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

The low-redshift intergalactic medium as seen in archival legacy Hubble/STIS and FUSE data

We present a comprehensive catalog of ultraviolet HST/STIS and FUSE absorbers in the low-redshift IGM at z<0.4. The catalog draws from the extensive literature on IGM absorption, and it reconciles discrepancies among previous catalogs through a critical evaluation of all reported absorption features in light of new HST/COS data. We report on 746 HI absorbers down to a rest-frame equivalent width of 12 milliAngstroms over a maximum redshift path length Deltaz=5.38. We also confirm 111 OVI absorbers, 29 CIV absorbers, and numerous absorption features due to other metal ions. We characterize the distribution of absorber line frequency as a function of column density as a power law, dN/dz \propto N^{-beta}, where beta=2.08+-0.12 for OVI and beta=1.68+-0.03 for HI. Utilizing a more sophisticated accounting technique than past work, the catalog accounts for ~43% of the baryons: 24+-2% in the photoionized Ly-alpha forest and 19+-2% in the WHIM as traced by OVI. We discuss the large systematic effects of various assumed metallicities and ionization states on these calculations, and we implement recent simulation results in our estimates.Comment: 18 pages, 9 figures, 2 machine readable tables; Submitted to The Astrophysical Journa

Tracing the Cosmic Metal Evolution in the Low-Redshift Intergalactic Medium

Using the Cosmic Origins Spectrograph aboard the Hubble Space Telescope, we measured the abundances of six ions (C III, C IV, Si III, Si IV, N V, O VI) in the low-redshift (z < 0.4) intergalactic medium and explored C and Si ionization corrections from adjacent ion stages. Both C IV and Si IV have increased in abundance by a factor of ~10 from z = 5.5 to the present. We derive ion mass densities, (rho_ion) = (Omega_ion)(rho_cr) with Omega_ion expressed relative to closure density. Our models of the mass-abundance ratios, (Si III / Si IV) = 0.67(+0.35,-0.19), (C III / C IV) = 0.70(+0.43,-0.20), and (Omega_CIII + Omega_CIV) / (Omega_SiIII + Omega_SiIV) = 4.9(+2.2,-1.1), are consistent with a hydrogen photoionization rate Gamma_H = (8 +/- 2) x 10^{-14} s^{-1} at z < 0.4 and specific intensity I_0 = (3 +/- 1) x 10^{-23} erg/(cm^2 s Hz sr) at the Lyman limit. We find mean photoionization parameter log U = -1.5 +/- 0.4, baryon overdensity Delta_b = 200 +/- 50, and Si/C enhanced to three times its solar ratio (enhancement of alpha-process elements). We compare these metal abundances to the expected IGM enrichment and abundances in higher photoionized states of carbon (C V) and silicon (Si V, Si VI, Si VII). Our ionization modeling infers IGM metal densities of (5.4 +/- 0.5) x 10^5 M_sun / Mpc^3 in the photoionized Lya forest traced by the C and Si ions and (9.1 +/- 0.6) x 10^5 M_sun / Mpc^3 in hotter gas traced by O VI. Combining both phases, the heavy elements in the IGM have mass density rho_Z = (1.5 +/- 0.8) x 10^6 M_sun / Mpc^3 or Omega_Z = 10^{-5}. This represents 10 +/- 5 percent of the metals produced by (6 +/- 2) x 10^8 M_sun / Mpc^3 of integrated star formation with yield y_m = 0.025 +/- 0.010. The missing metals at low redshift may reside within galaxies and in undetected ionized gas in galaxy halos and circumgalactic medium.Comment: 19 pages, 7 figures. Accepted to Astrophysical Journa

Prepared for Bioterrorism Events? A Study of the Grain and Oilseed Sector

One of the most crucial problems facing the U.S. economy is the possibility of a terrorist attack on its food sector. The implications can be profound for its stakeholders, who are highly dependent on this sector for their economic livelihood as well as their food supplies. The U.S. Bioterrorism Act of 2002 was enacted to improve the ability of the United States to prevent, prepare for and respond to bioterrorism and other public health emergencies. One of the important features of the U.S. Bioterrorism Act of 2002 is its emphasis on prevention, a change from prior legislation that focused on punishments after an incidence had occurred. The U.S. Bioterrorism Act does not address food safety issues in general; its focus is to prevent intentional contamination. The objective of this study was to assess the preparedness to potential bioterrorism in the grain and oilseed sector based on facility security expenditures and history of security breaches. The study was conducted as a research activity under the multistate project NC-1016 “Economic Assessment of Changes in Trade Arrangements, Bio-terrorism Threats and Renewable Fuels Requirements on the U.S. Grain and Oilseed Sector.” In addition to assessing preparedness, the study investigated the relationship between adoption of security measures and breaches in facility security. Finally the study documents, for a small sample, the extent to which grain and oilseed facilities appear to be following regulations that implement the U.S. Bioterrorism Act.bioterrorism, grain and oilseed sector, Bioterrorism Act of 2002, NC-1016, Agribusiness, Agricultural and Food Policy, Marketing, Political Economy, I18, K23, Q13, Q18,

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