The Influence of the Photoionizing Radiation Spectrum on Metal-Line Ratios in Ly alpha Forest Clouds

Abstract

Recent measurements of Si IV/C IV ratios in the high-redshift Ly-alpha forest (Songaila & Cowie, AJ, 112, 335 [1996a]; Savaglio et al., A&A, in press [1997]) have opened a new window on chemical enrichment and the first generations of stars. However, the derivation of accurate Si/C abundances requires reliable ionization corrections, which are strongly dependent on the spectral shape of the metagalactic ionizing background and on the ``local effects'' of hot stars in nearby galaxies. Recent models have assumed power-law quasar ionizing backgrounds plus a decrement at 4 Ryd to account for He II attenuation in intervening clouds. However, we show that realistic ionizing backgrounds based on cosmological radiative transfer models produce more complex ionizing spectra between 1-5 Ryd that are critical to interpreting ions of Si and C. We also make a preliminary investigation of the effects of He II ionization front non-overlap. Because the attenuation and re-emission by intervening clouds enhance Si IV relative to C IV, the observed high Si IV/C IV ratios do not require an unrealistic Si overproduction [Si/C $\ge 3(Si/C{)}_{\odot }$]. If the ionizing spectrum is dominated by ``local effects'' from massive stars, even larger Si IV/C IV ratios are possible. However, unless stellar radiation dominates quasars by more than a factor of 10, we confirm the evidence for some Si overproduction by massive stars; values Si/C $\approx 2(Si/C{)}_{\odot }$ fit the measurements better than solar abundances. Ultimately, an adequate interpretation of the ratios of C IV, Si IV, and C II may require hot, collisionally ionized gas in a multiphase medium.

18 pages, latex, 8 figures, postscript; accepted by Astronomical Journal, to appear in vol. 113, no. 5 (May, 1997)

Cited by in corpus (56)

• On the properties of massive Population III stars and metal-free stellar populations
• Absorption-Line Probes of Gas and Dust in Galactic Superwinds
• The thermal history of the intergalactic medium
• The observed ionization rate of the intergalactic medium and the ionizing emissivity at z >5: Evidence for a photon starved and extended epoch of reionization
• The Physics of the Intergalactic Medium
• Characterizing the Circumgalactic Medium of Nearby Galaxies with HST/COS and HST/STIS Absorption-Line Spectroscopy
• Observational Signatures of the First Quasars
• The Ultraviolet Spectroscopic Properties of Local Starbursts: Implications at High-Redshift
• Zero-Metallicity Stars and the Effects of the First Stars on Reionization
• HeII Reionization and its Effect on the IGM
• The Low-z Intergalactic Medium. III. HI and Metal Absorbers at z<0.4
• The Escape of Ionizing Photons from OB Associations in Disk Galaxies: Radiation Transfer Through Superbubbles
• The High-Redshift He II Gunn-Peterson Effect: Implications and Future Prospects
• A Large Reservoir of Ionized Gas in the Galactic Halo: Ionized Silicon in High-Velocity and Intermediate-Velocity Clouds
• The Local Lyman-alpha Forest. II: Distribution of HI Absorbers, Doppler Widths, and Baryon Content
• The Redshift Evolution of the Metagalactic Ionizing Flux Inferred from Metal Line Ratios in the Lyman Forest
• Metals in the IGM approaching the re-ionization epoch: results from X-shooter at the VLT
• Highly Ionized High Velocity Clouds: Intergalactic Gas in the Local Group or Distant Gas in the Galactic Halo?
• The Impact of Starbursts on the Circumgalactic Medium
• A Low Upper Limit to the Lyman Continuum Emission of two galaxies at z 3
• The Statistical and Physical Properties of the Low Redshift Lyman Alpha Forest Observed with HST/STIS
• Temperature fluctuations in the intergalactic medium
• Characterizing the Circumgalactic Medium of Nearby Galaxies with HST/COS and HST/STIS Absorption-Line Spectroscopy: II. Methods and Models
• Highly Ionized High-Velocity Clouds toward PKS 2155-304 and Markarian 509
• An ionizing UV background dominated by massive stars
• A Lower Limit to the Universal Density of Metals at z \sim 3
• On the importance of local sources of radiation for quasar absorption line systems
• Lyman Continuum Escape from Inhomogeneous ISM
• Spatial fluctuations in the spectral shape of the UV background at 2<z<3 and the reionization of helium
• The Contribution of Population III to the Enrichment and Preheating of the Intracluster Medium
• The evolution of the physical state of the IGM
• A Cluster of Low-Redshift Lyman-alpha Clouds toward PKS 2155-304. I. Limits on Metals and D/H
• Properties of QSO Metal Line Absorption Systems at High Redshifts: Nature and Evolution of the Absorbers and New Evidence on Escape of Ionizing Radiation from Galaxies
• Extreme Ultraviolet Absorption Lines in LyA Forest Absorbers and the Oxygen Abundance in the Intergalactic Medium
• HST/COS Spectra of Three QSOs That Probe The Circumgalactic Medium of a Single Spiral Galaxy: Evidence for Gas Recycling and Outflow
• Spectral shape of the UV ionizing background and HeII absorption at redshifts 1.8 < z < 2.9
• The Ionizing Efficiency of the First Stars
• The Implications of Gunn-Peterson Troughs in the HeII Lyman-alpha Forest
• The Absorption and Emission Kinematics in the z=0.7450 Mg II Absorber Towards Q1331+17
• Spectral energy distribution of the metagalactic ionizing radiation field from QSO absorption spectra
• Patchy He II reionization and the physical state of the IGM
• Distribution of Faint Atomic Gas in Hickson Compact Groups
• The Metallicity of Intergalactic Gas in Cosmic Voids
• The impact of spatial fluctuations in the ultra-violet background on intergalactic carbon and silicon
• Inhomogeneous metal enrichment at z \sim 1.9: the Lyman limit systems in the spectrum of the HDF-S quasar
• Chemical Abundances and the Metagalactic Radiation Field at High Redshift
• Absorption Line Signatures of Gas in Mini Dark Matter Halos
• Spectral shape of the UV ionizing background and OVI absorbers at z ~ 1.5 towards HS0747+4259
• High Metallicity Mg II Absorbers in the z < 1 Lyman alpha Forest of PKS 0454+039: Giant LSB Galaxies?
• A Direct Detection of Gas Accretion: The Lyman Limit System in 3C 232
• Sources for Metals in the Intergalactic Medium
• The HI Column Density Distribution of the Galactic Disk and Halo
• Towards a Direct Detection of Warm Gas in Galactic Haloes at Cosmological Distances
• A Detection of Gas Associated with the M 31 Stellar Stream
• UV background fluctuations traced by metal ions at $z\approx 3$
• Fluctuations of the intergalactic ionization field at redshift z ~ 2