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Aims. Doubly ionized silicon (Si III) is a powerful tracer of diffuse ionized gas inside and outside of galaxies. It can be observed in the local Universe in ultraviolet (UV) absorption against bright extragalactic background sources. We here present an extensive study of intervening Si III-selected absorbers and study the properties of the warm circumgalactic medium (CGM) around low-redshift (z <= 0.1) galaxies. Methods. We analyzed the UV absorption spectra of 303 extragalactic background sources, as obtained with the Cosmic Origins Spectrograph (COS) on-board the Hubble Space Telescope (HST). We developed a geometrical model for the absorption-cross section of the CGM around the local galaxy population and compared the observed Si III absorption statistics with predictions provided by the model. We also compared redshifts and positions of the absorbers with those of similar to 64 000 galaxies using archival galaxy-survey data to investigate the relation between intervening Si III absorbers and the CGM. Results. Along a total redshift path of Delta z approximate to 24, we identify 69 intervening Si III systems that all show associated absorption from other low and high ions (e.g., H I, Si II, Si IV, C II, C IV). We derive a bias-corrected number density of dN/dz(Si III) = 2.5 +/- 0.4 for absorbers with column densities log N(Si III) > 12.2, which is similar to 3 times the number density of strong Mg II systems at z = 0. This number density matches the expected cross section of a Si III absorbing CGM around the local galaxy population with a mean covering fraction of < f(c)> = 0.69. For the majority (similar to 60 percent) of the absorbers, we identify possible host galaxies within 300 km s(-1) of the absorbers and derive impact parameters rho < 200 kpc, demonstrating that the spatial distributions of Si III absorbers and galaxies are highly correlated. Conclusions. Our study indicates that the majority of Si III-selected absorbers in our sample trace the CGM of nearby galaxies within their virial radii at a typical covering fraction of similar to 70 percent. We estimate that diffuse gas in the CGM around galaxies, as traced by Si III, contains substantially more (more than twice as much) baryonic mass than their neutral interstellar medium.
We present a systematic study of weak intervening CaII absorbers at low redshift (z < 0.5), based on the analysis of archival high-resolution (R >= 45 000) optical spectra of 304 quasars and active galactic nuclei observed with VLT/UVES. Along a total redshift path of Delta z approximate to 100 we detected 23 intervening CaII absorbers in both the CaII H & K lines, with rest frame equivalent widths W-r,W-3934 = 15-799 m angstrom and column densities log N(CaII) = 11.25-13.04 (obtained by fitting Voigt-profile components). We obtain a bias-corrected number density of weak intervening CaII absorbers of dN/dz = 0.117 +/- 0.044 at < z(abs)> = 0.35 for absorbers with log N(CaII) >= 11.65 (W-r,W-3934 >= 32 m angstrom). This is similar to 2.6 times the value obtained for damped Lyman alpha absorbers (DLAs) at low redshift. All CaII absorbers in our sample show associated absorption by other low ions such as MgII and FeII; 45 percent of them have associated NaI absorption. From ionization modelling we conclude that intervening CaII absorption with log N(CaII) >= 11.5 arises in DLAs, sub-DLAs and Lyman-limit systems (LLS) at HI column densities of log N(HI) >= 17.4. Using supplementary HI information for nine of the absorbers we find that the CaII/HI ratio decreases strongly with increasing HI column density, indicating a column-density-dependent dust depletion of Ca. The observed column density distribution function of CaII absorption components follows a relatively steep power law, f(N) proportional to N-beta, with a slope of -beta = -1.68, which again points towards an enhanced dust depletion in high column density systems. The relatively large cross section of these absorbers together with the frequent detection of CaII absorption in high-velocity clouds (HVCs) in the halo of the Milky Way suggests that a considerable fraction of the intervening CaII systems trace (partly) neutral gas structures in the halos and circumgalactic environment of galaxies (i.e., they are HVC analogs). Based on the recently measured detection rate of CaII absorption in the Milky Way HVCs we estimate that the mean (projected) CaII covering fraction of galaxies and their gaseous halos is < f(c,CaII)> = 0.33. Using this value and considering all galaxies with luminosities L >= 0.05 L-star we calculate that the characteristic radial extent of (partly) neutral gas clouds with log N(HI) >= 17.4 around low-redshift galaxies is R-HVC approximate to 55 kpc.
We report on the detection of strongly varying intergalactic He II absorption in HST/COS spectra of two z(em) similar or equal to 3 quasars. From our homogeneous analysis of the He II absorption in these and three archival sightlines, we find a marked increase in the mean He II effective optical depth from <tau(eff, He II)> similar or equal to 1 at z similar or equal to 2.3 to <tau(eff, He II)> greater than or similar to 5 at z similar or equal to 3.2, but with a large scatter of 2 less than or similar to tau(eff, He II) less than or similar to 5 at 2.7 < z < 3 on scales of similar to 10 proper Mpc. This scatter is primarily due to fluctuations in the He II fraction and the He II-ionizing background, rather than density variations that are probed by the coeval Hi forest. Semianalytic models of He II absorption require a strong decrease in the He II-ionizing background to explain the strong increase of the absorption at z greater than or similar to 2.7, probably indicating He II reionization was incomplete at z(reion) greater than or similar to 2.7. Likewise, recent three-dimensional numerical simulations of He II reionization qualitatively agree with the observed trend only if He II reionization completes at z(reion) similar or equal to 2.7 or even below, as suggested by a large tau(eff, He II) greater than or similar to 3 in two of our five sightlines at z < 2.8. By doubling the sample size at 2.7 less than or similar to z less than or similar to 3, our newly discovered He II sightlines for the first time probe the diversity of the second epoch of reionization when helium became fully ionized.
We present a new approach to observationally constraining the spectral energy distribution of the intergalactic UV background by studying metal absorption systems. We study single-component metal line systems that exhibit various well-measured species. Among the observed transitions, at least two ratios of ionization stages from the same element are required, e. g. C III/C IV and Si III/Si IV. For each system photoionization models are constructed by varying the spectrum of the ionizing radiation. The spectral energy distribution can then be constrained by comparing the models with the observed column density ratios. Extensive tests with artificial absorbers show that the spectrum of the ionizing radiation cannot be reconstructed unambiguously, but it is possible to constrain the main characteristics of the spectrum. Furthermore, the resulting physical parameters of the absorber, such as ionization parameter, metallicity, and relative abundances, may depend strongly on the adopted ionizing spectrum. Even in case of well-fitting models, the uncertainties can be as high as similar to 0.5 dex for the ionization parameter and up to similar to 1.5 dex for the metallicity. Therefore, it is essential to know the hardness of the UV background when estimating the metallicity of the intergalactic medium. Applying the procedure to a small sample of 3 observed single-component metal line systems yields a soft ionizing radiation at z > 2 and a slightly harder spectrum at z < 2. The resulting energy distributions exhibit strong He II Lya re-emission features, suggesting that reprocessing by intergalactic He II is important. Comparing the observed systems to UV background spectra from the literature indicates that a recent model that includes sawtooth modulation due to reprocessing by intergalactic He II with delayed helium reionization fits the investigated systems very well.
To explore the ionization conditions in highly-ionized absorbers at high redshift, we study in detail two intervening O vi absorbers at z approximate to 2 toward the quasar PKS 1448-232, based on high (R approximate to 75 000) and intermediate (R approximate to 45 000) resolution optical VLT/UVES spectra. We find that both absorption systems are composed of several narrow subcomponents with typical Civ/O VI Doppler-parameters of b < 10 km s(-1). This implies that the gas temperatures are T < 10(5) K and that the absorbers are photoionized by the UV background. The system at z = 2.1098 represents a simple, isolated O VI absorber that has only two absorption components and is relatively metal-rich (Z similar to 0.6 solar). Ioinization modeling implies that the system is photoionized with O VI, C IV, and H I coexisting in the same gas phase. The second system at z = 2.1660 represents a complicated, multi-component absorption system with eight O VI components spanning almost 300 km s(-1) in radial velocity. The photoionization modeling implies that the metallicity is non-uniform and relatively low (<= 0.1 solar) and that the O VI absorption must arise in a gas phase that differs from that traced by C IV, C III, and H I. Our detailed study of the two O VI systems towards PKS 1448-232 shows that multi-phase, multi-component high-ion absorbers similar to the one at z = 2.1660 can be described by applying a detailed ionization modeling of the various subcomponents to obtain reliable measurements of the physical conditions and the metal abundances in the gas.
We present a multi-wavelength study of the Magellanic Stream (MS), a massive gaseous structure in the Local Group that is believed to represent material stripped from the Magellanic Clouds. We use ultraviolet, optical and radio data obtained with HST/COS, VLT/UVES, FUSE, GASS, and ATCA to study metal abundances and physical conditions in the Stream toward the quasar Fairall 9. Line absorption in the MS from a large number of metal ions and from molecular hydrogen is detected in up to seven absorption components, indicating the presence of multi-phase gas. From the analysis of unsaturated S II absorption, in combination with a detailed photoionization model, we obtain a surprisingly high alpha abundance in the Stream toward Fairall 9 of [S/H] = -0.30 +/- 0.04 (0.50 solar). This value is five times higher than what is found along other MS sightlines based on similar COS/UVES data sets. In contrast, the measured nitrogen abundance is found to be substantially lower ([N/H] = -1.15 +/- 0.06), implying a very low [N/alpha] ratio of -0.85 dex. The substantial differences in the chemical composition of MS toward Fairall 9 compared to other sightlines point toward a complex enrichment history of the Stream. We favor a scenario, in which the gas toward Fairall 9 was locally enriched with a elements by massive stars and then was separated from the Magellanic Clouds before the delayed nitrogen enrichment from intermediate-mass stars could set in. Our results support (but do not require) the idea that there is a metal-enriched filament in the Stream toward Fairall 9 that originates in the LMC.