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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 International Ultraviolet Explorer and Far Ultraviolet Spectroscopic Explorer 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 ± 0.57 kpc (Gaia parallax) and 2.99 ± 0.14 kpc (photometric) using a standard grid of absolute magnitudes. For 29 O stars in Car OB1 associated with Trumpler 16, Trumpler 14, Trumpler 15, and Collinder 228 star clusters, we find 2.87 ± 0.73 kpc (Gaia) and 2.60 ± 0.28 kpc (photometric). Using an alternative grid of O-star absolute magnitudes 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. [ABSTRACT FROM AUTHOR]

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 NH 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 Å). For 158 absorbers with log NH i ≥ 15, the mean frequency is over path length Δz = 31.94 (0.24 ≤ z ≤ 0.84). We identify eight Lyman limit systems (LLS, log NH i ≥ 17.2) and 54 partial systems (pLLS) with 16.0 ≤ log NH i < 17.2. Toward 159 AGNs in the range 0.01 < zabs < 0.84 with Δz ≈ 48, we find four damped Lyα absorbers (DLA) with at . The mean LLS frequency in z = 0.24–0.48 is fitted to . For 54 pLLSs, we find at , a frequency consistent with gaseous halo sizes R ≈ 100 h−1 kpc for (0.3–3L*) galaxies. A maximum-likelihood analysis yields a distribution with β = 1.48 ± 0.05 and for 15 ≤ log NH i ≤ 17.5. The far-UV opacity gradient is d τeff/dz ≈ (0.444)(1 + z)1.14 over the range 15 ≤ log NH i ≤ 17, implying mean LyC optical depth τeff ≈ 0.3–0.5 toward sources at z = 1–2. [ABSTRACT FROM AUTHOR]

We use high-quality, medium-resolution Hubble Space Telescope/Cosmic Origins Spectrograph (HST/COS) observations of 82 UV-bright active galactic nuclei (AGNs) at redshifts zAGN < 0.85 to construct the largest survey of the low-redshift intergalactic medium (IGM) to date: 5138 individual extragalactic absorption lines in H i and 25 different metal-ion species grouped into 2611 distinct redshift systems at zabs < 0.75 covering total redshift pathlengths ΔzH i = 21.7 and ΔzO vi = 14.5. Our semi-automated line-finding and measurement technique renders the catalog as objectively defined as possible. The cumulative column density distribution of H i systems can be parametrized = , with C14 = 25 ± 1 and β = 1.65 ± 0.02. This distribution is seen to evolve both in amplitude, , and slope β(z) = 1.75–0.31 z for z ≤ 0.47. We observe metal lines in 418 systems, and find that the fraction of IGM absorbers detected in metals is strongly dependent on . The distribution of O vi absorbers appears to evolve in the same sense as the Lyα forest. We calculate contributions to Ωb from different components of the low-z IGM and determine the Lyα decrement as a function of redshift. IGM absorbers are analyzed via a two-point correlation function in velocity space. We find substantial clustering of H i absorbers on scales of Δv = 50–300 km s−1 with no significant clustering at Δv ≳ 1000 km s−1. Splitting the sample into strong and weak absorbers, we see that most of the clustering occurs in strong, NH i ≳ 1013.5 cm−2, metal-bearing IGM systems. The full catalog of absorption lines and fully reduced spectra is available via the Mikulski Archive for Space Telescopes (MAST) as a high-level science product at http://archive.stsci.edu/prepds/igm/. [ABSTRACT FROM AUTHOR]

The rest-frame ultraviolet (UV) spectra of active galactic nuclei (AGNs) are important diagnostics of both accretion disk physics and their contribution to the metagalactic ionizing UV background. Though the mean AGN spectrum is well characterized with composite spectra at wavelengths greater than 912 Å, the shorter-wavelength extreme-UV (EUV) remains poorly studied. In this third paper in a series on the spectra of AGNs, we combine 11 new spectra taken with the Cosmic Origins Spectrograph on the Hubble Space Telescope with archival spectra to characterize the typical EUV spectral slope of AGNs from λrest ∼ 850 Å down to λrest ∼ 425 Å. Parameterizing this slope as a power law, we obtain Fν ∝ ν−0.72±0.26, but we also discuss the limitations and systematic uncertainties of this model. We identify broad emission features in this spectral region, including emission due to ions of O, Ne, Mg, and other species, and we limit the intrinsic He i 504 Å photoelectric absorption edge opacity to τHe i < 0.047. [ABSTRACT FROM AUTHOR]

Recent suggestions of a “photon underproduction crisis” have generated concern over the intensity and spectrum of ionizing photons in the metagalactic ultraviolet background (UVB). The balance of hydrogen photoionization and recombination determines the opacity of the low-redshift intergalactic medium (IGM). We calibrate the hydrogen photoionization rate () by comparing Hubble Space Telescope spectroscopic surveys of the low-redshift column density distribution of H i absorbers and the observed () mean Lyα flux decrement, , to new cosmological simulations. The distribution, , is consistent with an increased UVB that includes contributions from both quasars and galaxies. Our recommended fit, for , corresponds to unidirectional LyC photon flux cm−2 s−1 at z = 0. This flux agrees with observed IGM metal ionization ratios (C iii/C iv and Si iii/Si iv) and suggests a 25%–30% contribution of Lyα absorbers to the cosmic baryon inventory. The primary uncertainties in the low-redshift UVB are the contribution from massive stars in galaxies and the LyC escape fraction (), a highly directional quantity that is difficult to constrain statistically. We suggest that both quasars and low-mass starburst galaxies are important contributors to the ionizing UVB at . Their additional ionizing flux would resolve any crisis in photon underproduction. [ABSTRACT FROM AUTHOR]