the group. Since many O VI nondetections in our sample occur at comparably close impact parameters as those of the metal-bearing absorbers, some more pristine intergalactic material appears to be accreting onto groups where it can mix with metal-bearing clouds.

We analyze the sources of free electrons that produce the large dispersion measures, (in units of cm−3 pc), observed toward fast radio bursts (FRBs). Individual galaxies typically produce from ionized gas in their disk, disk-halo interface, and circumgalactic medium. Toward an FRB source at redshift z, a homogeneous intergalactic medium (IGM) containing a fraction of cosmological baryons will produce , where . A structured IGM of photoionized Lyα absorbers in the cosmic web produces similar dispersion, modeled from the observed distribution, , of H i (Lyα-forest) absorbers in column density and redshift with ionization corrections and scaling relations from cosmological simulations. An analytic formula for DM(z) applied to observed FRB dispersions suggests that for an IGM containing a significant baryon fraction, . Future surveys of the statistical distribution, DM( , of FRBs identified with specific galaxies and redshifts can be used to calibrate the IGM baryon fraction and distribution of Lyα absorbers. Fluctuations in DM at the level ±10 cm−3 pc will arise from filaments and voids in the cosmic web.

To establish the connection between galaxies and UV-detected absorption systems in the local universe, a deep (g <= 20) and wide (similar to 20' radius) galaxy redshift survey is presented around 47 sight lines to UV-bright AGNs observed by the Cosmic Origins Spectrograph (COS). Specific COS science team papers have used this survey to connect absorbers to galaxies, groups of galaxies, and large-scale structures, including voids. Here we present the technical details of the survey and the basic measurements required for its use, including redshifts for individual galaxies and uncertainties determined collectively by spectral class (emission-line, absorption-line, and composite spectra) and completeness for each sight line as a function of impact parameter and magnitude. For most of these sight lines, the design criteria of > 90% completeness over a > 1 Mpc region down to less than or similar to 0.1 L* luminosities at z <= 0.1 allows a plausible association between low=z absorbers and individual galaxies. Lya covering fractions are computed to approximate the star-forming and passive galaxy populations using the spectral classes above. In agreement with previous results, the covering fraction of star-forming galaxies with L greater than or similar to 0.3 L* is consistent with unity inside one virial radius and declines slowly to >50% at four virial radii. On the other hand, passive galaxies have lower covering fractions (similar to 60%) and a shallower decline with impact parameter, suggesting that their gaseous halos are patchy but have a larger scale-length than star-forming galaxies. All spectra obtained by this project are made available electronically for individual measurement and use.

It has been known for decades that the observed number of baryons in the local Universe falls about 30–40 per cent short 1 , 2 of the total number of baryons predicted 3 by Big Bang nucleosynthesis, as inferred 4 , 5 from density fluctuations of the cosmic microwave background and seen during the first 2–3 billion years of the Universe in the so-called ‘Lyman α forest’ 6 , 7 (a dense series of intervening H i Lyman α absorption lines in the optical spectra of background quasars). A theoretical solution to this paradox locates the missing baryons in the hot and tenuous filamentary gas between galaxies, known as the warm–hot intergalactic medium. However, it is difficult to detect them there because the largest by far constituent of this gas—hydrogen—is mostly ionized and therefore almost invisible in far-ultraviolet spectra with typical signal-to-noise ratios 8 , 9 . Indeed, despite large observational efforts, only a few marginal claims of detection have been made so far 2 , 10 . Here we report observations of two absorbers of highly ionized oxygen (O vii ) in the high-signal-to-noise-ratio X-ray spectrum of a quasar at a redshift higher than 0.4. These absorbers show no variability over a two-year timescale and have no associated cold absorption, making the assumption that they originate from the quasar’s intrinsic outflow or the host galaxy’s interstellar medium implausible. The O vii systems lie in regions characterized by large (four times larger than average 11 ) galaxy overdensities and their number (down to the sensitivity threshold of our data) agrees well with numerical simulation predictions for the long-sought warm–hot intergalactic medium. We conclude that the missing baryons have been found.

We present a blind search for doublet intergalactic metal absorption with a method dubbed 'agnostic stacking'. Using a forward-modelling framework, we combine this with direct detections in the literature to measure the overall metal population. We apply this novel approach to the search for Ne VIII absorption in a set of 26 high-quality COS spectra. We probe to an unprecedented low limit of log N> 12.3 at 0.47 <= z <= 1.34 over a path-length Delta z = 7.36. This method selects apparent absorption without requiring knowledge of its source. Stacking this mixed population dilutes doublet features in composite spectra in a deterministic manner, allowing us to measure the proportion corresponding to Ne VIII absorption. We stack potential Ne VIII absorption in two regimes: absorption too weak to be significant in direct line studies (12.3 < log N < 13.7), and strong absorbers (log N > 13.7). We do not detect Ne VIII absorption in either regime. Combining our measurements with direct detections, we find that the Ne VIII population is reproduced with a power-law column density distribution function with slope beta = -1.86(0.26)(+0.18) and normalization log f(13.7) = -13.99(-0.23)(+0.20), leading to an incidence rate of strong Ne VIII absorbers dn/dz = 1.38(-0.82)(+0.97). We infer a cosmic mass density for Ne VIII gas with 12.3 < log N < 15.0 of Omega(Ne VIII) = 2.2(-1.2)(+1.6) x 10(-8), a value significantly lower that than predicted by recent simulations. We translate this density into an estimate of the baryon density Omega(b) approximate to 1.8 x 10(-3), constituting 4 per cent of the total baryonic mass.

We present an ultraviolet spectroscopic survey of strong H I absorbers in the intergalactic medium, probing their evolution over the last 6-7 Gyr at redshifts 0.24 <= z <= 0.84. We measure column densities N-H I (cm(-2)) from the pattern of Lyman-series absorption lines and flux decrement at the Lyman limit (LL) when available. We analyzed 220 H I absorbers in ultraviolet spectra of 102 active galactic nuclei (AGNs) taken by the Cosmic Origins Spectrograph on board the Hubble Space Telescope with G130M/G160M gratings (1134-1795 angstrom). For 158 absorbers with log N-H (I) >= 15, the mean frequency is dN/dz= 4.95 +/- 0.39 over path length Delta z = 31.94 (0.24 <= z <= 0.84). We identify eight Lyman limit systems (LLS, log N-H I >= 17.2) and 54 partial systems (pLLS) with 16.0 <= log N-H (I) < 17.2. Toward 159 AGNs in the range 0.01 < z(abs) < 0.84 with Delta z approximate to 48, we find four damped Lya absorbers (DLA) with (dN/dz)(DLA) 0.083(-0.040)(+0.066) at < z > = 0.18. The mean LLS frequency in z = 0.24- 0.48 is (dN/dz)(LLs) = 0.36(-0.13)(+0.20) fitted to N(z) = (0.25(-0.09) (+0.13)) (1 + z)(1.14). For 54 pLLSs, we find (dN/dz)(pLLS) = 1.69 +/- 0.23 at < z > = 0.39, a frequency consistent with gaseous halo sizes R approximate to 100 h(-1) kpc for (0.3-3L*) galaxies. A maximum-likelihood analysis yields a distribution f(N, z)= C0N-beta(1+ z)(gamma) with beta = 1.48 +/- 0.05 and gamma = 1.14(-0.89)(+0.88) for 15 <= log N-H (I) <= 17.5. The far-UV opacity gradient is d tau(eff)/dz approximate to (0.444)(1 + z)(1.14) over the range 15 <= log N-H (I) <= 17, implying mean LyC optical depth tau(eff) approximate to 0.3-0.5 toward sources at z = 1-2.

In preparation for a Hubble Space Telescope (HST) observing project using the Cosmic Origins Spectrograph (COS), the positions of all AGN targets having high-S/N far-UV G130M spectra were cross-correlated with a large catalog of low-redshift galaxy groups homogenously selected from the spectroscopic sample of the Sloan Digital Sky Survey (SDSS). Searching for targets behind only those groups at z = 0.1-0.2 (which places the O VI doublet in the wavelength region of peak COS sensitivity), we identified only one potential S/N = 15-20 target, FBQS 1010+3003. An O VI-only absorber was found in its G130M spectrum at z = 0.11326, close to the redshift of a foreground small group of luminous galaxies at z = 0.11685. Because there is no associated Ly alpha absorption, any characterization of this absorber is necessarily minimal; however, the O VI detection likely traces "warm" gas in collisional ionization equilibrium at T approximate to 3 x 10(5) K. While this discovery is consistent with being interface gas between cooler, photoionized clouds and a hotter intra-group medium, it could also be warm, interface gas associated with the circumgalactic medium (CGM) of the single closest galaxy. In this case, a detailed analysis of the galaxy distribution (complete to 0.2 L*) strongly favors the individual galaxy association. This analysis highlights the necessity of both high-S/N > 20 COS data and a deep galaxy redshift survey of the region in order to test more rigorously the association of O VI-absorbing gas with a galaxy group. A Cycle 23 HST/COS program is currently. targeting 10 UV-bright AGN behind 12 low-redshift galaxy groups to test the warm, group gas hypothesis.

We present basic data and modeling for a survey of the cool, photoionized circumgalactic medium (CGM) of low-redshift galaxies using far-UV QSO absorption-line probes. This survey consists of “targeted” and “serendipitous” CGM subsamples, originally described in Stocke et al. (Paper I). The targeted subsample probes low-luminosity, late-type galaxies at z < 0.02 with small impact parameters ( 〈 ρ 〉 = 71 kpc), and the serendipitous subsample probes higher luminosity galaxies at z ≲ 0.2 with larger impact parameters ( 〈 ρ 〉 = 222 kpc). Hubble Space Telescope and FUSE UV spectroscopy of the absorbers and basic data for the associated galaxies, derived from ground-based imaging and spectroscopy, are presented. We find broad agreement with the COS-Halos results, but our sample shows no evidence for changing ionization parameter or hydrogen density with distance from the CGM host galaxy, probably because the COS-Halos survey probes the CGM at smaller impact parameters. We find at least two passive galaxies with H i and metal-line absorption, confirming the intriguing COS-Halos result that galaxies sometimes have cool gas halos despite no on-going star formation. Using a new methodology for fitting H i absorption complexes, we confirm the CGM cool gas mass of Paper I, but this value is significantly smaller than that found by the COS-Halos survey. We trace much of this difference to the specific values of the low- z metagalactic ionization rate assumed. After accounting for this difference, a best-value for the CGM cool gas mass is found by combining the results of both surveys to obtain log ( M / M ⊙ ) = 10.5 ± 0.3 , or ∼30% of the total baryon reservoir of an L ≥ L * , star-forming galaxy.