Chemical enrichment and physical conditions in I Zw 18

Abridged. Low-metallicity star-forming dwarf galaxies are prime targets to understand the chemical enrichment of the interstellar medium. The HI region provides important constraints on the dispersal and mixing of heavy elements released by successive star-formation episodes. Our primary objective is to study the enrichment of the HI region and the interplay between star-formation history and metallicity evolution. We observed the most metal-poor star-forming galaxy in the Local Universe, I Zw 18, with Hubble/COS. The abundances in the neutral gas are derived from far-UV absorption-lines (HI, CII, CII*, NI, OI, ...) and are compared to the abundances in the HII region. Models are constructed to calculate the ionization structure and the thermal processes. We investigate the gas cooling in the HI region through physical diagnostics drawn from the fine-structure level of C+. We find that HI region abundances are lower by a factor of ~2 as compared to the HII region. There is no differential depletion on dust between the HI and HII region. Using sulfur as a metallicity tracer, we calculate a metallicity of 1/46 solar (vs. 1/31 in the HII region). From the study of abundance ratios, we propose that C, N, O, and Fe are mainly produced in massive stars. We argue that the HI envelope may contain pockets of pristine gas with a metallicity essentially null. Finally, we derive the physical conditions in the HI region by investigating the CII* absorption line. The cooling rate derived from CII* is consistent with collisions with H atoms in the diffuse neutral gas. We calculate the star-formation rate from the CII* cooling rate assuming that photoelectric effect on dust is the dominant gas heating mechanism. Our determination is in good agreement with the values in the literature if we assume a low dust-to-gas ratio (~2000 times lower than the Milky Way value).

[1]  P. Kroupa,et al.  DID THE INFANT R136 AND NGC 3603 CLUSTERS UNDERGO RESIDUAL GAS EXPULSION? , 2013, 1301.3491.

[2]  E. Jenkins,et al.  THE FRACTIONAL IONIZATION OF THE WARM NEUTRAL INTERSTELLAR MEDIUM , 2013, 1301.3144.

[3]  P. Goldsmith,et al.  COLLISIONAL EXCITATION OF THE [C ii] FINE STRUCTURE TRANSITION IN INTERSTELLAR CLOUDS , 2012, 1209.4536.

[4]  M. Krumholz.  STAR FORMATION IN ATOMIC GAS , 2012, 1208.1504.

[5]  J. Xavier Prochaska,et al.  METALLICITY EVOLUTION OF DAMPED Lyα SYSTEMS OUT TO z ∼ 5 , 2012, 1205.5047.

[6]  Tucson,et al.  Globules and pillars seen in the [CII] 158 μm line with SOFIA , 2012, 1203.6307.

[7]  A. Dekel,et al.  METALLICITY-DEPENDENT QUENCHING OF STAR FORMATION AT HIGH REDSHIFT IN SMALL GALAXIES , 2011, 1106.0301.

[8]  D. O. Astronomy,et al.  I Zw 18 as morphological paradigm for rapidly assembling high-z galaxies , 2011, 1110.4393.

[9]  F. Fraternali,et al.  Dynamics of starbursting dwarf galaxies: I Zw 18 , 2011, 1110.6042.

[10]  A. Saha,et al.  SPATIAL DISTRIBUTION AND EVOLUTION OF THE STELLAR POPULATIONS AND CANDIDATE STAR CLUSTERS IN THE BLUE COMPACT DWARF I ZWICKY 18 , 2011, 1106.5613.

[11]  S. Glover,et al.  Approximations for modelling CO chemistry in GMCs: a comparison of approaches , 2011, 1102.0670.

[12]  Manuel Peimbert,et al.  ON THE O/H, Mg/H, Si/H, AND Fe/H GAS AND DUST ABUNDANCE RATIOS IN GALACTIC AND EXTRAGALACTIC H ii REGIONS , 2010, 1006.0692.

[13]  F. Mannucci,et al.  A fundamental relation between mass, SFR and metallicity in local and high redshift galaxies , 2010, 1005.0006.

[14]  M. Asplund,et al.  The chemical composition of the Sun , 2009, 0909.0948.

[15]  Wolfgang L. Wiese,et al.  Critically Evaluated Atomic Transition Probabilities for Sulfur S I – S XV , 2009 .

[16]  Edward B. Jenkins,et al.  A UNIFIED REPRESENTATION OF GAS-PHASE ELEMENT DEPLETIONS IN THE INTERSTELLAR MEDIUM , 2009, 0905.3173.

[17]  C. Leitherer,et al.  OBSERVATIONS OF STARBURST GALAXIES WITH FAR-ULTRAVIOLET SPECTROGRAPHIC EXPLORER: GALACTIC FEEDBACK IN THE LOCAL UNIVERSE , 2009 .

[18]  U. Virginia,et al.  Metal enrichment of the neutral gas of blue compact dwarf galaxies : the compelling case of Pox 36 , 2008, 0812.2911.

[19]  Iap,et al.  Galaxies with Wolf-Rayet signatures in the low-redshift Universe. A survey using the Sloan Digital Sky Survey , 2008, 0805.1073.

[20]  D. Péquignot.  Heating of blue compact dwarf galaxies: gas distribution and photoionization by stars in I Zw 18 , 2007, 0710.5082.

[21]  S. Smartt,et al.  The VLT FLAMES Survey of Massive Stars: Rotation and Nitrogen Enrichment as the Key to Understanding Massive Star Evolution , 2007, 0711.2267.

[22]  J. Bernard-Salas,et al.  Chemical Composition and Mixing in Giant H II Regions: NGC 3603, 30 Doradus, and N66 , 2007, 0710.4549.

[23]  P. Chanial,et al.  Stellar Evolutionary Effects on the Abundances of Polycyclic Aromatic Hydrocarbons and Supernova-Condensed Dust in Galaxies , 2007, 0708.0790.

[24]  USA,et al.  MMT Observations of New Extremely Metal-poor Emission-Line Galaxies in the Sloan Digital Sky Survey , 2007, 0704.3842.

[25]  I. Hubeny,et al.  A Grid of NLTE Line-blanketed Model Atmospheres of Early B-Type Stars , 2006, astro-ph/0611891.

[26]  G. Stinson,et al.  The Origin and Evolution of the Mass-Metallicity Relationship for Galaxies: Results from Cosmological N-Body Simulations , 2006, astro-ph/0609620.

[27]  Á. López-Sánchez,et al.  The Localized Chemical Pollution in NGC 5253 Revisited: Results from Deep Echelle Spectrophotometry , 2006, astro-ph/0609498.

[28]  W. P. Chen,et al.  Triggered Star Formation by Massive Stars , 2005, astro-ph/0509315.

[29]  R. Peletier,et al.  Stellar Populations as Building Blocks of Galaxies , 2007 .

[30]  S. F. Sanchez,et al.  IFU observations of the core of Abell 2218 , 2006, Proceedings of the International Astronomical Union.

[31]  A. D. Etangs,et al.  Interstellar abundances in the neutral and ionized gas of NGC 604 , 2006, astro-ph/0608445.

[32]  J. Prochaska,et al.  The N/O Plateau of Blue Compact Galaxies: Monte Carlo Simulations of the Observed Scatter , 2006, astro-ph/0605265.

[33]  D. Casebeer,et al.  On the Determination of N and O Abundances in Low-Metallicity Systems , 2006, astro-ph/0603654.

[34]  J. Bernard-Salas,et al.  Mid-Infrared Properties of Low-Metallicity Blue Compact Dwarf Galaxies from the Spitzer Infrared Spectrograph , 2005, astro-ph/0510856.

[35]  M. Pettini,et al.  A Comparison of Absorption- and Emission-Line Abundances in the Nearby Damped Lyα Galaxy SBS 1543+593 , 2005, astro-ph/0509002.

[36]  E. al.,et al.  The Arecibo Legacy Fast ALFA Survey. I. Science Goals, Survey Design, and Strategy , 2005, astro-ph/0508301.

[37]  V. Lebouteiller,et al.  Phosphorus in the diffuse interstellar medium , 2005, astro-ph/0507404.

[38]  Ukraine,et al.  High-Ionization Emission in Metal-deficient Blue Compact Dwarf Galaxies , 2005, astro-ph/0507209.

[39]  Kyiv,et al.  SBS 0335–052W: The Lowest Metallicity Star-forming Galaxy Known , 2005, astro-ph/0506498.

[40]  N. D. Gibson,et al.  Oscillator Strengths for Ultraviolet Transitions in Cl II and Cl III , 2005 .

[41]  K. Omukai,et al.  Thermal and Fragmentation Properties of Star-forming Clouds in Low-Metallicity Environments , 2005, astro-ph/0503010.

[42]  A. Jones,et al.  ISM properties in low-metallicity environments. III. The dust spectral energy distributions of II Zw 40, He 2-10 and NGC 1140 , 2005, astro-ph/0501632.

[43]  T. Thuan,et al.  Abundances in the H I Envelope of the Extremely Low Metallicity Blue Compact Dwarf Galaxy SBS 0335–052 from Far Ultraviolet Spectroscopic Explorer Observations , 2004, astro-ph/0411460.

[44]  K. Sembach,et al.  Probing the Multiphase Interstellar Medium of the Dwarf Starburst Galaxy NGC 625 with Far Ultraviolet Spectroscopic Explorer Spectroscopy , 2004, astro-ph/0409318.

[45]  B. Savage,et al.  C II Radiative Cooling of the Diffuse Gas in the Milky Way , 2004, astro-ph/0407363.

[46]  E. Skillman,et al.  Chemical Abundances of H II Regions in the Starburst Galaxy NGC 1705 , 2004, astro-ph/0406571.

[47]  Garching,et al.  Continuous star formation in IZw18 , 2004, astro-ph/0406484.

[48]  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.

[49]  F. Bauer,et al.  Chandra Observations of the Three Most Metal Deficient Blue Compact Dwarf Galaxies Known in the Local Universe, SBS 0335–052, SBS 0335–052W, and I Zw 18 , 2004, astro-ph/0401349.

[50]  D. Morton,et al.  Atomic Data for Resonance Absorption Lines. III. Wavelengths Longward of the Lyman Limit for the Elements Hydrogen to Gallium , 2003 .

[51]  A. Loeb,et al.  The formation of the first low-mass stars from gas with low carbon and oxygen abundances , 2003, Nature.

[52]  G. H'ebrard,et al.  Abundance differences between the neutral and the ionized gas of the dwarf galaxy IZw 36 , 2003, astro-ph/0310599.

[53]  C. Leitherer,et al.  Abundances in the Neutral Interstellar Medium of I Zw 18 from Far Ultraviolet Spectroscopic Explorer Observations , 2003 .

[54]  R. Schulte-Ladbeck,et al.  The H II Regions of the Damped Lyα Absorber SBS 1543+593 , 2003, astro-ph/0309792.

[55]  A. Vidal-Madjar,et al.  FUSE observations of the H I interstellar gas of I Zw 18 , 2003, astro-ph/0309766.

[56]  K. Lodders.  Solar System Abundances and Condensation Temperatures of the Elements , 2003 .

[57]  J. Prochaska,et al.  C II* Absorption in Damped Lyα Systems. II. A New Window on the Star Formation History of the Universe , 2003, astro-ph/0304042.

[58]  J. Prochaska,et al.  C II* Absorption in Damped Lyα Systems. I. Star Formation Rates in a Two-Phase Medium , 2003, astro-ph/0304040.

[59]  M. Pettini,et al.  Rest-Frame Ultraviolet Spectra of z ∼ 3 Lyman Break Galaxies , 2003, astro-ph/0301230.

[60]  E. Jessberger,et al.  Elemental Abundances and Mass Densities of Dust and Gas in the Local Interstellar Cloud , 2003 .

[61]  F. Castelli,et al.  Round Table Summary: Problems in Modelling Stellar Atmospheres , 2003 .

[62]  Ivan Hubeny,et al.  A Grid of Non-LTE Line-blanketed Model Atmospheres of O-Type Stars , 2002, astro-ph/0210157.

[63]  A. Davidsen,et al.  Extreme-Ultraviolet Absorption Lines in Lyα Forest Absorbers and the Oxygen Abundance in the Intergalactic Medium , 2002, astro-ph/0207301.

[64]  T. Heckman,et al.  Received; Accepted , 2001 .

[65]  G. Meynet,et al.  Stellar evolution with rotation - VIII. Models at Z = 10$^\mathsf{-5}$ and CNO yields for early galactic evolution , 2002, astro-ph/0205370.

[66]  D. York,et al.  Deuterium Abundance toward G191-B2B: Results from the FUSE Mission , 2002 .

[67]  S. Lacour,et al.  Deuterium Abundance toward WD 2211–495: Results from the FUSE Mission , 2002 .

[68]  A. Vidal-Madjar,et al.  Deuterium Abundance toward WD 1634–573: Results from the FUSE Mission , 2002 .

[69]  T. Thuan,et al.  Far Ultraviolet Spectroscopic Explorer Observations of the Blue Compact Dwarf Galaxy Markarian 59 , 2002 .

[70]  G. Meynet,et al.  The origin of primary nitrogen in galaxies , 2001, astro-ph/0111187.

[71]  Kyiv,et al.  A Spectroscopic Study of Component C and the Extended Emission around I Zw 18 , 2001, astro-ph/0109257.

[72]  C. Leitherer,et al.  FUSE Observations of Outflowing O VI in the Dwarf Starburst Galaxy NGC 1705 , 2001, astro-ph/0102283.

[73]  V. Lipovetsky,et al.  VLA H I Line Observations of the Extremely Metal-Poor Blue Compact Dwarf Galaxy SBS 0335−052 , 2000, astro-ph/0011291.

[74]  D. Schaerer,et al.  On Ionization Effects and Abundance Ratios in Damped Lyα Systems , 2000, astro-ph/0010643.

[75]  N. Fukuda,et al.  Sequential Star Formation Triggered by Expansion of an H II Region , 2000 .

[76]  M. Edmunds,et al.  On the Cosmic Origins of Carbon and Nitrogen , 2000, astro-ph/0004299.

[77]  F. Keenan,et al.  Modeling the Warm Ionized Interstellar Medium and Its Impact on Elemental Abundance Studies , 1999, astro-ph/9908051.

[78]  James C. Green,et al.  THE COSMIC ORIGINS SPECTROGRAPH , 1999, Astronomical Telescopes + Instrumentation.

[79]  Cambridge,et al.  The local environment of H ii galaxies , 1999, astro-ph/9903037.

[80]  G. Östlin,et al.  The most metal-poor galaxies , 1999, astro-ph/9911094.

[81]  M. Giavalisco,et al.  The Ultraviolet Spectrum of MS 1512–cB58: An Insight into Lyman-Break Galaxies , 1999, astro-ph/9908007.

[82]  Tucson,et al.  Helium Abundance in the Most Metal-deficient Blue Compact Galaxies: I Zw 18 and SBS 0335–052 , 1999, astro-ph/9907228.

[83]  Puebla,et al.  The evolution of superbubbles and the detection of Lyα in star-forming galaxies , 1999, astro-ph/9905324.

[84]  A. Aloisi,et al.  The Star Formation History of I Zw 18 , 1999, astro-ph/9904219.

[85]  Jeremiah P. Ostriker,et al.  Cosmic Chemical Evolution , 1999, astro-ph/9903207.

[86]  T. Thuan,et al.  Heavy-Element Abundances in Blue Compact Galaxies , 1998, astro-ph/9811387.

[87]  A. Ferrara,et al.  Starburst-driven Mass Loss from Dwarf Galaxies: Efficiency and Metal Ejection , 1998, astro-ph/9801237.

[88]  A. Cameron,et al.  Numerical Simulations of Triggered Star Formation. I. Collapse of Dense Molecular Cloud Cores , 1998 .

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

[90]  G. Ferland,et al.  CLOUDY 90: Numerical Simulation of Plasmas and Their Spectra , 1998 .

[91]  E. Fitzpatrick,et al.  A Reanalysis of the Carbon Abundance in the Translucent Cloud toward HD 24534 , 1998, astro-ph/9806401.

[92]  D. Schlegel,et al.  Maps of Dust Infrared Emission for Use in Estimation of Reddening and Cosmic Microwave Background Radiation Foregrounds , 1998 .

[93]  E. Jenkins,et al.  Interstellar Medium Absorption Profile Spectrograph Observations of Interstellar Neutral Argon and the Implications for Partially Ionized Gas , 1998 .

[94]  A. Whitworth,et al.  Dynamically triggered star formation in giant molecular clouds , 1998, astro-ph/9805001.

[95]  Jr.,et al.  The Global Schmidt law in star forming galaxies , 1997, astro-ph/9712213.

[96]  M. Haynes,et al.  The Complex Kinematics of the Neutral Hydrogen Associated with I Zw 18 , 1997, astro-ph/9712070.

[97]  E. Jenkins,et al.  IMAPS Observations of Interstellar Neutral Argon and the Implications for Partially Ionized Gas , 1997, astro-ph/9712260.

[98]  D. Schlegel,et al.  Maps of Dust IR Emission for Use in Estimation of Reddening and CMBR Foregrounds , 1997, astro-ph/9710327.

[99]  J. Cardelli,et al.  Carbon in the Diffuse Interstellar Medium , 1997 .

[100]  D. Garnett,et al.  High Carbon in I Zwicky 18: New Results from Hubble Space Telescope Spectroscopy , 1996, astro-ph/9611026.

[101]  V. Lipovetsky,et al.  The Primordial Helium Abundance: Systematic Effects and a New Determination , 1997 .

[102]  C. Taylor.  The H I Companions of H II Galaxies and Low Surface Brightness Dwarf Galaxies , 1996, astro-ph/9611142.

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

[104]  V. A. Lipovetsky,et al.  Hubble Space Telescope Observations of the Blue Compact Dwarf SBS 0335–052: A Probable Young Galaxy , 1996 .

[105]  V. Lipovetsky,et al.  Hubble Space Telescope Observations of the Unusual Blue Compact Dwarf Galaxy Markarian 996 , 1996 .

[106]  D. A. Verner,et al.  Atomic data for astrophysics. II. New analytic fits for photoionization cross sections of atoms and ions , 1996 .

[107]  A. Tielens,et al.  The neutral atomic phases of the interstellar medium , 1995 .

[108]  D. Hunter,et al.  The Massive Stars of I ZW 18 as Seen in Hubble Space Telescope Images , 1995 .

[109]  S. Woosley,et al.  The Evolution and Explosion of Massive Stars. II. Explosive Hydrodynamics and Nucleosynthesis , 1995 .

[110]  Ivan Hubeny,et al.  Non-LTE line-blanketed model atmospheres of hot stars. 1: Hybrid complete linearization/accelerated lambda iteration method , 1995 .

[111]  E.Terlevich,et al.  The evolution of C/O in dwarf galaxies from Hubble Space Telescope FOS observations , 1994, astro-ph/9411011.

[112]  A. Tielens,et al.  THE PHOTOELECTRIC HEATING MECHANISM FOR VERY SMALL GRAPHITIC GRAINS AND POLYCYCLIC AROMATIC HYDROCARBONS , 1994 .

[113]  R. Kennicutt,et al.  Spatially resolved optical and near infrared spectroscopy of I Zw 18 , 1993 .

[114]  J. Walsh,et al.  Abundances in the starburst galaxy II Zw 40 , 1993 .

[115]  A. Poglitsch,et al.  158 micron forbidden C II mapping of NGC 6946 - Probing the atomic medium , 1993 .

[116]  N. C. Rana.  Chemical Evolution of the Galaxy , 1991 .

[117]  Reginald J. Dufour,et al.  Extended emission and star formation in I Zw 18. , 1990 .

[118]  J. Walsh,et al.  Optical spectroscopic and abundance mapping of the amorphous galaxy NGC 5253 , 1989 .

[119]  W. Sargent,et al.  I ZW 18 and the Existence of Very Metal Poor Blue Compact Dwarf Galaxies , 1986 .

[120]  Charles H. Townes,et al.  Far-infrared spectroscopy of galaxies - The 158 micron C(+) line and the energy balance of molecular clouds , 1985 .

[121]  G. Martin,et al.  Blue compact dwarf galaxies. I. Neutral hydrogen observations of 115 galaxies , 1981 .

[122]  E. Jenkins.  A closer look at interstellar Lyman-alpha absorption , 1971 .

[123]  W. Sargent,et al.  ISOLATED EXTRAGALACTIC H II REGIONS. , 1970 .

[124]  Olin J. Eggen.  The System of Algol. , 1948 .