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Register a new userTracing the Warm-Hot Intergalactic Medium at Low Redshift: X-ray Forest Observations Towards H1821+643
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We present a high resolution (R~500) X-ray spectrum of the bright quasar H1821+643 (z=0.297), obtained in a 470 ksec Chandra observation. We search for X-ray absorption by highly ionized metal species, OVII and OVIII in particular, at the redshifts of the six intervening OVI absorption systems known from UV studies. We detect features with >~2-sigma significance at the predicted OVII and OVIII wavelengths of one OVI system, at the OVII wavelength of a second, and at the NeIX wavelength of a third. We find two additional features of comparable strength (one OVII and one OVIII) within 1000 km/s of OVI redshifts. The 1-sigma constraints in the two detected OVI systems imply gas overdensities lower than the values delta>100 expected in virialized systems, suggesting that the absorption arises in lower density, filamentary structures. At the 2-sigma level, however, the physical constraints are weak. If we treat our 2-sigma detections of known OVI systems as real, but assume minimal OVII and OVIII in the other systems, we estimate [f(OVI)+f(OVII)+f(OVIII)]/f(OVI) = 32 +/- 9 for the average ratio of all highly ionized oxygen species to OVI. Combined with estimates of the total column density of OVI absorption per unit redshift, this ratio implies a total baryon fraction associated with detected OVI absorbers Omega_b(OVI)~0.03/h_70, a substantial fraction of the baryon density predicted by BBN, and larger than that in known low redshift components. Because of the limited S/N of the detections, these results must be treated with caution. Nonetheless, the combination of the OVI data with these X-ray forest measurements provides the most direct evidence to date for the pervasive, moderate density, shock-heated intergalactic medium predicted by leading cosmological scenarios. (Abridged.)
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We observed the Seyfert I active galaxy/broad line radio galaxy 3C120 with the Chandra high energy transmission gratings and present an analysis of the soft X-ray spectrum. We identify the strongest absorption feature (detected at >99.9% confidence) with O VIII Lya (FWHM=1010^{+295}_{-265} km/s), blueshifted by -5500 +/- 140 km/s from systemic velocity. The absorption may be due to missing baryons in warm/hot intergalactic medium (WHIGM) along the line-of-sight to 3C 120 at z=0.0147 +/- 0.0005, or it could be intrinsic to the jet of 3C 120. Assuming metallicities of 0.1 solar, we estimate an ionic column density of N_{O VIII}>3.4 times 10^{16} cm^{-2} for WHIGM and a filament depth of <19 h^{-1}_{70} Mpc. We find a baryon overdensity >56 relative to the critical density of a $Lambda$-dominated cold dark matter universe, which is in reasonable agreement with WHIGM simulations. We detect, at marginal significance, absorption of O VIII Lya at zsim 0 due to a hot medium in the Local Group. We also detect an unidentified absorption feature at sim 0.71 keV. Absorption features which might be expected along with O VIII Lya, were not significant statistically. Relative abundances of metals in the WHIGM and local absorbers may therefore be considerably different from solar.
The Warm-Hot Intergalactic Medium (WHIM) arises from shock-heated gas collapsing in large-scale filaments and probably harbours a substantial fraction of the baryons in the local Universe. Absorption-line measurements in the ultraviolet (UV) and in the X-ray band currently represent the best method to study the WHIM at low redshifts. We here describe the physical properties of the WHIM and the concepts behind WHIM absorption line measurements of H I and high ions such as O VI, O VII, and O VIII in the far-ultraviolet and X-ray band. We review results of recent WHIM absorption line studies carried out with UV and X-ray satellites such as FUSE, HST, Chandra, and XMM-Newton and discuss their implications for our knowledge of the WHIM.
Recent Cosmological measurements indicate that baryons comprise about four percent of the total mass-energy density of the Universe, which is in accord with the predictions arising from studies of the production of the lightest elements. It also is in agreement with the actual number of baryons detected at early times (redshifts>2). However, close to our own epoch (z<2), the number of baryons actually detected add up to just over half (~55 percent) of the number seen at z>2, meaning that about 45 percent are missing. Here we report a determination of the mass-density of a previously undetected population of baryons, in the warm-hot phase of the intergalactic medium. We show that this mass-density is consistent, within the uncertainties, with the mass-density of the missing baryons.
Today, the majority of the cosmic baryons in the Universe are not observed directly, leading to an issue of missing baryons at low redshift. Cosmological hydrodynamical simulations have indicated that a significant portion of them will be converted into the so-called Warm-Hot Intergalactic Medium (WHIM), with gas temperature ranging between 10$^5$-10$^7$K. While the cooler phase of this gas has been observed using O VI and Ne VIII absorbers at UV wavelengths, the hotter fraction detection relies mostly on observations of O VII and O VIII at X-ray wavelengths. Here, we target the forbidden line of [Fe XXI] $lambda$ 1354$unicode{x212B}$ which traces 10$^7$K gas at UV wavelengths, using more than one hundred high-spectral resolution (R$sim$49,000) and high signal to noise VLT/UVES quasar spectra, corresponding to over 600 hrs of VLT time observations. A stack of these at the position of known DLAs lead to a 5-$sigma$ limit of $mathrm{log[N([Fe,XXI])]<}$17.4 (${EW_{rest}<22}$m$unicode{x212B}$), three orders of magnitude higher than the expected column density of the WHIM $mathrm{log[N([Fe,XXI])]<}$14.5. This work proposes an alternative to X-ray detected 10$^7$K WHIM tracers, by targeting faint lines at UV wavelengths from the ground benefiting from higher instrumental throughput, enhanced spectral resolution, longer exposure times and increased number of targets. The number of quasar spectra required to reach this theoretical column density with future facilities including 4MOST, ELT/HIRES, MSE and the Spectroscopic Telescope appears challenging at present. Probing the missing baryons is essential to constrain the accretion and feedback processes which are fundamental to galaxy formation.
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.
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