Fluctuations of the intergalactic ionization field at redshift z ~ 2

(Abridged) Aims. To probe the spectral energy distribution (SED) of the ionizing background radiation at z ~ 2 and to specify the sources contributing to the intergalactic radiation field. Methods. The spectrum of a bright quasar HS1103+6416 (zem = 2.19) contains five successive metal-line absorption systems at zabs = 1.1923, 1.7193, 1.8873, 1.8916, and 1.9410. The systems are optically thin and reveal multiple lines of different metal ions with the ionization potentials lying in the extreme ultraviolet (EUV) range (1 Ryd to 0.2 keV). For each system, the EUV SED of the underlying ionization field is reconstructed by means of a special technique developed for solving the inverse problem in spectroscopy. For the zabs = 1.8916 system, the analysis also involves the HeI resonance lines of the Lyman series and the HeI 504 A continuum, which are seen for the first time in any cosmic object except the Sun. Results. From one system to another, the SED of the ionizing continuum changes significantly, indicating that the intergalactic ionization field at z ~ 2 fluctuates at the scale of at least Delta_z ~ 0.004. This is consistent with Delta_z ~ 0.01 estimated from HeII and HI Lyman-alpha forest measurements between the redshifts 2 and 3.Comment: 29 pages, 18 figures, 3 tables, accepted for publication in A\&

Nitrogen-deficient and iron-rich associated absorbers with oversolar metallicities towards the quasar HE0141-3932

HE0141-3932 (zem=1.80) is a bright blue radio-quite quasar which reveals an emission line spectrum with an unusually weak Ly-alpha line. In addition, large redshift differences (Delta z=0.05) are observed between high ionization and low ionization emission lines. Absorption systems identified at z=1.78, 1.71, and 1.68 show mild oversolar metallicities (Z ~= 1-2Zsolar) and can be attributed to the associated gas clouds ejected from the circumnuclear region. The joint analysis of the emission and absorption lines leads to the conclusion that this quasar is seen almost pole-on. Its apparent luminosity may be Doppler boosted by ~10 times. The absorbing gas shows high abundance of Fe, Mg, and Al ([Fe, Mg, Al/C] ~= 0.15+/-0.10) along with underabundance of N ([N/C]<=-0.5). This abundance pattern is at variance with current chemical evolution models of QSOs predicting [N/C]>0 and [Fe/C]<0 at Z ~ Zsolar. Full details of this work are given in Reimers et al. (2005).Comment: 2 pages, to appear in the Proceed. of IAU Symp.228 "From Lithium to Uranium: Elemental Tracers of Early Cosmic Evolution", eds. V.Hill, P.Francois and F.Prima

HE 0141-3932: a bright QSO with an unusual emission line spectrum and associated absorption

HE 0141-3932 (zem = 1.80) is a bright blue radio-quite quasar with an unusually weak Ly-alpha emission line. Large redshift differences (Delta z = 0.05) are observed between high ionization and low ionization emission lines. Absorption systems identified at zabs = 1.78, 1.71, and 1.68 show mild oversolar metallicities (Z ~= 1-2Zsolar) and can be attributed to the associated gas clouds ejected from the circumnuclear region. The joint analysis of the emission and absorption lines leads to the conclusion that this quasar is seen almost pole-on. Its apparent luminosity may be Doppler boosted by ~10 times. The absorbing gas shows a high abundance of Fe, Mg and Al ([Fe, Mg, Al/C] ~= 0.15 +/- 0.10) along with underabundance of N ([N/C] <= -0.5). This abundance pattern is at variance with current chemical evolution models of QSOs predicting [N/C] >= 0$ and [Fe/C] < 0 at Z ~ Zsolar.Comment: 12 pages, 6 figures, accepted for publication in A&

Spectral shape of the UV ionizing background and HeII absorption at redshifts 1.8 < z < 2.9

The shape of the UV ionizing background is reconstructed from optically thin metal absorption-line systems identified in spectra of HE2347-4342, Q1157+3143, and HS1700+6416 in the redshift interval 1.8 < z < 2.9. The systems are analyzed by means of the Monte Carlo Inversion method completed with the spectral shape recovering procedure. The UVB spectral shape fluctuates at 2.4 < z < 2.9 mostly due to radiative transfer processes in the clumpy IGM. At z < 1.8, the IGM becomes almost transparent both in the HI and HeII Lyman continua and the variability of the spectral shape comes from diversity of spectral indices describing the QSO/AGN intrinsic radiation. At z > 2.4, the recovered spectral shapes show intensity depression between 3 and 4 Ryd due to HeII Ly-alpha absorption in the IGM clouds (line blanketing) and continuous medium (true Gunn-Petersen effect). The mean HeII Ly-alpha opacity estimated from the depth of this depression corresponds within 1-2sigma to the values directly measured from the HI/HeII Ly-alpha forest towards the quasars studied. The observed scatter in eta = N(HeII)/N(HI) and anti-correlation between N(HI) and eta can be explained by the combined action of variable spectral softness and differences in the mean gas density between the absorbing clouds. Neither of the recovered spectral shapes show features which can be attributed to the putative input of radiation from soft sources like starburst galaxies.Comment: 20 pages, 20 figures. Accepted for publication in A&

The neutron 'thunder' accompanying the extensive air shower

Simulations show that neutrons are the most abundant component among extensive air shower hadrons. However, multiple neutrons which appear with long delays in neutron monitors nearby the EAS core ('neutron thunder') are mostly not the neutrons of the shower, but have a secondary origin. The bulk of them is produced by high energy EAS hadrons hitting the monitors. The delays are due to the termalization and diffusion of neutrons in the moderator and reflector of the monitor accompanied by the production of secondary gamma-quanta. This conclusion raises the important problem of the interaction of EAS with the ground, the stuff of the detectors and their environment since they have often hydrogen containing materials like polyethilene in neutron monitors. Such interaction can give an additional contribution to the signal in the EAS detectors. It can be particularly important for the signals from scintillator or water tank detectors at km-long distances from the EAS core where neutrons of the shower become the dominant component after a few mcsec behind the EAS front.Comment: 12 pages, 4 figures, accepted by J.Phys.G: Nucl.Part.Phy

Metal-rich absorbers at high redshifts: abundance patterns

(Abbreviated) From six spectra of high-z QSOs, we select eleven metal-rich, Z>=Z_solar, and optically-thin to the ionizing radiation, N(HI)<10^17 cm^-2, absorption systems ranging between z=1.5 and z=2.9 and revealing lines of different ions in subsequent ionization stages. The majority of the systems (10 from 11) show abundance patterns which relate them to outflows from low and intermediate mass stars. All systems have sub-kpc linear sizes along the line-of-sight with many less than 20 pc. In several systems, silicon is deficient, presumably due to the depletion onto dust grains in the envelopes of dust-forming stars and the subsequent gas-dust separation. At any value of [C/H], nitrogen can be either deficient, [N/C]0, which supposes that the nitrogen enrichment occurs irregularly. In some cases, the lines of MgII 2796, 2803 appear to be shifted, probably as a result of an enhanced content of heavy isotopes 25Mg and 26Mg in the absorbing gas relative to the solar isotopic composition. Seven absorbers are characterized by low mean ionization parameter U, log U<-2.3, among them only one system has a redshift z>2 whereas all others are found at z ~= 1.8. Comparing the space number density of metal-rich absorbers with the comoving density of star-forming galaxies at z ~= 2, we estimate that the circumgalactic volume of each galaxy is populated by 10^7 - 10^8 such absorbers with total mass <=1/100th of the stellar galactic mass. Possible effects of high metal content on the peak values of star-forming and AGN activities at z~2 are discussed.Comment: 19 pages, 16 figures, 3 tables. Accepted for publication in A&

Spectral shape of the UV ionizing background and OVI absorbers at z ~ 1.5 towards HS0747+4259

We report on high resolution spectra of the bright QSO HS0747+4259 (zem = 1.90, V = 15.8) observed to search for intermediate redshift OVI absorption systems. The spectra were obtained by means of the Space Telescope Imaging Spectrograph (STIS) at the Hubble Space Telescope (HST) and the High Resolution Echelle Spectrometer (HIRES) at the W. M. Keck telescope. We identify 16 OVI systems in the range 1.07 <= z <= 1.87. Among them, six systems with zabs = 1.46-1.8 exhibit a sufficient number of lines of different ionic transitions to estimate the shape of the ionizing radiation field in the range 1 Ryd < E < 10 Ryd. All recovered UV ionizing spectra are characterized by the enhanced intensity at E > 3 Ryd compared to the model spectrum of Haardt and Madau (1996). This is in line with the observational evidence of a deficiency of strong Ly-alpha absorbers with N(HI) > 10^{15} cm^{-2}, at z < 2. The UV background shows significant local variations: the spectral shape estimated at z = 1.59 differs from that obtained at z = 1.81 and 1.73. A possible cause of these variations is the presence of a QSO/AGN at z ~= 1.54-1.59 close to the line of sight. No features favoring the input of stellar radiation to the ionizing background are detected, limiting the escape fraction of the galactic UV photons to f_esc < 0.05.Comment: 14 pages, 7 figures, 5 tables; accepted for publication in A&

Reionization and high-redshift galaxies: the view from quasar absorption lines

Determining when and how the first galaxies reionised the intergalactic medium promises to shed light on both the nature of the first objects and the cosmic history of baryons. Towards this goal, quasar absorption lines play a unique role by probing the properties of diffuse gas on galactic and intergalactic scales. In this review, we examine the multiple ways in which absorption lines trace the connection between galaxies and the intergalactic medium near the reionisation epoch. We first describe how the Ly α forest is used to determine the intensity of the ionising ultraviolet background and the global ionising emissivity budget. Critically, these measurements reflect the escaping ionising radiation from all galaxies, including those too faint to detect directly. We then discuss insights from metal absorption lines into reionisation-era galaxies and their surroundings. Current observations suggest a buildup of metals in the circumgalactic environments of galaxies over z ~ 6 to 5, although changes in ionisation will also affect the evolution of metal line properties. A substantial fraction of metal absorbers at these redshifts may trace relatively low-mass galaxies. Finally, we review constraints from the Ly α forest and quasar near zones on the timing of reionisation. Along with other probes of the high-redshift Universe, absorption line data are consistent with a relatively late end to reionisation (5.5 ≲ z ≲ 7); however, the constraints are still fairly week. Significant progress is expected to come through improved analysis techniques, increases in the number of known high-redshift quasars from optical and infrared sky surveys, large gains in sensitivity from next-generation observing facilities, and synergies with other probes of the reionisation era

Quasar spectral energy distribution in EUV restored from associated absorbers: indications to the HeII opacity of the quasar accretion disk wind

(abridged) Aims. To reconstruct the spectral shape of the quasar ionizing radiation in the extreme-UV range (1Ryd <= E < 10Ryd) from the analysis of narrow absorption lines (NAL) of the associated systems. Methods. Computational technique for inverse spectroscopic problems - Monte Carlo Inversion augmented by procedure of the spectral shape recovering and modified to account for the incomplete coverage of the light source. Results. The ionizing spectra responsible for the ionization structure of the NAL systems require an intensity depression at E > 4Ryd which is attributed to the HeII Lyman continuum opacity (tau^HeII_c ~ 1). A most likely source of this opacity is a quasar accretion disk wind. The corresponding column density of HI in the wind is estimated as a few times 10^16 cm^-2. This amount of neutral hydrogen should cause a weak continuum depression at lamb <= 912A (rest-frame), and a broad and shallow absorption in HI Ly-alpha. If metallicity of the wind is high enough, other resonance lines of OVI, NeVI-NeVIII, etc. are expected. In the analyzed QSO spectra we do observe broad (stretching over 1000s km/s) and shallow (tau << 1) absorption troughs of HI Ly-alpha and OVI 1031,1037A...Comment: 18 pages, 14 figures, 5 tables, accepted for publication in A&