Lyman-Continuum Emission from Galaxies at z ≃ 3.4

We report the detection of significant Lyman-continuum flux in the composite spectrum of 29 Lyman-break galaxies (LBGs) with redshifts ⟨z⟩ = 3.40 ± 0.09. After correction for opacity due to intervening absorption using a new composite QSO spectrum evaluated at the same redshift, the ratio of emergent flux density at 1500 Å in the rest frame to that in the Lyman continuum is L(1500 )/L(900 ) = 4.6 ± 1.0. If the relative intensity of the inferred escaping Lyman-continuum radiation is typical of LBGs at z ~ 3 (the galaxies in this sample are drawn from the bluest quartile of LBG spectral energy distributions due to known selection effects), then observed LBGs produce about 5 times more H-ionizing photons per unit comoving volume than QSOs at z ~ 3. The associated contribution to the metagalactic ionizing radiation field is Jν(912 Å) ≈ 1.2 ± 0.3 × 10-21 ergs s-1 cm-2 Hz-1 sr-1 at z ~ 3, very close to most estimates of the radiation field background based on the "proximity effect." A preliminary analysis of the density of faint QSOs in our Lyman-break galaxy survey indicates that the standard extrapolated QSO luminosity function may slightly overpredict the QSO contribution to Jν(912 ) at z ~ 3. We briefly discuss the implications of a galaxy-dominated UV background at high redshifts.

[1]  J. Hutchings,et al.  Far Ultraviolet Spectroscopic Explorer Observations of the Stellar Winds of Two O7 Supergiants in the Magellanic Clouds , 2000, astro-ph/0005039.

[2]  J. Bechtold,et al.  A Uniform Analysis of the Lyα Forest at z = 0-5. II. Measuring the Mean Intensity of the Extragalactic Ionizing Background Using the Proximity Effect , 2000, astro-ph/0004155.

[3]  P. Madau.  Cosmological reionization , 2000, Philosophical Transactions of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences.

[4]  C. C. Steidel,et al.  Multiwavelength Observations of Dusty Star Formation at Low and High Redshift , 2000, astro-ph/0001126.

[5]  M. Giavalisco,et al.  Lyα Imaging of a Proto-Cluster Region at ⟨z⟩ = 3.09 , 1999, astro-ph/9910144.

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

[7]  Timothy M. Heckman,et al.  Dust Absorption and the Ultraviolet Luminosity Density at z ≈ 3 as Calibrated by Local Starburst Galaxies , 1999, astro-ph/9903054.

[8]  Denis Foo Kune,et al.  Starburst99: Synthesis Models for Galaxies with Active Star Formation , 1999, astro-ph/9902334.

[9]  M. Bershady,et al.  The Distribution of High-Redshift Galaxy Colors: Line-of-Sight Variations in Neutral Hydrogen Absorption , 1999, astro-ph/9901197.

[10]  Cambridge,et al.  ∼ 4 and the Evolution of the Uv Luminosity Density at High Redshift , 2022 .

[11]  Martin J. Rees,et al.  Radiative Transfer in a Clumpy Universe. III. The Nature of Cosmological Ionizing Sources , 1998, astro-ph/9809058.

[12]  Z. Haiman,et al.  Constraints from the Hubble Deep Field on High-Redshift Quasar Models , 1998, astro-ph/9805258.

[13]  J. Graham,et al.  Seeking the Ultraviolet Ionizing Background at z ≈ 3 with the Keck Telescope , 1998, astro-ph/9808111.

[14]  M. Giavalisco,et al.  Infrared Observations of Nebular Emission Lines from Galaxies at z ≃ 3 , 1998, astro-ph/9806219.

[15]  P. Jelinsky,et al.  Reexamining the Lyman Continuum in Starburst Galaxies Observed with the Hopkins Ultraviolet Telescope , 1997, astro-ph/9703041.

[16]  D. Calzetti.  Reddening and star formation in starburst galaxies , 1996, astro-ph/9610184.

[17]  M. Steinmetz,et al.  QSO Metal Absorption Systems at High Redshift and the Signature of Hierarchical Galaxy Formation , 1996, astro-ph/9609083.

[18]  R. Bohlin,et al.  AN ATLAS OF OB SPECTRA FROM 1000A TO 1200A , 1996 .

[19]  M. Giavalisco,et al.  Spectroscopic Confirmation of a Population of Normal Star-forming Galaxies at Redshifts z > 3 , 1996, astro-ph/9602024.

[20]  S. Djorgovski,et al.  The Luminosity Function of z>4 Quasars from the Second Palomar Sky Survey , 1995 .

[21]  Henry C. Ferguson,et al.  The Lyman Continuum in Starburst Galaxies Observed with the Hopkins Ultraviolet Telescope , 1995 .

[22]  P. Madau,et al.  Radiative Transfer in a Clumpy Universe. II. The Ultraviolet Extragalactic Background , 1995, astro-ph/9509093.

[23]  James E. Gunn,et al.  Spectrscopic CCD Surveys for Quasars at Large Redshift.IV.Evolution of the Luminosity Function from Quasars Detected by Their Lyman-Alpha Emission , 1995 .

[24]  C. Steidel,et al.  The Hubble Space Telescope quasar absorption line key project. v. redshift evolution of lyman limit absorption in the spectra of a large sample of quasars , 1995 .

[25]  Harland W. Epps,et al.  THE KECK LOW-RESOLUTION IMAGING SPECTROMETER , 1995 .

[26]  Piero Madau,et al.  Radiative transfer in a clumpy universe: The colors of high-redshift galaxies , 1995 .

[27]  Y. Pei.  The Luminosity Function of Quasars , 1995 .

[28]  P. Madau,et al.  On the photoionization of the intergalactic medium by quasars at high redshift , 1993 .

[29]  Donald Hamilton,et al.  Deep imaging of high redshift QSO fields below the Lyman limit. II - Number counts and colors of field galaxies , 1993 .

[30]  J. Ostriker,et al.  What Produces the Ionizing Background at Large Redshift , 1990 .

[31]  S. Lilly,et al.  Galaxy Formation and the Origin of the Ionizing Flux at Large Redshift , 1990 .

[32]  C. Steidel,et al.  QSO heavy element absorption systems and the nature of the metagalactic ionizing flux at high redshift , 1989 .

[33]  C. Steidel,et al.  A survey of Lyman-limit absorption in the spectra of 59 high-redshift QSOs , 1989 .

[34]  J. Ostriker,et al.  Quasar ionization of Lyman-alpha clouds - The proximity effect, a probe of the ultraviolet background at high redshift , 1988 .

[35]  M. Donahue,et al.  Can quasars ionize the intergalactic medium , 1987 .

[36]  P. Shapiro,et al.  COSMOLOGICAL H II REGIONS AND THE PHOTOIONIZATION OF THE INTERGALACTIC MEDIUM. , 1986 .

[37]  M. Malkan,et al.  Integrated ultraviolet radiation field from QSOs , 1986 .

[38]  J. B. Oke,et al.  Absolute spectrophotometry of very large redshift quasars , 1982 .

[39]  A. Boksenberg,et al.  The distribution of Lyman-alpha absorption lines in the spectra of six QSOs: evidence for an intergalactic origin. , 1980 .

[40]  P. Schechter.  An analytic expression for the luminosity function for galaxies , 1976 .

[41]  Bruce A. Peterson,et al.  On the Density of Neutral Hydrogen in Intergalactic Space , 1965 .