Although galaxies, groups, and clusters contain ∼10% of the baryons, many more reside in the photoionized and shocked-heated intergalactic medium (IGM) and in the circumgalactic medium (CGM). We update the baryon census in the (H i) Lyα forest and warm-hot IGM (WHIM) at 105–6 K traced by O vi λ1032, 1038 absorption. From Enzo cosmological simulations of heating, cooling, and metal transport, we improve the H i and O vi baryon surveys using spatially averaged corrections for metallicity (Z/Z☉) and ionization fractions (fH i, fO vi). Statistically, the O vi correction product correlates with column density, (Z/Z☉)fO vi ≈ (0.015)(NO vi/1014 cm−2)0.70, with an NO vi-weighted mean of 0.01, which doubles previous estimates of WHIM baryon content. We also update the Lyα forest contribution to baryon density out to z = 0.4, correcting for the (1 + z)3 increase in absorber density, the (1 + z)4.4 rise in photoionizing background, and cosmological proper length dℓ/dz. We find substantial baryon fractions in the photoionized Lyα forest (28% ± 11%) and WHIM traced by O vi and broad-Lyα absorbers (25% ± 8%). The collapsed phase (galaxies, groups, clusters, CGM) contains 18% ± 4%, leaving an apparent baryon shortfall of 29% ± 13%. Our simulations suggest that ∼15% reside in hotter WHIM (T ⩾ 106 K). Additional baryons could be detected in weaker Lyα and O vi absorbers. Further progress requires higher-precision baryon surveys of weak absorbers, down to minimum column densities NH i ⩾ 1012.0 cm−2, NO vi ⩾ 1012.5 cm−2, NO vii ⩾ 1014.5 cm−2, using high signal-to-noise data from high-resolution UV and X-ray spectrographs.
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.
We present far-UV HST/COS spectra of four nearby BL Lac Objects. BL Lac spectra are dominated by a smooth, power-law continuum which arises in a relativistic jet. However, the spectra are not necessarily featureless; weak, broad- and/or narrow-line emission is sometimes seen in high-quality optical spectra. We present detections of Lya emission in HST/COS spectra of Mrk421 (z=0.030) and PKS2005-489 (z=0.071) as well as an archival HST/GHRS observation of Mrk501 (z=0.0337). Archival HST/STIS observations of PKS2155-304 (z=0.116) show no Lya emission to a very low upper limit. Using the assumption that the broad-line region (BLR) clouds are asymmetrically placed around the AGN, we use these measured Lya emission features to constrain either the relativistic Gamma values for the ionizing continuum produced by the jet (in the ionization-bounded case) or the mass of warm gas (in the density-bounded case). While realistic Gamma values can be obtained for all four cases, the values for Mrk421 and PKS2155-304 are high enough to suggest that covering factors of broad-line-region clouds of ~1-2% might be required to provide consistency with earlier values of Doppler boosting and viewing angles suggested for this class of BL Lacs. This discrepancy also exists in the case of M87, where the amount of Doppler boosting in our direction is expected to be minimal, again suggestive of a small covering factor of broad-line-region clouds. If, as these small covering factors might suggest, the assumptions of a density-bounded model could be more correct, then the observed Lya luminosities require that BL\,Lac/FRI nuclei possess very little warm gas (10^-4 to 10^-5 Msun) as suggested by Guilbert, Fabian & McCray (1983). If these clouds are in pressure balance with a hotter (~10^6 K) gas, the BLR contains too little mass to power the AGN by accretion alone.
We analysed data from the Cosmic Origins Spectrograph (COS) to characterize the spectral properties and outflows of the active galactic nucleus (AGN), Markarian 1513. Further investigation using previous data collected by the Space Telescope Imaging Spectrograph (STIS), Goddard High Resolution Spectrograph (GHRS), and International Ultraviolet Explorer (IUE) was used to examine variability in the outflows along with the AGN emission and continuum luminosity spanning 32 yr. The COS data contained two sets of intrinsic absorption systems. The first, which is associated with an outflow, was observed in Lyman α, N v, Si iv, and C iv, with an outflow velocity of −1521 ± 20 km s−1. This absorption system prevailed through the historical Hubble Space Telescope observations spanning 15 yr, and COS data revealed a previously unobserved Si iv outflow absorption feature. A second absorption system was observed at −17 ± 20 km s−1 in Lyman α, N v, and the blue component of C iv (1548.2 Å), indicating the presence of an intervening cloud, but not necessarily an outflow. Variability was also observed in the continuum levels of the AGN spectrum, which dropped by nearly a factor of 2 in both the power-law index and flux level between the GHRS and COS data.
We have observed a sample of 14 nearby () star-forming blue compact galaxies (BCGs) in the rest-frame far-UV (∼1150–2200 Å) using the Cosmic Origins Spectrograph on the Hubble Space Telescope. We have also generated a grid of stellar population synthesis models using the Starburst99 evolutionary synthesis code, allowing us to compare observations and theoretical predictions for the Si iv_1400 and C iv_1550 UV indices; both are comprised of a blend of stellar wind and interstellar lines and have been proposed as metallicity diagnostics in the UV. Our models and observations both demonstrate that there is a positive linear correlation with metallicity for both indices, and we find generally good agreement between our observations and the predictions of the Starburst99 models (with the models slightly under-estimating the value of the indices due to contributions from interstellar lines not simulated by a stellar population synthesis code). By combining the rest-frame UV observations with pre-existing rest-frame optical spectrophotometry of our BCG sample, we also directly compare the predictions of metallicity and extinction diagnostics across both wavelength regimes. This comparison reveals a correlation between the UV absorption and optical strong-line diagnostics, offering the first means of directly comparing interstellar medium (ISM) properties determined across different rest-frame regimes. Finally, using our Starburst99 model grid, we determine theoretical values for the short-wavelength UV continuum slope, , which can be used for determining extinction in rest-frame UV spectra of star-forming galaxies. We consider the implications of these results and discuss future work aimed at parameterizing these and other environmental diagnostics in the UV (a suite of diagnostics that could offer particular utility in the study of star-forming galaxies at high redshift) as well as the development of robust comparisons between ISM diagnostics across a broad wavelength baseline.
