Lyα Forest Clouds Research Articles

Supersolar Metallicity in Weak MgiiAbsorption Systems atz∼ 1.71

Through photoionization modeling, constraints on the physical conditions of three z ~ 1.7 single-cloud weak Mg II systems [Wr(2796) ≤ 0.3 A] are derived. Constraints are provided by high-resolution R = 45,000, high-S/N spectra of the three quasars HE 0141–3932, HE 0429–4091, and HE 2243–6031, which we have obtained from the ESO archive of VLT UVES. Results are as follows: (1) The single-cloud weak Mg II absorption in the three z ~ 1.7 systems is produced by clouds with ionization parameters of -3.8 < log U − 2.0 and sizes of 1-100 pc. (2) In addition to the low-ionization phase Mg II clouds, all systems need an additional one to three high-ionization phase C IV clouds within 100 km s−1 of the Mg II component. The ionization parameters of the C IV clouds range from -1.9 < log U < − 1.0, with sizes of tens of parsecs to kiloparsecs. (3) Two of the three single-cloud weak Mg II absorbers have near-solar or supersolar metallicities, if we assume a solar abundance pattern. Although such large metallicities have been found for z < 1 weak Mg II absorbers, these are the first high metallicities derived for such systems at higher redshifts. (4) Two of the three weak Mg II systems also need additional low-metallicity, broad Lyα absorption lines, offset in velocity from the metal-line absorption, in order to reproduce the full Lyα profile. (5) Metallicities in single-cloud weak Mg II systems are more than an order of magnitude larger than those in DLA systems at z ~ 1.7. In fact, there appears to be a gradual decrease in metallicity with increasing NH I, from these, the most metal-rich Lyα forest clouds, to Lyman limit systems, to sub-DLAs, and finally to the DLAs.

Connecting Lost Baryons and Dark Galaxies via QSO Absorption Lines

AbstractQSO absorption lines are sensitive to very low density gas, and consequently, QSO spectroscopy provides a powerful tool for measuring the distribution, physical conditions, and metal enrichment of baryons in the intergalactic medium and in “dark galaxies”. However, ultraviolet spectroscopy is required to study QSO absorbers in the nearby universe where the connections between the absorbing gas and galaxies/environment can be probed. This talk reviewed several recent studies of low-zQSO absorbers in order to demonstrate the value of combining high-resolution ultraviolet spectroscopy with complementary information on the absorber environment, e.g., mapping of 21 cm emission in the vicinity of the absorbers; some notes on these examples are presented in this paper. The absorbers probed range from high-N(H I) damped Lyman α absorbers down to very low-column Lyα forest clouds with logN(H I) &lt; 13.5. The high-N(H I) systems are candidate “dark galaxies” – these systems are more metal enriched than the high-zIGM, possibly due to in-situ star formation, and some of the absorbers are highly metal enriched. However, we have obtained follow-up 21cm emission mapping with the VLA and deep optical imaging, and while we do find H I clouds associated with the absorbers in 21cm emission, we often do not find any evidence of in-situ stars. At this juncture, it seems more likely that these low-zabsorbers were enriched with metals within galaxies and were subsequently transported out into the IGM, e.g., by tidal stripping or galactic winds. This contribution also summarizes some recent results on the search for lost baryons in the “warm-hot” (shock-heated) low-redshift IGM.

The Impact of Stochastic Attenuation on Photometric Redshift Estimates

AbstractWe model the stochastic attenuation by HI absorbers in the intergalactic medium (IGM), such as Lyα Forest clouds, and absorbers associated with galaxies, such as Lyman Limit systems (LLS) and Damped Lyman Alpha absorbers (DLAs), and compute an ensemble of 4 · 103 attenuated Spectral Energy Distributions (SEDs) in the Johnson system for the spectrum of a galaxy with a constant star formation rate (CSFR). Using these, we asses the impact of the stochastic attenuation on the estimates of photometric redshifts for this type of galaxy by comparison with model SEDs that include only a mean attenuation.

Expansion and Collapse in the Cosmic Web

We study the kinematics of the gaseous cosmic web at high redshift using Lyα forest absorption in multiple QSO sight lines. Observations of the projected velocity shifts between Lyα absorbers common to the lines of sight to a gravitationally lensed QSO and three more widely separated QSO pairs are used to directly measure the expansion of the cosmic web in units of the Hubble velocity, as a function of redshift and spatial scale. The lines of sight used span a redshift range from about 2 to 4.5 and represent transverse scales from the subkiloparsec range to about 300 h physical kpc. Using a simple analytic model and a cosmological hydrodynamic simulation, we constrain the underlying three-dimensional distribution of expansion velocities from the observed line-of-sight distribution of velocity shear across the plane of the sky. The shape of the shear distribution and its width (14.9 km s-1 rms for a physical transverse separation of 61 h kpc at z = 2, 30.0 km s-1 for 261 h kpc at z = 3.6) are found to be in good agreement with the IGM undergoing large-scale motions dominated by the Hubble flow, making this one of the most direct observations possible of the expansion of the universe. However, modeling the Lyα clouds with a simple expanding pancake model, the average expansion velocity of the gaseous structures causing the Lyα forest in the lower redshift (z ~ 2) smaller separation (61 kpc) sample appears about 20% lower than the local Hubble expansion velocity. In order to understand the observed velocity distribution further we investigated the statistical distribution of expansion velocities in cosmological Lyα forest simulations. The mean expansion velocity in the (z ~ 2, separation ~ 60 kpc) simulation is indeed somewhat smaller than the Hubble velocity, as found in the real data. We interpret this finding as tentative evidence for some Lyα forest clouds breaking away from the Hubble flow and undergoing the early stages of gravitational collapse. However, the distribution of velocities is highly skewed, and the majority of Lyα forest clouds at all redshifts from 2 to 3.8 expand with super-Hubble velocities, typically about 5%-20% faster than the Hubble flow. This behavior is explained if most Lyα forest clouds in the column density range typically detectable are expanding filaments that stretch and drain into more massive nodes. The significant difference seen in the velocity distributions between the high- and low-redshift samples may conceivably reflect actual peculiar deceleration, the differences in spatial scale, or our selecting higher densities at lower redshift for a given detection threshold for Lyα forest lines. We also investigate the alternative possibility that the velocity structure of the general Lyα forest could have an entirely different, local origin, as expected if the Lyα forest were produced or at least significantly modified by galactic feedback, e.g., winds from star-forming galaxies at high redshift. However, we find no evidence that the observed distribution of velocity shear is significantly influenced by processes other than Hubble expansion and gravitational instability. To avoid overly disturbing the IGM, galactic winds may be old and/or limp by the time we observe them in the Lyα forest, or they may occupy only an insignificant volume fraction of the IGM. We briefly discuss the observational evidence usually presented in favor of an IGM afflicted by high-redshift extragalactic superwinds and find much of it ambiguous. During the hierarchical buildup of structure, galaxies are expected to spill parts of their interstellar medium and to heat and stir the IGM in ways that make it hard to disentangle this gravitational process from the effects of winds.

Sharp HiEdges at Highz: The Gas Distribution from Damped Lyα to Lyman Limit Absorption Systems

We derive the distribution of neutral and ionized gas in high-redshift clouds, which are optically thick to hydrogen ionizing radiation, using published data on Lyman limit and damped Lyα absorption systems in the redshift range 1.75 ≤ z < 3.25. We assume that the distribution of the hydrogen total (H I+H II) column density in the absorbers, NH, follows a power law KN, whereas the observed H I column density distribution deviates from a pure power law as a result of ionization from a background radiation field. We use an accurate radiative transfer code for computing the rapidly varying ratio NH/N as a function of NH. Comparison of the models and observations gave excellent fits with maximum likelihood solutions for the exponent α and X, the value of log when the Lyman limit optical depth along the line of sight is τLL = 1. The slope of the total gas column density distribution with its relative 3 σ errors is α = 2.7 and X = 2.75 ± 0.35. This value of X is much lower than what would be obtained for a gaseous distribution in equilibrium under its own gravity. The ratio η0 of dark matter to gas density, however, is not well constrained since log (η0) = 1.1 ± 0.8. An extrapolation of our derived power-law distribution toward systems of lower column density, the Lyα forest, tends to favor models with log(η0) 1.1 and α ~ 2.7-3.3. With α appreciably larger than 2, Lyman limit systems contain more gas than damped Lyα systems and Lyα forest clouds even more. Estimates of the cosmological gas and dark matter density due to absorbers of different column density at z ~ 2.5 are also given.

Chemical enrichment at high redshifts

We have tried to understand the recent observations related to metallicity in Ly-α forest clouds in the framework of the two component model. The model consists of mini-halos having circular velocities smaller than ∼ 55 km s−1, with no star formation and galactic halos with higher circular velocities ≤ 250 km s−1, having clouds, star formation and consequent metal enrichment. We find that even if the mini-halos were chemically enriched by an earlier generation of stars, to have [C/H] ≃ −2.5, the number of C IV lines with column density >1012 cm−2, contributed by the mini-halos, at the redshift of 3, would be only about 10% of the total number of lines, for a chemical enrichment rate of (1 + z)−3 in the galaxies. Recently reported absence of heavy element lines associated with most of the Ly-α lines with HI column density between 1013.5 cm−2 and 1014 cm−2 by Lu et al [13], if correct, gives an upper limit on [C/H]= −3.7, not only in the mini-halos, but also in the outer parts of galactic halos. However, the mean value of 7×10−3 for the column density ratio of C IV and HI, determined by Cowie and Songaila (1998) for low Ly-α optical depths, implies an abundance of [C/H]= −2.5 in mini-halos as well as most of the region in galactic halos. The redshift and column density distribution of C IV has been shown to be in reasonable agreement with the observations.

Limits on dust and metallicity evolution of Lyα forest clouds from COBE

We consider the possible observational consequences of dust and metals in Lyα forest clouds. We relate the dust content, ΟdLyα, to the metal evolution of the absorbers and assume that dust is heated by the ultraviolet background radiation and by the cosmic microwave background (CMB). We find that the dust temperature deviates from TCMB by at most 10 per cent at redshift z=0. The Lyα cloud dust opacity to redshift ∼ 5 sources around the observed wavelength λ0∼ 1 μ m is τ∼ 0.13, and could affect observations of the distant universe in that band. The expected CMB spectral distortions arising from high-z dust in Ly α clouds is ∼ 1.25--10 smaller than the current COBE upper limit, depending on the metallicity evolution of the clouds. If Ly α clouds are clustered, the corresponding CMB anisotropy due to dust is ∼ 10-1 on angular scales θ≤ 10 arcsec at frequencies probed by various future/ongoing far-infrared (FIR) missions, which makes these fluctuations potentially detectable in the near future. Emission from C ii fine-structure transitions could considerably contribute to the submillimetre (submm) range of the FIR background radiation. Depending on the ionization of carbon and on the density of metal-enriched regions, this contribution can be comparable with the observed residual flux at λ≈ 0.15 mm, after CMB subtraction. We argue that constraints on metal evolution versus redshift can be obtained from the observed flux in that range.

The High-Redshift H[CLC]e[/CLC] [CSC]ii[/CSC] Gunn-Peterson Effect: Implications and Future Prospects

Absorption due to He II Lyα has now been detected at low resolution in the spectra of four quasars between redshifts z = 2.74 and z = 3.29. We assess these observations, giving particular attention to the radiative transfer of the ionizing background radiation, cloud diffuse emission and ionization physics, and statistical fluctuations. We use high-resolution observations of H I absorption toward quasars to derive an improved model for the opacity of the intergalactic medium (IGM) from the distribution of absorbing clouds in column density and redshift. We use these models to calculate the H I and He II photoionization history, the ratio η = He II/H I in both optically thin and self-shielded clouds, and the average line-blanketing contribution of the clouds to He II absorption. The derived ionization rate, ΓH I = (1–3) × 10-12 s-1 (z = 2–4), is consistent with the ionizing background intensity inferred from the proximity effect, but it remains larger than that inferred by N-body hydrodynamic simulations of the Lyα absorber distribution. The He II observations are consistent with line blanketing from clouds having NH ≥ 1012 cm-2, although a contribution from a more diffuse IGM would help to explain the observed opacity. We compute the expected He II optical depth, τHe I(z), and examine the implications of the sizable fluctuations that arise from variations in the cloud numbers and ionizing radiation field. We assess how He II absorption constrains the intensity and spectrum of the ionizing radiation and the fractional contributions of the dominant sources (quasars and starburst galaxies). Finally, we demonstrate how high-resolution ultraviolet observations can distinguish between absorption from the diffuse IGM and the Lyα forest clouds and determine the source of the ionizing background.

High‐Metallicity MgiiAbsorbers in thez&lt; 1 Lyα Forest of PKS 0454+039: Giant Low Surface Brightness Galaxies?

We report the discovery of two iron-group enhanced high-metallicity Mg II absorbers in a search through 28 Lyα forest clouds along the PKS 0454+039 sight line. Based upon our survey and the measured redshift number densities of Wr(Mg II) ≤ 0.3 A absorbers and Lyα absorbers at z ~ 1, we suggest that roughly 5% of Lyα absorbers at z ≤ 1 will exhibit weak Mg II absorption to a 5 σ Wr(λ2796) detection limit of 0.02 A. The two discovered absorbers, at redshifts z = 0.6248 and z = 0.9315, have Wr(Lyα) = 0.33 and 0.15 A, respectively. Based upon photoionization modeling, the H I column densities are inferred to be in the range 15.8 ≤ log N(H I) ≤ 16.8 cm-2. For the z = 0.6428 absorber, if the abundance pattern is solar, then the cloud has [Fe/H] > -1; if its gas-phase abundance follows that of depleted clouds in our Galaxy, then [Fe/H] > 0 is inferred. For the z = 0.9315 absorber, the metallicity is [Fe/H] > 0, whether the abundance pattern is solar or suffers depletion. Imaging and spectroscopic studies of the PKS 0454+039 field reveal no candidate luminous objects at these redshifts. We discuss the possibility that these Mg II absorbers may arise in the class of giant low surface brightness galaxies, which have [Fe/H] ≥ -1, and even [Fe/H] ≥ 0, in their extended disks. We tentatively suggest that a substantial fraction of these weak Mg II absorbers may select low surface brightness galaxies out to z ~ 1.

Cosmological Conductive/Cooling Fronts as Lyα Forest Clouds

We propose a simple model for the origin and evolution of \lya clouds based on cosmological conductive/cooling fronts. In this model the \lya arises in the interfaces between the IGM and cold clouds that could be tentatively identified with protogalaxies. Most of the properties of the \lya absorbers are reproduced with a very restricted number of assumptions. Among these are the correct range of HI column density, cloud sizes and redshift and HI column density distributions for the absorbers. Several predictions and implications of the model are briefly discussed.

Radiative Transfer in a Clumpy Universe. II. The Ultraviolet Extragalactic Background

view Abstract Citations (1427) References (99) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radiative Transfer in a Clumpy Universe. II. The Ultraviolet Extragalactic Background Haardt, Francesco ; Madau, Piero Abstract The integrated ultraviolet flux arising from QSOs and/or hot, massive stars in metal-producing young galaxies is likely responsible for maintaining the intergalactic diffuse gas and the Lyα forest clouds in a highly ionized state. The spectrum and intensity of such UV background have generally been obtained by modeling the reprocessing due to intervening material as a pure photoelectric absorption process. However, owing to the emission from radiative recombinations within the absorbing clouds, a photoionized clumpy medium could contribute substantially to the metagalactic flux. In other words, QSO absorption- line systems are sources, not just sinks of ionizing photons. We present a detailed calculation of the propagation of AGN-like ionizing radiation through the intergalactic space. We model the ionization state of absorbing clouds, and show that the universe will be more opaque above 4 Ry than previously estimated. Singly ionized helium in Lyα forest clouds and Lyman-limit systems is found to be very efficient in reprocessing soft X-ray, helium-ionizing photons into ultraviolet, hydrogen-ionizing ones. We demonstrate that a significant fraction of the absorbed primary photons (emitted, e.g., by quasar sources) will be reradiated by the photoionized gas through Lyα line emission, two-photon continuum, and recombination continuum radiation. In the light of new data and recent studies, we also reassess the contribution of the QSOs observed in optical surveys to the UV extragalactic background, and find that the stochastic reprocessing of quasar Lyman continuum radiation by hydrogen and helium along the line of sight will significantly affect the amplitude, spectral shape, and fluctuation properties of the metagalactic flux. In a scenario in which QSOs are the primary source of ionizing photons in the universe, the integrated H I Lyα emission at z = 0 from photoionized Lyα clouds and Lyman limit systems is found to be at a level of less than 5% of current observational limits on the far-UV extragalactic radiation flux. We show that J_912_ increases from ~10^-23^ ergs cm^-2^ s^-1^ Hz^- 1^ sr^-1^ at the present epoch to ~5 x 10^-22^ ergs cm^-2^ s^-1^ Hz^-1^ sr^-1^ at z = 2.5. The attenuated direct quasar emission plus recombination radiation from intergalactic gas appear to provide enough hydrogen-ionizing photons to satisfy the proximity effect at large redshift. The He II/H I ratio in the diffuse intergalactic medium and the Lyα clouds increases from ~25 at z = 0 to ~45 at z = 2.5, to decrease again below 30 for z >~ 4.5. The spectrum of the ionizing background at high redshift is shown to have a hump at energies below 40.8 eV due to redshift-smeared He Il Lyα line and two-photon continuum emission. We propose that observations of low-ionization species such as O II in metal-line absorption systems may be able to test the presence of such a prominent feature in the UV background spectrum. We also note that, if the metagalactic flux is dominated by QSOs, the suggested steep decline of their ionizing emissivity beyond z ~ 4 should produce an increase in the observed rate of incidence of Lyα forest clouds at these redshifts relative to an extrapolation from the intermediate-z regime, as observed by Williger et al. (1994). Publication: The Astrophysical Journal Pub Date: April 1996 DOI: 10.1086/177035 arXiv: arXiv:astro-ph/9509093 Bibcode: 1996ApJ...461...20H Keywords: COSMOLOGY: THEORY; COSMOLOGY: DIFFUSE RADIATION; GALAXIES: INTERGALACTIC MEDIUM; GALAXIES: QUASARS: ABSORPTION LINES; RADIATIVE TRANSFER; Astrophysics E-Print: revised version submitted to ApJ 20 sept 95, full text sources arXiv | ADS | data products SIMBAD (4) Related Materials (3) Part 1: 1995ApJ...441...18M Part 3: 1999ApJ...514..648M Part 4: 2012ApJ...746..125H

Evolution of the EUV Background from Quasar Absorption Line Studies

The integrated extreme ultraviolet (EUV) radiation from quasars and other high redshift sources provides an ambient ionizing radiation field which may photoionize the gas seen as quasar absorption lines. In particular, the observed evolution of the Lyα forest clouds probably results in part from the evolution of the EUV metagalactic field. Estimates of the EUV field as a function of redshift can be made from measuring the “proximity effect” in quasar spectra; uncertainties in these estimates may be large. Given the uncertainties, the estimated EUV field at z≈3 derived from the proximity effect is in reasonable agreement with the expected contribution from luminous quasars.

Implications of Nonthermal Motions for Statistics of Lyman-Alpha Forest Clouds

view Abstract Citations (5) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Implications of Nonthermal Motions for Statistics of Lyman-Alpha Forest Clouds Kulkarni, Varsha P. ; Fall, S. Michael Abstract We investigate the effects of nonthermal bulk motions within Lyα forest clouds on some observationally derived statistics of these clouds. We calculate the absorption-line profiles and curves of growth for three illustrative cases of single-peaked, non-Maxwellian velocity distributions. We then calculate the distributions in H I column density f(N) and velocity dispersion f(o) for Lyα forest clouds, using the corresponding distributions derived under the Maxwellian assumption, while also maintaining consistency with measurable line properties (Lyα central intensity and Lyα equivalent width, or Lyman alpha and Lyβ equivalent widths). We find that, for systems with saturated Lyα lines, in the case of velocity distributions with shorter tails than the Maxwellian, f(N) is steeper at log N ≳ 15.2 than f(N) in the Maxwellian case. On the other hand, in the case of velocity distributions with longer tails than the Maxwellian, f(N) is shallower at log N ≳ 15.9 than f(N) in the Maxwellian case. The effect found here is not expected to be relevant for log N ≳ 17.2, where f(N) is derived from quantities independent of the velocity distribution. The f(σ) also depends on the velocity distribution, but less so than f(N). Finally, using some published high-resolution observations, we find that, even for samples that show no N-σ correlation in the case of the Maxwellian velocity distribution, a significant positive correlation exists in the cases of the velocity distributions with shorter tails than the Maxwellian, while a significant negative correlation exists in the cases of the velocity distributions with longer tails than the Maxwellian. Our results imply that various properties derived from f(N), such as the effective photoelectric opacity of the Lyα forest clouds to the ionizing background at high redshifts, could differ from previous estimates if the velocity distribution within Lyα forest clouds were non-Maxwellian. Publication: The Astrophysical Journal Pub Date: November 1995 DOI: 10.1086/176370 Bibcode: 1995ApJ...453...65K Keywords: GALAXIES: INTERGALACTIC MEDIUM; GALAXIES: QUASARS: ABSORPTION LINES full text sources ADS |

A spectroscopic method for measuring turbulence in QSO Ly? forest clouds

The effects of a finite correlation length of a turbulent velocity field on the line formation process are considered for hydrogen absorption lines observed in quasar spectra. It is shown that both thermal and large-scale stochastic motions with a finite correlation length (0<l<∞) lead in general tosmaller equivalent widths of hydrogen lines than in the case when macroscopic motions are assumed to be completely uncorrelated (l≡0). Different influence of this effect on optically thin and thick absorption lines allows us to measure eventually turbulent characteristics of the gas in the Lyα forest clouds. It is emphasized that possible measurements of the deuterium abundance in the early Universe could yield systematically higher ratios of D/H if the finite-correlation-length effects are ignored.

Intergalactic Helium Absorption toward High-Redshift Quasars

view Abstract Citations (33) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Intergalactic Helium Absorption toward High-Redshift Quasars Giroux, Mark L. ; Fardal, Mark A. ; Shull, J. Michael Abstract The recent Hubble Space Telescope (HST) observations of the zq = 3.286 quasar Q0302 -003 (Jakobsen et al. 1994) and the zq = 3.185 quasar Q1935 - 67 by Tytler (1995) show absorption edges at the redshifted wavelength of He II 304 Å. A key goal is to distinguish between contributions from discrete Lyα forest clouds and a smoothly distributed intergalactic medium (IGM). We model the contributions from each of these sources of He II absorption, including the distribution of line Doppler widths and column densities, the "He II proximity effect" from the quasar, and a self-consistent derivation of the He II opacity of the universe as a function of the spectrum of ionizing sources, with the assumption that both the clouds and the IGM are photoionized. The He II edge can be fully accounted for by He II line blanketing for reasonable distributions of line widths and column densities in the Lyman alpha forest, provided that the ionizing sources have spectral index αs > 1.5, and any He II proximity effect is neglected. Even with some contribution from a diffuse IGM, it is difficult to account for the edge observed by Jakobsen et al. (1994) with a "hard" source spectrum (αs < 1.3). The proximity effect modifies the relative contributions of the clouds and IGM to τHe II near the quasar (z ≲ zq) and markedly increases the amount of He II absorption required. This implies, for example, that to account for the He 11 edge with line blanketing alone, the minimum spectral index αs must be increased from 1.5 to 1.9. We demonstrate the need for higher resolution observations that characterize the change in transmission as z → zq and resolve line-free gaps in the continuum. We set limits on the density of the diffuse IGM and suggest that the IGM and Lyman alpha clouds are likely to be a significant repository for dark baryons. Publication: The Astrophysical Journal Pub Date: October 1995 DOI: 10.1086/176236 Bibcode: 1995ApJ...451..477G Keywords: GALAXIES: EVOLUTION; GALAXIES: INTERGALACTIC MEDIUM; GALAXIES: QUASARS: ABSORPTION LINES full text sources ADS | data products SIMBAD (1) NED (1)

Toward unbiased determination of the redshift evolution of Lyman-alpha forest clouds

view Abstract Citations (12) References (34) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Toward Unbiased Determination of the Redshift Evolution of Lyman-Alpha Forest Clouds Lu, Limin ; Zuo, Lin Abstract The possibility of using D_A_, the mean depression of a quasar spectrum due to Lyα forest absorption, to study the number density evolution of the Lyα forest clouds is examined in some detail. Current D_A_ measurements are made against a continuum that is a power- law extrapolation from the continuum longward of Lyα emission. Compared to the line-counting approach, the D_A_-method has the advantage that the D_A_ measurements are not affected by line-blending effects. However, we find using low-redshift quasar spectra obtained with the Hubble Space Telescope (HST), where the true continuum in the Lyα forest can be estimated fairly reliably because of the much lower density of the Lyα forest lines, that the extrapolated continuum often deviates systematically from the true continuum in the forest region. Such systematic continuum errors introduce large errors in the D_A_ measurements. The current D_A_ measurements may also be significantly biased by the possible presence of the Gunn-Peterson absorption. We propose a modification to the existing D_A_-method, namely, to measure D_A_ against a locally established continuum in the Lyα forest. Under conditions that the quasar spectrum has good resolution and S/N to allow for a reliable estimate of the local continuum in the Lyα forest, the modified D_A_ measurements should be largely free of the systematic uncertainties suffered by the existing D_A_ measurements. We also introduce a formalism based on the work of Zuo (1993) to simplify the application of the D_A_-method(s) to real data. We discuss the merits and limitations of the modified D_A_-method, and conclude that it is a useful alternative. Our findings that the extrapolated continuum from longward of Lyα emission often deviates systematically from the true continuum in the Lyα forest present a major problem in the study of the Gunn- Peterson absorption. Publication: The Astrophysical Journal Pub Date: May 1994 DOI: 10.1086/174086 Bibcode: 1994ApJ...426..502L Keywords: Absorption Spectra; Intergalactic Media; Line Spectra; Lyman Alpha Radiation; Quasars; Red Shift; Spectrum Analysis; Bias; Cosmology; Errors; Estimates; Evolution (Development); Astronomy; COSMOLOGY: OBSERVATIONS; GALAXIES: INTERGALACTIC MEDIUM; GALAXIES: QUASARS: ABSORPTION LINES full text sources ADS | data products SIMBAD (7) NED (7) MAST (1) ESA (1)

Diffuse H alpha Emission in the Virgo Cluster Associated with the 3C 273 Lyman-Alpha Clouds

view Abstract Citations (9) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Diffuse H alpha Emission in the Virgo Cluster Associated with the 3C 273 Lyman-Alpha Clouds Williams, T. B. ; Schommer, R. A. Abstract We have discovered diffuse Hα emission at a velocity of 1713 +.- 4 km s^-1^ in the vicinity of the line of sight to the quasar 3C 273. The flux of this line emission is 11.7 +/- 1.8 x 10^-19^ ergs cm^-2^ sec^-1^ arcsec^-1^, and appears to be uniformly distributed over a region of at least 7.7' diameter about the quasar. The velocity of the emission agrees with that of one of the Lyα forest clouds along the line of sight to the quasar and thus is presumably part of the structure responsible for this Lyα absorption. The emission seems to arise from diffuse ionized gas within the Virgo cluster of galaxies, which is along the sightline to 3C 273. Publication: The Astrophysical Journal Pub Date: December 1993 DOI: 10.1086/187135 Bibcode: 1993ApJ...419L..53W Keywords: GALAXIES: QUASARS: ABSORPTION LINES; GALAXIES: INTERGALACTIC MEDIUM; GALAXIES: QUASARS: INDIVIDUAL ALPHANUMERIC: 3C 273; GALAXIES: CLUSTERS: INDIVIDUAL NAME: VIRGO full text sources ADS | data products SIMBAD (2) NED (1)

Lyman alpha emission from thick clouds photoionized by the metagalactic radiation

We present calculations of the expected Lyα surface brightness and emergent spectral line profiles from thick (N H ≥10 20 cm −2 ), massive (≥ 10 10 M ○ .) clouds at high redshift (z ≥ 3) photoionized by the metagalactic background radiation. Our results directly apply to the damped Lyα systems and to the «pancakes» that are expected to form at large redshift in the Sunyaev-Zel'dovich theory of cluster formation. We consider both quasar-dominated and stellar-dominated metagalactic fluxes. For the quasar-dominated metagalactic flux, we use two energy distributions: a power law with index − 1, which is representative of a «raw» (unfiltered) quasar energy distribution, and a distribution from Miralda-Escude and Ostriker [ApJ, 350, 1 (1990)] in which the ionizing background arises from quasar emission filtered by dust intervening galaxies and by Lyα forest clouds

Equilibrium slab models of Lyman-alpha clouds

We model the Lyα clouds as slabs of hydrogen with an ionizing extragalactic radiation field incident from both sides. In general, the equilibrium configuration of a slab at redshift z &lt;~ 5 is determined by a balance of the gas pressure, gravity (including the effects of a dark matter halo), and the pressure exerted by the inter-galactic medium, P_ext_. These models have been used to make predictions of the number of slabs as a function of the neutral hydrogen column density, N_H_. A break in the curve is predicted at the transition between regimes where gravity and pressure are the dominant confining forces, with a less rapid decrease at larger NH. The transition from optically thin to optically thick slabs leads to a gap in the distribution, whose location is governed largely by the spectrum of ionizing radiation. Observations provide us with the distribution of N_H_ at z ~ 2.5, from the Lyα forest through the damped Lyα clouds. Recent HST data also provide a rough count of forest clouds at present and an indication of the rate of evolution. We define the parameters ζ ~ (1 + z)^j^ and P_ext_ ~ (1 + z)^P^, representing the time-dependent ionization rate and external pressure. Comparisons with various observations are used to constrain these parameters- assuming that the population of clouds is described by a power-law distribution of total column densities (1) The best fit is provided by P_ext_/k = 366 cm^-3^ K near z ~ 2.5, based on the current observed value of the relative number of damped and forest clouds. If damped systems are undercounted due to dust obscuration, a smaller value of P_ext_/k will result. (2) At z ~ 2.5 the number of Lyα forest clouds is observed to decrease rapidly, with the number dN/dz ~ (1 + z)^2.4^. This constrains the clouds to be pressure confined at that time, since gravity-confined clouds would evolve too slowly. (3) The large number of observed low z forest clouds (slower evolution since z ~ 2.5) requires a large value of P_ext_/k and j ~&gt; 3. Gravity confinement may lead to evolution that is too slow, unless j &lt;~ 0 or unless the gravity-confined clouds are mixed with some more rapidly evolving pressure-confined clouds. Combining the various arguments we find the best model has P_ext_/k ~ 1 cm^-3^ K and ζ ~ 10^-14^ s^-1^ at present. Much smaller values of the pressure seem unlikely. There are certain parallels between lines of sight through the outer HI disk of spiral galaxy with increasing radius, and the progression from damped, to Lyman limit, to forest clouds. We discuss briefly the possibility that at least some of the observed low z forest clouds may be a separate population associated with galaxies, as suggested by the observations of Bahcall et al. This population could dominate the forest at present if the dark matter attached to galaxies should lead to gravity confinement for this disk population, while the isolated clouds remain pressure confined. The formalism developed in this paper will allow a more detailed study. We also discuss a more general parameter study of the equilibrium configuration of slabs, including mock gravity and Lyα photon trapping. Although mock gravity on dust does not impact the analysis of Lyα clouds at z &lt;~ 5, it may affect early growth of small scale structure at higher z.

The redshift distribution of Ly-alpha clouds and the proximity effect

The results of a systematic analysis of the redshift distribution of Lyα forest clouds are presented. We form a Lyα line sample from 38 QSO spectra which exhibit neither broad absorption line systems nor the "associated" Z_abs_ ~ Z_em_ metal line systems. The absorption line sample includes only Lyα lines which occur between Lyβ and Lyα emission, which have rest frame equivalent width greater than 0.36 A, and which are not associated with any known metal line systems. The sample contains 950 lines, the largest by far in this kind of study. We confirm that there is overwhelming evidence for the evolution in number density of Lyα forest lines: for the functional form dN/dz is proportional to (1 + z)<SUP>gamma</SUP>^, we find γ = 2.37 +/- 0.26. Using two mutually independent samples we confirm the existence of an inverse effect or proximity effect in that the Lyα line number density tends to be systematically lower near the QSO emission redshifts than elsewhere. We find no positive evidence that the proximity effect is correlated significantly with the QSO luminosities, contrary to previous results. Explanations are sought to reconcile this paradox. We also find that the proximity effect is uncorrelated with the radio properties of QSOs. We show that the data are entirely consistent with a model in which the inverse effect is due to the enhanced ionization of the Lyα clouds by the nearby QSOs. Assuming the model is correct, the background mean intensity, J_v_, is found to be log J_v_ ~ 21.0, where J_v_ is the Lyman limit intensity in unit ergs cm^-2^ s^-1^ Hz^-1^ sr^-1^. This value of J_v_ is consistent with a previous result from a similar study (BDO). When the proximity effect is taken into consideration, the redshift index is found to be γ = 2.75 +/- 0.29. This value should be considered as a more realistic estimate for the redshift evolution of Lyα clouds than the previously quoted value of γ = 2.37. In addition, we consider two other alternative explanations for the inverse effect, i.e., (1) the Lyα clouds may not be big enough to cover the entire broad emission line regions, or (2) the functional form, (1 + z)<SUP>gamma</SUP>^, may be inadequate to describe the redshift distribution of the Lyα clouds, thus causing the inverse effect. We find no evidence in the data, however, that support either of them. Further tests are suggested.