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

We conduct an ultraviolet (HST and FUSE) spectroscopic survey of HI (Lyman lines) and seven metal ions (OVI, NV, CIV, CIII, SiIV, SiIII, FeIII) in the low-redshift intergalactic medium (IGM) at z<0.4. We analyzed 650 Lya absorbers over redshift pathlength Delta z=5.27, detecting numerous absorbers: 83 OVI systems, 39 CIII, 53 SiIII, 24 CIV, 24 NV, and so on. Our survey yields distributions in column density and estimates of the IGM baryon content and metallicities of C, N, O in the IGM. In the low-z IGM, we have accounted for ~40% of the baryons: 30% in the photoionized Lya forest and 10% in the (T=10^5-6 K) warm-hot intergalactic medium (WHIM) traced by OVI. Statistical metallicities of C, N, O ions are consistent with the canonical (z=0) value of 10% solar, with considerable scatter. Improved statistics for weak OVI absorbers allows us to estimate Omega_WHIM/Omega_b=0.073+-0.008 down to logN_OVI=13.4. NV absorption is well-correlated with OVI and both ions show similarly steep power-law indices dN/dz N^-beta with beta_OVI beta_NV 2 while beta_HI=1.7. We conclude that OVI and NV are reliable tracers of the portion of the WHIM at T=10^5-6 K. CIV may be present in both collisional and photoionized phases; N_CIV correlates poorly with both N_HI and N_OVI and beta_HI<beta_CIV<beta_OVI. The ions CIII, SiIII, and SiIV are well correlated with HI and show patterns typical of photoionization. Adjacent ion stages of the same element (CIII/IV and SiIII/IV) provide useful constraints on the photoionization parameter, logU=-1.5+-0.5. Comparison of SiIV and CIV with high-z surveys shows a modest increase in line density, consistent with increasing IGM metallicity at recent epochs.Comment: Submitted to ApJ, 27 pages in ApJ format (figure and discussion added

HST-COS Observations of AGN. I. Ultraviolet Composite Spectra of the Ionizing Continuum and Emission Lines

The ionizing fluxes from quasars and other active galactic nuclei (AGN) are critical for interpreting the emission-line spectra of AGN 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 AGN. Over the redshift range 0.026 < z < 1.44, COS samples the Lyman continuum and many far-UV emission lines (Lya 1216, C IV 1549, Si IV/OIV] 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, F_nu ~ nu^{alpha_nu}, shows a break at wavelengths below 1000 A, with spectral index alpha_nu = -0.68 +/- 0.14 in the FUV (1200-2000 A) steepening to alpha_nu = -1.41 +/- 0.21 in the EUV (500-1000 A). The COS EUV index is similar to that of radio-quiet AGN in the 2002 HST/FOS survey (alpha_nu = -1.57 +/- 0.17). We see no Lyman edge (tau_HI < 0.03) or He I 584 emission in the AGN composite. Our 22 AGN exhibit a substantial range of FUV/EUV spectral indices and a correlation with AGN luminosity and redshift, likely due to observing below the 1000 A break.Comment: 17 pages, 10 figs, accepted to Astrophysical Journal (revised AGN luminosities and fluxes, updated figures

Probing the Hydrodesulfurization Properties of Nickel-Rich Bimetallic Phosphides: Supported Catalysts and Encapsulated Nanoparticles

The need for improved hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) catalysts is being driven by increasingly demanding requirements for ultralow sulfur fuels and by the fact that sulfur levels in crude oil have been trending upward over recent decades. Current industrial catalysts are based on molybdenum sulfide (MoS2) and its highly anisotropic structure severely limits the number of exposed active sites. Transition metal phosphides (e.g. Ni2P/SiO2) are a new class of materials with promise to have improved properties relative to sulfided Ni-Mo/Al2O3 catalysts. The addition of a second metal can have large effects on the electronic and surface properties of a catalyst, which in turn can heavily influence its catalytic properties. Nickel-cobalt and nickel-iron phosphide catalysts (Ni2-xMxP, 0 ≤ x ≤ 2) are being investigated to determine the effect of cobalt and iron on the HDS properties of bimetallic phosphides of nickel. Catalysts prepared via conventional preparation methods using metal phosphate precursors supported on silica, and reduced by temperature programed reduction (TPR) are compared with solution-prepared metal phosphide nanoparticles that were subsequently encapsulated in a mesoporous silica shell to form core@shell nanostructures. Infrared spectra of adsorbed CO on the Ni2-xMxP catalyst surfaces show two significant peaks, one is attributed to CO bound to surface nickel atoms, while the second, smaller peak is attributed to CO bound to surface phosphorous atoms. As the second metal is added into the crystal lattice there is a shift to lower wavenumbers of the absorbance due to CO bonded to surface nickel atoms. This may indicate electron donation from cobalt to nickel, making for more electron-rich binding sites. The catalysts were tested for HDS activity and selectivity using dibenzothiophene and 4,6-dimethyldibenzothiophene. The trends in catalyst activities and turnover frequencies with Co and Fe content will be described and discussed in the context of the characterization results