Two-phase equilibrium and molecular hydrogen formation in damped Lyman-alpha systems

Abstract

Molecular hydrogen is quite underabundant in damped Lyman-alpha systems at high redshift, when compared to the interstellar medium near the Sun. This has been interpreted as implying that the gas in damped Lyman-alpha systems is warm. like the nearby neutral intercloud medium, rather than cool, as in the clouds which give rise to most H I absorption in the Milky Way. Other lines of evidence suggest that the gas in damped Lyman-alpha systems -- in whole or part -- is actually cool; spectroscopy of neutral and ionized carbon, discussed here, shows that the damped Lyman-alpha systems observed at lower redshift z $<$ 2.3 are largely cool, while those seen at z $>$ 2.8 are warm (though not devoid of H2). To interpret the observations of carbon and hydrogen we constructed detailed numerical models of H2 formation under the conditions of two-phase thermal equilibrium, like those which account for conditions near the Sun, but with varying metallicity, dust-gas ratio, $etc$. We find that the low metallicity of damped Lyman-alpha systems is enough to suppress H2 formation by many orders of magnitude even in cool diffuse clouds, as long as the ambient optical/uv radiation field is not too small. For very low metallicity and under the most diffuse conditions, H2 formation will be dominated by slow gas-phase processes not involving grains, and a minimum molecular fraction in the range $10^{-8}-10^{-7}$ is expected.

13 pages, 7 figures; accepted 2002-04-30 by Astronomy and Astrophysics

References in corpus (8)

• The microwave background temperature at the redshift of 2.33771
• Molecular hydrogen, deuterium and metal abundances in the damped Ly-alpha system at z = 3.025 toward QSO 0347-3819
• The Spin Temperature of Warm Interstellar H I
• Ionization Properties and Elemental Abundances in Damped Ly alpha Systems
• Thermal Instability-Induced Interstellar Turbulence
• The cosmic microwave background radiation temperature at z = 3.025 toward QSO 0347--3819
• Implications of 21cm observations for damped Ly-$\alpha$ systems
• HI 21cm absorption in low z damped lyman-alpha systems

Cited by in corpus (35)

• Damped Lyman alpha Systems
• The Atomic to Molecular Transition in Galaxies. II: HI and H_2 Column Densities
• The Big Problems in Star Formation: the Star Formation Rate, Stellar Clustering, and the Initial Mass Function
• The VLT-UVES survey for molecular hydrogen in high-redshift damped Lyman-alpha systems
• Probing the ISM Near Star Forming Regions with GRB Afterglow Spectroscopy: Gas, Metals, and Dust
• The Atomic to Molecular Transition in Galaxies. I: An Analytic Approximation for Photodissociation Fronts in Finite Clouds
• The Atomic-to-Molecular Transition in Galaxies. III. A New Method for Determining the Molecular Content of Primordial and Dusty Clouds
• The VLT-UVES survey for molecular hydrogen in high-redshift damped Lyman-alpha systems: Physical conditions in the neutral gas
• CII* Absorption in Damped Lyman Alpha Systems: (I) Star Formation Rates in a Two-Phase Medium
• A deep search for 21cm absorption in high redshift damped Lyman-$\alpha$ systems
• CII* Absorption in Damped Lyman Alpha Systems: (II) A New Window on the Star Formation History of the Universe
• Effects of dust grains on early galaxy evolution
• A High Resolution Study of the HI-H2 Transition across the Perseus Molecular Cloud
• Molecular Hydrogen in the Damped Ly alpha Absorber of Q1331+170
• Thermal Phases of the Neutral Atomic Interstellar Medium -- from Solar Metallicity to Primordial Gas
• The First Observations of Low Redshift Damped Lyman-alpha Systems with the Cosmic Origins Spectrograph
• Gas-phase recombination, grain neutralization and cosmic-ray ionization in diffuse gas
• The early build-up of dust in galaxies: A study of Damped Ly alpha Systems
• Detection of molecular hydrogen at z=1.15 toward HE 0515-4414
• An accurate measurement of the anisotropies and mean level of the Cosmic Infrared Background at 100 and 160 um
• Molecular hydrogen in damped Ly-alpha systems: clues to interstellar physics at high-redshift
• HI 21cm absorption at $z\sim 3.39$ towards PKS 0201+113
• The Spitzer Space Telescope First-Look Survey: Neutral Hydrogen Emission
• Time-dependent H2 formation and protonation
• Molecular fraction limits in damped Lyman-alpha absorption systems
• GASKAP Pilot Survey Science II: ASKAP Zoom Observations of Galactic 21-cm Absorption
• Molecular Hydrogen in Damped Ly-alpha Systems: Spatial Distribution
• Cosmological Evolution of Heavy Element and Molecular Hydrogen Abundances
• The far-UV Interstellar Radiation Field in Galactic Disks: Numerical and Analytic Models
• Metallicity Dependence of Molecular Cloud Hierarchical Structure at Early Evolutionary Stages
• A search for molecules in damped Lyman-alpha absorbers occulting millimetre-loud quasars
• CII*/CII ratio in high-redshift DLAs: ISM phase separation drives the observed bimodality of [CII] cooling rates
• Effect of a partial coverage of quasar broad-line regions by intervening H${}_2$-bearing clouds
• A neutral hydrogen absorption study of cold gas in the outskirts of the Magellanic Clouds using the GASKAP-HI survey
• A 22GHz search for molecular absorption at z~3 with the upgraded ATCA