Molecular Hydrogen in high redshift Damped Lyman-α systems

We have searched for molecular hydrogen in damped Lyman α (DLA) and sub-DLA systems at high redshift (z abs > 1.8) using the Ultraviolet and Visible Echelle Spectrograph (UVES) at the Very Large Telescope (VLT) down to a detection limit of typically N(H 2 ) = 2 × 10 14 cm -2 . Out of the 33 systems in our sample, eight have firm and two have tentative detections of associated H 2 absorption lines. Considering that three detections were already known from past searches, molecular hydrogen is detected in 13-20 per cent of the newly surveyed systems. We report new detections of molecular hydrogen at z abs = 2.087 and 2.595 toward, respectively, Q 1444+014 and Q 0405-443, and also reanalyse the system at z abs = 3.025 toward Q 0347-383. In all of the systems, we measure metallicities relative to solar, [X/H] (with either X = Zn, S or Si), and depletion factors of Fe, [X/Fe], supposedly on to dust grains, and compare the characteristics of our sample with those of the global population of DLA systems (60 systems in total). We find that there is a correlation between the metallicity and the depletion factor in both our sample and also the global population of DLA systems. Although H 2 molecules are detected in systems with [Zn/Fe] as small as 0.3, the DLA and sub-DLA systems where H 2 is detected are usually amongst those having the highest metallicities and the largest depletion factors. In particular, H 2 is detected in the five systems having the largest depletion factors. Moreover, the individual components where H 2 is detected have depletion factors systematically larger than other components in the profiles. In two different systems, one of the H 2 -detected components even has [Zn/Fe]≥1.4. These are the largest depletion factors ever seen in DLA systems. All of this clearly demonstrates the presence of dust in a large fraction of the DLA systems. The mean H 2 molecular fraction, f= 2N (H 2 )/[2N(H 2 ) +N(H i)], is generally small in DLA systems (typically log f 2 and the H i column density. In fact, two systems where H 2 is detected have log N(H i) 2 on to dust grains is reduced in those systems, probably because the gas is warm (T > 1000 Κ) and/or the ionizing flux is enhanced relative to what is observed in our Galaxy.

[1]  J. Lawler,et al.  Radiative lifetimes, branching rations, and absolute transition probabilities in Cr II and Zn II , 1993 .

[2]  J. Prochaska,et al.  The UCSD HIRES/Keck I Damped Lyα Abundance Database. II. The Implications , 2001, astro-ph/0110351.

[3]  J. Prochaska,et al.  The UCSD HIRES/Keck I Damped Lyα Abundance Database. I. The Data , 2001, astro-ph/0110350.

[4]  P. Petitjean,et al.  The microwave background temperature at the redshift of 2.33771 , 2000 .

[5]  J. Lawler,et al.  Absorption f-Values of Ti II Vacuum Ultraviolet Resonance Transitions , 2001 .

[6]  D. York,et al.  Accurate oscillator strengths for ultraviolet lines of Ar I - Implications for interstellar material , 1992 .

[7]  D. York,et al.  The Diffuse Interstellar Clouds toward 23 Orionis , 1999, astro-ph/9905234.

[8]  D. Morton.  Atomic data for resonance absorption lines. I, Wavelengths longward of the Lyman limit , 1991 .

[9]  P. Petitjean,et al.  The cosmic microwave background radiation temperature at a redshift of 2.34 , 2000, Nature.

[10]  E. Jenkins,et al.  A survey of interstellar C I - Insights on carbon abundances, UV grain albedos, and pressures in the interstellar medium , 1979 .

[11]  C. Steidel,et al.  Metal Abundances at z < 1.5: Fresh Clues to the Chemical Enrichment History of Damped Lyα Systems , 1998, astro-ph/9808017.

[12]  S. M. Fall,et al.  Confirmation of Dust in Damped Lyman-Alpha Systems , 1991 .

[13]  R. C. Bohlin,et al.  A survey of interstellar molecular hydrogen. I , 1977 .

[14]  S. Levshakov,et al.  Molecular hydrogen in the Z=2.811 absorbing material toward the quasar PKS 0528-250 , 1985 .

[15]  M. Steinmetz,et al.  Damped Lyα Absorber and the Faint End of the Galaxy Luminosity Function at High Redshift , 1999, astro-ph/9911447.

[16]  H. Liszt,et al.  Two-phase equilibrium and molecular hydrogen formation in damped Lyman-alpha systems , 2002, astro-ph/0204515.

[17]  J. Lawler,et al.  Absolute UV and Vacuum UV Oscillator Strengths for Ni II , 1999 .

[18]  D. York,et al.  A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in the Small and Large Magellanic Clouds , 2001, astro-ph/0110262.

[19]  Stephen C. Russell,et al.  Abundances of the heavy elements in the Magellanic Clouds. III - Interpretation of results , 1992 .

[20]  Giovanni Vladilo,et al.  Dust and Elemental Abundances in Damped Lyα Absorbers , 1997, astro-ph/9710026.

[21]  P. Petitjean,et al.  Detection of molecular hydrogen in a near Solar-metallicity damped Lyman-α system at $z_\mathsf{abs}$ ≈ 2 toward Q 0551-366 , 2002 .

[22]  Molecular Hydrogen Absorption in the z = 1.97 Damped Lyα Absorption System toward Quasi-Stellar Object Q0013–004* , 1997, astro-ph/9701041.

[23]  D. Morton,et al.  Interstellar molecular hydrogen toward zeta Puppis , 1976 .

[24]  E. Jenkins,et al.  Molecular Hydrogen in the Direction of ζ Orionis A , 1996, astro-ph/9609144.

[25]  Institut d'Astrophysique de Paris,et al.  Dust depletion and abundance pattern in damped Lyα systems: A sample of Mn and Ti abundances at z < 2.2 , 2002 .

[26]  Kenneth R. Sembach,et al.  INTERSTELLAR ABUNDANCES FROM ABSORPTION-LINE OBSERVATIONS WITH THE HUBBLE SPACE TELESCOPE , 1996 .

[27]  J. Maza,et al.  Slit spectroscopy of nine new $\vec{z > 3}$ southern QSOs: Damped Ly$\alpha$ and Lyman-limit systems , 2001 .

[28]  B. Savage,et al.  Molecular Hydrogen in High-Velocity Clouds , 2001, astro-ph/0110279.

[29]  J. Lawler,et al.  Vacuum Ultraviolet Resonance Absorption f-values for Ni II , 2000 .

[30]  S. M. Fall,et al.  Obscuration of quasars by dust in damped Lyman-alpha systems , 1993 .

[31]  A Physical Upper Limit on the H I Column Density of Gas Clouds , 2001, astro-ph/0109280.

[32]  J. Ge,et al.  H2, C I, Metallicity, and Dust Depletion in the z = 2.34 Damped Lyα Absorption System toward QSO 1232+0815 , 2001 .

[33]  Distinct abundance patterns in multiple damped ly alpha galaxies: evidence for truncated star formation? , 2003, astro-ph/0303441.

[34]  S. M. Fall,et al.  Are High-Redshift Damped Lyα Galaxies Lyman Break Galaxies? , 2002, astro-ph/0203361.

[35]  Simon J. E. Radford,et al.  Highly Redshifted Radio Lines , 1999 .

[36]  D. Weinberg,et al.  Simulations of Damped Lyα and Lyman Limit Absorbers in Different Cosmologies: Implications for Structure Formation at High Redshift , 1999, astro-ph/9911343.

[37]  Molecular hydrogen at zabs=1.973 toward Q 0013−004: dust depletion pattern in damped Lyman α systems , 2002, astro-ph/0201477.

[38]  Limin Lu,et al.  Abundances at High Redshifts: The Chemical Enrichment History of Damped Lyα Galaxies , 1996, astro-ph/9606044.

[39]  P. Bonifacio,et al.  UVES Observations of QSO 0000–2620: Argon and Phosphorus Abundances in the Dust-free Damped Lyα System at zabs = 3.3901 , 2000, astro-ph/0010434.

[40]  E. Fitzpatrick,et al.  Composition of Interstellar Clouds in the Disk and Halo. IV. HD 215733 , 1993 .

[41]  C. Foltz,et al.  Molecules at early epochs. II. H/sub 2/ and CO toward PHL 957 , 1987 .

[42]  C. Foltz,et al.  Molecules at early epochs. IV. Confirmation of the detection of H2 toward PKS 0528 - 250 , 1988 .

[43]  I. Hook,et al.  The corals survey I: new estimates of the number density and gas content of damped lyman alpha systems free from dust bias , 2001, astro-ph/0109205.

[44]  Garching,et al.  Molecular hydrogen, deuterium and metal abundances in the damped Ly-alpha system at z = 3.025 toward QSO 0347-3819 , 2001, astro-ph/0105529.

[45]  J. Shull,et al.  Molecules in the early universe , 1984 .

[46]  J. Lawler,et al.  Oscillator strengths of the Si II 181 nanometer resonance multiplet , 1993 .

[47]  Molecular hydrogen formation in the interstellar medium , 2002, astro-ph/0207035.

[48]  M. Dopita,et al.  H I Shells in the Large Magellanic Cloud , 1999 .

[49]  P. Møller,et al.  Probing the faint end of the Galaxy luminosity function at z=3 with Ly-alpha emission , 2001, astro-ph/0105399.

[50]  J. Prochaska,et al.  Chemical Abundances of the Damped Lyα Systems at z > 1.5 , 1998, astro-ph/9810381.

[51]  S. M. Fall,et al.  Accepted for Publication in the Astrophysical Journal Metallicity Evolution of Damped Lyman-Alpha Galaxies , 2002 .

[52]  Max Pettini,et al.  The Metallicity of High-Redshift Galaxies: The Abundance of Zinc in 34 Damped Lyα Systems from z = 0.7 to 3.4 , 1997 .