Charles W. Danforth

THE COSMIC ORIGINS SPECTROGRAPH

The Cosmic Origins Spectrograph (COS) is a moderate-resolution spectrograph with unprecedented sensitivity that was installed into the Hubble Space Telescope (HST) in 2009 May, during HST Servicing Mission 4 (STS-125). We present the design philosophy and summarize the key characteristics of the instrument that will be of interest to potential observers. For faint targets, with flux F ? 1.0 ? 10?14?erg?cm?2?s?1 ??1, COS can achieve comparable signal to noise (when compared to Space Telescope Imaging Spectrograph echelle modes) in 1%-2% of the observing time. This has led to a significant increase in the total data volume and data quality available to the community. For example, in the first 20 months of science operation (2009 September-2011 June) the cumulative redshift pathlength of extragalactic sight lines sampled by COS is nine times than sampled at moderate resolution in 19 previous years of Hubble observations. COS programs have observed 214 distinct lines of sight suitable for study of the intergalactic medium as of 2011 June. COS has measured, for the first time with high reliability, broad Ly? absorbers and Ne VIII in the intergalactic medium, and observed the He II reionization epoch along multiple sightlines. COS has detected the first CO emission and absorption in the UV spectra of low-mass circumstellar disks at the epoch of giant planet formation, and detected multiple ionization states of metals in extra-solar planetary atmospheres. In the coming years, COS will continue its census of intergalactic gas, probe galactic and cosmic structure, and explore physics in our solar system and Galaxy.

The Low-z Intergalactic Medium. III. H I and Metal Absorbers at z < 0.4

We conduct an ultraviolet (HST and FUSE) spectroscopic survey of H I (Lyman lines) and seven metal ions (O VI, N V, C IV, C III, Si IV, Si III, Fe III) in the low-redshift IGM at z < 0.4. We analyzed 650 Lyα absorbers over redshift path length Δ z = 5.27, detecting numerous absorbers: 83 O VI systems, 39 C III, 53 Si III, 24 C IV, 24 N V, and so on. In the low-z IGM, we have accounted for ~40% of the baryons: 30% in the photoionized Lyα forest and 10% in the (T = 105–106) WHIM traced by O VI. Statistical metallicities are consistent with the canonical value of 10% solar, with considerable scatter. Improved statistics for weak absorbers allows us to estimate ΩWHIM/Ωb = 0.073 ± 0.008 down to log NO VI = 13.4 and 0.086 ± 0.008 down to log NO VI = 13.0. The O VI absorber line frequency, d/dz = 40−8+14, down to 10 mA equivalent width suggests a 250-300 kpc extent of metals around dwarf galaxies. Many absorbers appear to contain multiphase gas, with both collisional ionization and photoionization determining the ionization state. N V absorption is well correlated with O VI, and both ions show similarly steep power-law indices d/dz ∝ N−β with βO VI ≈ βN V ≈ 2 while βH I. We conclude that O VI and N V are reliable tracers of the portion of the WHIM at T ≈ 105–106 K. C IV may be present in both collisional and photoionized phases; NC IV correlates poorly with both NH I and NO VI and βH I < βC IV < βO VI. The ions C III, Si III, and Si IV are well correlated with H I and show patterns typical of photoionization. Adjacent ion stages of the same element (C III/IV and Si III/IV) provide useful constraints on the photoionization parameter, log U ≈ − 1.5 ± 0.5.

The Galaxy Environment of O VI Absorption Systems

We combine a FUSE sample of O VI 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 ≥ 1013.2 cm-2 lie within 800 h kpc of the nearest galaxy, with no compelling evidence for O VI absorbers in voids. The O VI absorbers often appear to be associated with environments of individual galaxies. Gas with 10% ± 5% solar metallicity (O VI and C III) has a median spread in distance of 350-500 h kpc around L galaxies and 200-270 h kpc around 0.1L galaxies (ranges reflect uncertain metallicities of gas undetected in Lyα absorption). In order to match the O VI line frequency, (d/dz) ≈ 20 for N ≥ 1013.2 cm-2, galaxies with L ≤ 0.1L must contribute to the cross section. Lyα absorbers with N ≥ 1013.2 cm-2 cover ~50% of the surface area of typical galaxy filaments. Two-thirds of these show O VI and/or C III absorption, corresponding to a 33%-50% covering factor at 0.1 Z☉ and suggesting that metals are spread to a maximum distance of 800 h kpc, within typical galaxy supercluster filaments. Approximately 50% of the O VI absorbers have associated Lyα line pairs with separations (Δv)Lyα = 50-200 km s-1. These pairs could represent shocks at the speeds necessary to create copious O VI, located within 100 h kpc of the nearest galaxy and accounting for much of the two-point correlation function of low-z Lyα forest absorbers.

A LARGE RESERVOIR OF IONIZED GAS IN THE GALACTIC HALO: IONIZED SILICON IN HIGH-VELOCITY AND INTERMEDIATE-VELOCITY CLOUDS

The low Galactic halo is enveloped by a sheath of ionized, low-metallicity gas, which can provide a substantial (1 M_sun/yr) cooling inflow to replenish star formation in the disk. Using absorption spectra from the HST and FUSE toward 37 active galactic nuclei at high latitude, we detect widespread interstellar SiIII 1206.5 absorption: 61 high-velocity clouds (HVCs) along 30 sight lines and 22 intermediate-velocity clouds (IVCs) along 20 sight lines. We find a segregation of redshifted and blueshifted absorbers across the Galactic rotation axis at l=180, consistent with a lag in the rotation velocity above the Galactic plane. The HVC sky coverage is large (81+-5% for 30/37 directions) with SiIII optical depth typically 4-5 times that of OVI 1032. The mean HVC column density per sight line, =13.42+-0.21, corresponds to total column density N_HII~6x10^18)/(Z_Si/0.2Z_sun) of ionized low-metallicity gas, similar to that inferred in OVI. This reservoir could total 10^8 M_sun and produce a mass infall rate ~1 M_sun/yr. By modeling SiII, SiIII, SiIV, and HI in a subset of absorbers, we constrain the mean photoionization parameter in the low halo, = -3.0 (+0.3,-0.4), approximately ten times lower than observed in the low-redshift intergalactic medium. The metallicities in some HVCs, derived from [SiII/HI], are 10-30% solar, whereas values found from all three silicon ions are lower in the pure-photoionization models. Thse formally lower metallicities are highly uncertain, since some of the higher ions may be collisionally ionized. The mean Si metallicities, = -2.1(+1.1,-0.3) in 17 HVCs and -1.0(+0.6,-1.0) in 19 IVCs, are somewhat uncertain owing to ionization modeling, but consistent with the median photometric metallicity, [Fe/H]=-1.46+-0.30, for ~2e5 halo F/G stars in SDSS.

HST-COS OBSERVATIONS OF AGNs. I. ULTRAVIOLET COMPOSITE SPECTRA OF THE IONIZING CONTINUUM AND EMISSION LINES*

The ionizing fluxes from quasars and other active galactic nuclei (AGNs) are critical for interpreting the emission-line spectra of AGNs and for photoionization and heating of the intergalactic medium. Using ultraviolet spectra from the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST), we have directly measured the rest-frame ionizing continua and emission lines for 22 AGNs. Over the redshift range 0.026 < z < 1.44, COS samples the Lyman continuum and many far-UV emission lines (Ly? ?1216, C IV ?1549, Si IV/O IV] ?1400, N V ?1240, O VI ?1035). Strong EUV emission lines with 14-22 eV excitation energies (Ne VIII ??770, 780, Ne V ?569, O II ?834, O III ?833, ?702, O IV ?788, 608, 554, O V ?630, N III ?685) suggest the presence of hot gas in the broad emission-line region. The rest-frame continuum, , shows a break at wavelengths ? < 1000 ?, with spectral index ?? = ?0.68 ? 0.14 in the FUV (1200-2000 ?) steepening to ?? = ?1.41 ? 0.21 in the EUV (500-1000 ?). The COS EUV index is similar to that of radio-quiet AGNs in the 2002 HST/FOS survey (?? = ?1.57 ? 0.17). We see no Lyman edge (?H I < 0.03) or He I ?584 emission in the AGN composite. Our 22 AGNs exhibit a substantial range of FUV/EUV spectral indices and a correlation with AGN luminosity and redshift, likely due to observing below the 1000 ? spectral break.

Quasar outflows and AGN feedback in the extreme UV: HST/COS observations of HE 0238−1904

Spectroscopic observations of quasar outflows at rest-frame 500-1000 Angstrom have immense diagnostic power. We present analyses of such data, where absorption troughs from three important ions are measured: first, O IV and O IV* that allow us to obtain the distance of high ionization outflows from the AGN; second, Ne VIII and Mg X that are sensitive to the very high ionization phase of the outflow. Their inferred column densities, combined with those of troughs from O VI, N IV, and H I, yield two important results: 1) The outflow shows two ionization phases, where the high ionization phase carries the bulk of the material. This is similar to the situation seen in x-ray warm absorber studies. Furthermore, the low ionization phase is inferred to have a volume filling factor of 10^(-5)-10^(-6). 2) From the O IV to O IV* column density ratio, and the knowledge of the ionization parameter, we determine a distance of 3000 pc. from the outflow to the central source. Since this is a typical high ionization outflow, we can determine robust values for the mass flux and kinetic luminosity of the outflow: 40 solar masses per year and 10^45 ergs/s, respectively, where the latter is roughly equal to 1% of the bolometric luminosity. Such a large kinetic luminosity and mass flow rate measured in a typical high ionization wind suggests that quasar outflows are a major contributor to AGN feedback mechanisms.

The Low-z Intergalactic Medium. II. Lyβ, O VI, and C III Forest

We present the results of a large survey of H I, O VI, and C III absorption lines in the low-redshift (z 30 m?, we calculate a typical IGM absorber size r0 ~ 400 kpc, similar to scales derived by other means. The COG-derived b-values show that H I samples material with T < 105 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 O VI and C III observations with a single phase. Instead, the observations require a multiphase IGM in which H I and C III arise in photoionized regions, while O VI is produced primarily through shocks. From the multiphase ratio N/N, we infer the IGM metallicity to be ZC = 0.12 Z?, similar to our previous estimate of ZO = 0.09 Z? from O VI.

THE FIRM REDSHIFT LOWER LIMIT OF THE MOST DISTANT TeV-DETECTED BLAZAR PKS 1424+240

We present the redshift lower limit of z ≥ 0.6035 for the very high energy (VHE; E ≥ 100 GeV) emitting blazar PKS 1424+240 (PG 1424+240). This limit is inferred from Lyβ and Lyγ absorption observed in the far-ultraviolet spectra from the Hubble Space Telescope/Cosmic Origins Spectrograph. No VHE-detected blazar has shown solid spectroscopic evidence of being more distant. At this distance, VHE observations by VERITAS are shown to sample historically large gamma-ray opacity values at 500 GeV, extending beyond τ = 4 for low-level models of the extragalactic background light (EBL) and beyond τ = 5 for high levels. The majority of the z = 0.6035 absorption-corrected VHE spectrum appears to exhibit a lower flux than an extrapolation of the contemporaneous Large Area Telescope power-law fit beyond 100 GeV. However, the highest energy VERITAS point is the only point showing agreement with this extrapolation, possibly implying the overestimation of the gamma-ray opacity or the onset of an unexpected VHE spectral feature. A curved log parabola is favored when fitting the full range of gamma-ray data (0.5-500 GeV). While fitting the absorption-corrected VHE data alone results in a harder differential power law than that from the full range, the indices derived using three EBL models are consistent with the physically motivated limit set by Fermi acceleration processes.

HST-COS Observations Of AGNs. II. Extended Survey Of Ultraviolet Composite Spectra From 159 Active Galactic Nuclei

The ionizing fluxes from quasars and other active galactic nuclei (AGNs) are critical for interpreting their emission-line spectra and for photoionizing and heating the intergalactic medium. Using far-ultraviolet (FUV) spectra from the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST), we directly measure the rest-frame ionizing continua and emission lines for 159 AGNs at redshifts 0.001 < z AGN < 1.476 and construct a composite spectrum from 475 to 1875 A. We identify the underlying AGN continuum and strong extreme ultraviolet (EUV) emission lines from ions of oxygen, neon, and nitrogen after masking out absorption lines from the H I Lyα forest, 7 Lyman-limit systems ( cm–2) and 214 partial Lyman-limit systems (). The 159 AGNs exhibit a wide range of FUV/EUV spectral shapes, , typically with –2 ≤ αν ≤ 0 and no discernible continuum edges at 912 A (H I) or 504 A (He I). The composite rest-frame continuum shows a gradual break at λbr ≈ 1000 A, with mean spectral index αν = –0.83 ± 0.09 in the FUV (1200-2000 A) steepening to αν = –1.41 ± 0.15 in the EUV (500-1000 A). We discuss the implications of the UV flux turnovers and lack of continuum edges for the structure of accretion disks, AGN mass inflow rates, and luminosities relative to Eddington values.

AN HST/COS SURVEY OF THE LOW-REDSHIFT INTERGALACTIC MEDIUM. I. SURVEY, METHODOLOGY, AND OVERALL RESULTS*

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 z(AGN) \textless 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 z(abs) \textless 0.75 covering total redshift pathlengths Delta z(HI) = 21.7 and Delta z(O 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 dN (\textgreater N)/dz = C-14 (N/10(14) cm(-2))(-(beta-1)), with C-14 = 25 +/- 1 and beta = 1.65 +/- 0.02. This distribution is seen to evolve both in amplitude, C-14 infinity (1+z)(2.3 +/- 0.1), and slope beta(z) = 1.75-0.31 z for z \textless= 0.47. We observe metal lines in 418 systems, and find that the fraction of IGM absorbers detected in metals is strongly dependent on N-H I. The distribution of O VI absorbers appears to evolve in the same sense as the Ly alpha forest. We calculate contributions to Omega(b) from different components of the low-z IGM and determine the Ly alpha 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 Delta v = 50-300 km s(-1) with no significant clustering at Delta(v) greater than or similar to 1000 km s(-1). Splitting the sample into strong and weak absorbers, we see that most of the clustering occurs in strong, N-H I greater than or similar to 10(13.5) 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/.