A homogeneous sample of sub-damped Lyman systems – IV. Global metallicity evolution

An accurate method to measure the abundance of high-redshift galaxies involves the observation of absorbers along the line of sight towards a background quasar. Here, we present abundance measurements of 13 z≥ 3 sub-damped Lyman α (sub-DLA) systems (quasar absorbers with H i column density in the range 19 3. These new data, combined with other sub-DLA measurements from the literature, confirm the stronger evolution of metallicity with redshift for sub-DLAs than for the classical damped Lyman α absorbers. In addition, these observations are used to compute for the first time, using photoionization modelling in a sample of sub-DLAs, the fraction of gas that is ionized. Based on these results, we calculate that sub-DLAs contribute no more than 6 per cent of the expected amount of metals at z∼ 2.5. We therefore conclude that, even if sub-DLAs are found to be more metal-rich than classical DLAs, their contribution is insufficient to solve the so-called ‘missing-metals’ problem.

[1]  Gary J. Ferland,et al.  Hazy, A Brief Introduction to Cloudy 96 , 1983 .

[2]  C. Foltz,et al.  The Large Bright QSO Survey for Damped LY alpha Absorption Systems , 1995 .

[3]  J. Prochaska.  The Physical Nature of the Lyman Limit Systems , 1998, astro-ph/9811357.

[4]  J. Walsh,et al.  Chemical Evolution from Zero to High Redshift , 1999 .

[5]  C. Steidel,et al.  Si and Mn Abundances in Damped Lyα Systems with Low Dust Content , 1999, astro-ph/9910131.

[6]  Pasadena,et al.  The evolution of ΩHI and the epoch of formation of damped Lyman α absorbers , 2001, astro-ph/0107045.

[7]  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.

[8]  P. Petitjean,et al.  Inhomogeneous metal enrichment at z 1.9: The Lyman limit systems in the spectrum of the HDF-S quasar ? , 2001, astro-ph/0102491.

[9]  UK.,et al.  A homogeneous sample of sub-damped Lyman α systems- I. Construction of the sample and chemical abundance measurements , 2003, astro-ph/0307049.

[10]  Cambridge,et al.  A homogeneous sample of sub-damped Lyman α systems – II. Statistical, kinematic and chemical properties , 2003, astro-ph/0307050.

[11]  J. Prochaska,et al.  Elemental Abundances in Two High Column Density Damped Lyα Systems at z < 1.5* , 2004, astro-ph/0409695.

[12]  J. Brinkmann,et al.  The Origin of the Mass-Metallicity Relation: Insights from 53,000 Star-forming Galaxies in the Sloan Digital Sky Survey , 2004, astro-ph/0405537.

[13]  A comprehensive set of elemental abundances in damped Lyα systems: Revealing the nature of these high-redshift galaxies , 2003, astro-ph/0312210.

[14]  The early build-up of dust in galaxies: A study of damped Ly α systems , 2004, astro-ph/0403237.

[15]  France,et al.  Where Are the Missing Cosmic Metals? , 2005, astro-ph/0510525.

[16]  C. Péroux,et al.  The dust obscuration bias in damped Lyman α systems , 2005, astro-ph/0502137.

[17]  K. Sembach,et al.  A Near-Solar Metallicity, Nitrogen-deficient Lyman Limit Absorber Associated with Two S0 Galaxies , 2005, astro-ph/0501475.

[18]  A homogeneous sample of sub-damped Lyman α systems — III. Total gas mass ΩH i+He ii at z > 2⋆ , 2005, astro-ph/0507353.

[19]  The missing metal problem — I. How many metals are in submillimetre galaxies? , 2005, astro-ph/0509005.

[20]  Patrick Petitjean,et al.  The VLT-UVES survey for molecular hydrogen in high-redshift damped Lyman α systems: physical conditions in the neutral gas , 2005 .

[21]  H.-W. Chen,et al.  ApJ in press Preprint typeset using L ATEX style emulateapj v. 9/08/03 THE GEMINI DEEP DEEP SURVEY. VII. THE REDSHIFT EVOLUTION OF THE MASS-METALLICITY RELATION 1,2 , 2005 .

[22]  S. Arnouts,et al.  The fabulous destiny of galaxies : bridging past and present , 2006 .

[23]  Eső,et al.  The missing metals problem: II. How many metals are in z ~ 2.2 galaxies? , 2005, astro-ph/0511698.

[24]  U. Michigan,et al.  Supersolar Super-Lyman Limit Systems , 2006, astro-ph/0606573.

[25]  D. York,et al.  The nature of damped Lyman a and sub-damped Lyman a absorbers , 2006, astro-ph/0608127.

[26]  The UCSD Radio-selected Quasar Survey for Damped Lyα Systems , 2006, astro-ph/0604334.

[27]  Interstellar abundances in the neutral and ionized gas of NGC 604 , 2006, astro-ph/0608445.

[28]  D. York,et al.  The most metal-rich intervening quasar absorber known , 2006, astro-ph/0601079.

[29]  M. Zwaan,et al.  ACCEPTED FOR PUBLICATION IN APJ Preprint typeset using L ATEX style emulateapj v. 6/22/04 WHERE IS THE MOLECULAR HYDROGEN IN DAMPED LY α ABSORBERS? , 2006 .

[30]  Metal-rich damped/subdamped Lyman α quasar absorbers at z < 1 , 2006, astro-ph/0607561.

[31]  P. Petitjean,et al.  The most metal-poor damped Ly alpha system at z<3: constraints on early nucleosynthesis , 2006, astro-ph/0602121.

[32]  The enrichment history of baryons in the Universe , 2006, astro-ph/0608268.

[33]  D. York,et al.  The Role of Sub-Damped Lyα Absorbers in the Cosmic Evolution of Metals , 2006, astro-ph/0608126.

[34]  The missing metals problem – III. How many metals are expelled from galaxies? , 2007, astro-ph/0703509.

[35]  New abundance determinations in z < 1.5 QSO absorbers: seven sub-DLAs and one DLA , 2007, astro-ph/0702297.

[36]  Hot halos around high redshift protogalaxies : Observations of O VI and N V absorption in damped Lyman-α systems , 2007, astro-ph/0701392.