Damped Absorbers Research Articles

The JWST-PRIMAL archival survey. A JWST/NIRSpec reference sample for the physical properties and Lyman-alpha absorption and emission of ~ 600 galaxies at z = 5:0 - 13:4

One of the surprising early findings with has been the discovery of a strong ``roll-over'' or a softening of the absorption edge of in a large number of galaxies at \(z 6\), in addition to systematic offsets from photometric redshift estimates and fundamental galaxy scaling relations. This has been interpreted as strong cumulative damped absorption (DLA) wings from high column densities of neutral atomic hydrogen ( signifying major gas accretion events in the formation of these galaxies. To explore this new phenomenon systematically, we assembled the PRImordial gas Mass AssembLy (PRIMAL) legacy survey of 584 galaxies at $z=5.0-13.4$, designed to study the physical properties and gas in and around galaxies during the reionization epoch. We characterized this benchmark sample in full and spectroscopically derived the galaxy redshifts, metallicities, star formation rates, and ultraviolet (UV) slopes. We defined a new diagnostic, the damping parameter to measure and quantify the net effect of emission strength, the fraction in the intergalactic medium, or the local column density for each source. The survey is based on the spectroscopic DAWN Archive (DJA-Spec). We describe DJA-Spec in this paper, detailing the reduction methods, the post-processing steps, and basic analysis tools. All the software, reduced spectra, and spectroscopically derived quantities and catalogs are made publicly available in dedicated repositories. We find that the fraction of galaxies showing strong integrated DLAs with $N_ HI $ only increases slightly from $ 60<!PCT!>$ at $z 6$ up to $ 65-90<!PCT!>$ at $z>8$. Similarly, the prevalence and prominence of emission is found to increase with decreasing redshift, in qualitative agreement with previous observational results. Strong emitters (LAEs) are predominantly found to be associated with low-metallicity and UV faint galaxies. By contrast, strong DLAs are observed in galaxies with a variety of intrinsic physical properties, but predominantly at high redshifts and low metallicities. Our results indicate that strong DLAs likely reflect a particular early assembly phase of reionization-era galaxies, at which point they are largely dominated by pristine gas accretion. At $z=8-10$, this gas gradually cools and forms into stars that ionize their local surroundings, forming large ionized bubbles and producing strong observed emission at $z<8$.

Vibration transmission and energy dissipation of roll controlled by particle damping absorber

The vibration of rolling mill in rolling process and its harm to rolling equipment have become an international problem in steel rolling industry. The essence of the vibration behavior of a mechanical structure is the transfer of energy in the system. In this work, for a proposed particle damped absorber (PDA) applied to roll vibration control, the influences of the PDA parameters on the vibration transmission characteristics of the system and the energy consumption characteristics of particle groups are studied from the standpoint of energy mechanism. Considering the nonlinearity of rolling interface and particle impact, a non-smooth four-degree-of-freedom vertical vibration model is established. The vibration displacement and velocity of the system are obtained by the incremental harmonic balance method (IHBM). The influence laws of the stiffness coefficients and damping coefficients in the PDA on the power flow are investigated according to the solutions of the system at different excitation frequencies. The effects of particle parameters on the energy consumption behavior of the particle groups are investigated to reveal the energy dissipation mechanism of PDA by using PFC3D software. The results show that the power flow transfer in Roll-PDA system has the targeting performance. This means that the energy loss share of each mechanism in the PDA can be effectively controlled by adjusting the parameters of the PDA. The energy consumption of the particle group increases in step form, which is more obvious in the case of low frequency vibration. In addition, the effects of particle material, particle size, filling rate and particle mixing on vibration suppression of PDA are investigated by experiments. The experimental results are compared with the simulation results to verify the reliability of the established model and the validity of the theoretical analysis. The findings of this study can provide insight into the selection of parameters for PDAs applied to different excitation frequencies.

Random vibrations of a structure modified by damped absorbers

The problem of reducing the level of vibrations in various constructions has been considered for many years. This is an important element of analyzing the technical condition of buildings and the comfort of users. Various ways and means of preventing unacceptable vibrations are known and the search for an optimal absorber is still ongoing. Different kinds of vibration absorbers are applied for reducing the vibration level in various engineering structures. Vibrations caused by traffic do not pose a threat to a structure, but are often a nuisance for people. There is a need to reduce them, especially in cities where there is more traffic. In order to be able to analyze an excited structure, traffic was modeled. A model that is a structure and an absorber system with two degrees of freedom is proposed in the paper. The first eigenfrequency of the structure was included in the analysis. As a result, the authors obtained closed equations of motion. The work examines the problem of the absorber effect on the vibrations of traffic-induced structures. It was assumed that the shocks occurring at random moments, and which form the Poisson process, are determined by random amplitude, a deterministic shape function and two different forms of envelope functions. The problem was discussed in the field of correlation theory and spectral density function. Numerical analysis of the displacement of the two-degree system is considered.

The Cosmic Ultraviolet Baryon Survey (CUBS). II. Discovery of an H2-bearing DLA in the Vicinity of an Early-type Galaxy at z = 0.576*

Abstract We report the serendipitous detection of an H2-bearing damped Lyα absorber at z = 0.576 in the spectrum of the QSO J0111–0316 in the Cosmic Ultraviolet Baryon Survey. Spectroscopic observations from Hubble Space Telescope-COS in the far-ultraviolet reveal a damped absorber with log[N(H i)/cm−2] = 20.1 ± 0.2 and log[N(H2)/cm−2] . The diffuse molecular gas is found in two velocity components separated by Δ ν ≈ 60 km s−1, with >99.9% of the total H2 column density concentrated in one component. At a metallicity of ≈50% of solar, there is evidence for Fe enhancement and dust depletion, with a dust-to-gas ratio κ O ≈ 0.4. A galaxy redshift survey conducted with IMACS and LDSS-3C on Magellan reveals an overdensity of nine galaxies at projected distance d ≤ 600 proper kpc (pkpc) and line-of-sight velocity offset Δ ν g ≤ 300 km s−1 from the absorber. The closest is a massive, early-type galaxy at d = 41 pkpc that contains ≈70% of the total stellar mass identified at d ≤ 310 pkpc of the H2 absorber. The close proximity of the H2-bearing gas to the quiescent galaxy and the Fe-enhanced chemical abundance pattern of the absorber suggest a physical connection, in contrast to a picture in which DLAs are primarily associated with gas-rich dwarfs. This case study illustrates that deep galaxy redshift surveys are needed to gain insight into the diverse environments that host dense and potentially star-forming gas.

Suppression of chaos in nonlinear oscillators using a linear vibration absorber

This paper proposes a nonfeedback control to suppress the chaotic response of nonlinear oscillators. A linear vibration absorber is used as nonfeedback method, whose key idea is to find conditions under which the nonlinear oscillator response converges to an equilibrium point or stay oscillating around it. Theoretical results show that if the ratio between the natural frequencies of the primary system $$(\omega _1)$$ and the undamped absorber $$(\omega _2)$$ , that is, $$\omega _r=\frac{\omega _2}{\omega _1}$$ is tuned to be equal to the excitation frequency $$(\Omega )$$ , then the chaotic behavior of a nonlinear oscillator is driven to a stable hyperbolic equilibrium point. Numerical results are presented for the Duffing oscillator shown that if $$\omega _r$$ is tuned close to excitation frequency, then the chaotic response of the Duffing oscillator is driven to periodic orbits. In the case of damped absorber, $$\omega _r$$ to be tuned close to the excitation frequency does not guarantee that the response of the primary system is periodic, it is also necessary to take into account the amplitude of the excitation.

Control of Beam Vibrations using Viscoelastically Damped Absorber System

The aim of this paper is to study the use of viscoelasically damped vibration absorber systems to optimally control the vibrations of a fixed-fixed beam. In this paper, a main beam with fixed-fixed boundary condition with viscoelstically damped cantilever beam as absorber is taken for the analysis. The paper includes optimum design & analysis of vibration absorber with viscoelastic damping. The equations of motion of the system have been derived to find the vibration response with absorber and used for optimization of parameters. The classical and Den Hartong optimization methods are used to optimize the design parameters and optimum values of design are found out. Theoretical calculations have been done for a fixed-fixed beam with three types of absorber beams (undamped, unconstrained treated damped absorber beam and constrained treated absorber beam). To validate the theoretical calculations, experiments have performed and deviation from theoretical data is discussed.

Stellar masses, metallicity gradients, and suppressed star formation revealed in a new sample of absorption selected galaxies

Context. Absorbing galaxies are selected via the detection of characteristic absorption lines which their gas-rich media imprint in the spectra of distant light-beacons. The proximity of the typically faint foreground absorbing galaxies to bright background sources makes it challenging to robustly identify these in emission, and hence to characterise their relation to the general galaxy population. Aims. We search for emission to confirm and characterise ten galaxies hosting damped, metal-rich quasar absorbers at redshift z < 1. Methods. We identified the absorbing galaxies by matching spectroscopic absorption -and emission redshifts and from projected separations. Combining emission-line diagnostics with existing absorption spectroscopy and photometry of quasar-fields hosting metal-rich, damped absorbers, we compare our new detections with reference samples and place them on scaling relations. Results. We spectroscopically confirm seven galaxies harbouring damped absorbers (a 70% success-rate). Our results conform to the emerging picture that neutral gas on scales of tens of kpc in galaxies is what causes the characteristic H I absorption. Our key results are: (I) Absorbing galaxies with log10[M⋆,(M⊙)] ≳ 10 have star formation rates that are lower than predicted for the main sequence of star formation. (II) The distribution of impact parameter with H I column density and with absorption-metallicity for absorbing galaxies at z ∼ 2–3 extends to z ∼ 0.7 and to lower H I column densities. (III) A robust mean metallicity gradient of ⟨Γ⟩ = −0.022 ± 0.001 dex kpc−1. (IV) By correcting absorption metallicities for ⟨Γ⟩ and imposing a truncation-radius at 12 kpc, absorbing galaxies fall on top of predicted mass-metallicity relations, with a statistically significant decrease in scatter.

ANALYSIS OF ENERGY ABSORBING DEVICE BASED ON DAMPED DYNAMIC VIBRATION ABSORBER

Vibrations exist widely in tall buildings, vehicle systems, and ocean platform suffering from environmental loading such as wind, wave or earthquake. With the global concern on energy and environmental issues, energy absorbing from large-scale vibrations becomes a research frontier. A type of damped dynamics vibration energy absorber is built and analyzed in this paper, in which an added mass is connected to the vibration system with the electromagnetic transducer and spring. The relationships between electrical damping ratio, excitation frequency ratio and dimensionless power are analyzed in frequency domain. The optimal parameters for maximizing the power output are discussed in analytical form while taking the inherent mechanical damping of the system into account. The results indicate that when the system has suitable optimal excitation frequency ratio and damping ratio and so on, more power can be obtained. It is helpful for the design of energy absorber devices.

Vibration Isolation of Lumped Masses Supported on Beam by Imposing Nodes Using Multiple Vibration Absorbers

<p class="1Body">In this paper, variable stiffness damped absorbers are used to isolate the substructures of Euler-Bernoulli beam, modelled as lumped masses, from vibrations. The novel algorithm is developed that can be used to determine the required absorber masses and resonance frequencies to impose nodes at selected locations on beam with the constraint of vibration amplitude of absorber mass. Numerical simulations are performed to show the effectiveness of the proposed algorithm. Experimental test is conducted on a cantilever beam with two absorbers to verify the numerical results.</p>

Dynamics Analysis of Energy Absorbing Device Based on Damped Dynamics Vibration Absorber

Nowadays, absorbing energy from vibration is one of the most promising technologies. In general, the vibrations may be very large, such as the vibrations of tall buildings, large flexible bridges, and ocean platform and so on in some environmental loading. With the global concern on energy and environmental issues, energy absorbing from large-scale vibrations for structural health monitoring purposes is more attractive and becomes a research frontier. A type of damped dynamics vibration energy absorber, where two masses are connected in series with the energy transducer and spring, is built and analyzed in this paper. The relationships among electrical damping ratio, excitation frequency ratio and dimensionless power are analyzed in frequency domain. The optimal parameters for maximizing the power output are discussed in analytical form while taking the parasitic mechanical damping of the system into account. In addition, the numerical simulations in time domain are calculated. The results indicate that when the system is excited by the larger peak of local optimal excitation frequency, more power can be obtained. It is helpful for design of energy absorber device.

Mitigation of post-flutter oscillations in suspension bridges by hysteretic tuned mass dampers

Systems of hysteretic tuned mass dampers are proposed for multi-mode flutter mitigation in long-span suspension bridges. The performance of the nonlinear absorbers is systematically compared with that of classical damped (linearly viscoelastic) absorbers. The equations of motion ensuing from a linearized parametric structural model of a suspension bridge are coupled with the equations governing the dynamics of the passive nonlinear control system and the time-dependent aerodynamic loads obtained through a quasi-steady nonlinear formulation and the unsteady indicial theory. The equations of the controlled aeroelastic system are reduced employing the Faedo-Galerkin method. Numerical simulations are performed to investigate the effectiveness of the vibration absorbers system comparing both the onset of flutter and the post-flutter behavior without the control devices with the corresponding responses obtained when multiple absorbers are installed.

1D25 Mass Measurement Using the Fixed-Point of a Spring-Mass System with a Dynamic Vibration Absorber and an Inertial-Mass Vibrator(The 12th International Conference on Motion and Vibration Control)

A mass-spring system with a dynamic vibration absorber has a fixed point in its frequency response curves. A method of measuring mass using this property was proposed. It has an advantage that mass is estimated independently of the damping in the absorber. The developed system was characterized by using a phase-looked loop to achieve the tuning condition. However, because the actuator producing the harmonic force was mounted on the base, the reaction force was transmitted to the surrounding structure. It may cause a severe vibration of the surrounding structure. Therefore, it is necessary to prevent transmission of force to the surrounding structure in measuring. In this paper, a new type of mass measurement system is proposed which uses an inertial-mass vibrator to reduce the transmitted vibration. The tuning conditions are derived for both systems with a damped absorber and with an undamped absorber. The principles of measurement are described based on a mathematical model. An experimental apparatus is designed and fabricated. The efficacy of the measurement system with an inertial mass-type vibrator is studied both analytically and experimentally.

The ESO UVES advanced data products quasar sample

Quasar foreground damped absorbers, associated with HI-rich galaxies allow to estimate the neutral gas mass over cosmic time, which is a possible indicator of gas consumption as star formation proceeds. The DLAs and sub-DLAs are believed to contain a large fraction of neutral gas mass in the Universe. In Paper I of the series, we present the results of a search for DLAs and sub-DLAs in the ESO-UVES Advanced Data Products dataset of 250 quasars. Here we use an unbiased sub-sample of sub-DLAs from this dataset. We build a subset of 122 quasars ranging from 1.5 <z_em < 5.0, suitable for statistical analysis. The statistical sample is analyzed in conjunction with other sub-DLA samples from the literature. This makes up a combined sample of 89 sub-DLAs over a redshift path of $\Delta z=193$. Redshift evolution of the number density and the line density are derived for sub-DLAs and compared with the LLSs and DLAs measurements from the literature. The results indicate that these three classes of absorbers are evolving in the redshift interval 1 < z < 5. The column density distribution, f(N,z), down to the sub-DLA limit is determined. The flattening of f_(N,z) in the sub-DLA regime is present in the observations. The redshift evolution of f_(N,z) down to sub-DLA regime is also presented, indicating the presence of more sub-DLAs at high-redshift as compared to low-redshift. f_(N,z) is further used to determine the neutral gas mass density, Omega_g, at 1.5 < z < 5.0. The complete sample shows that sub-DLAs contribute 8-20% to the total Omega_g from 1.5 < z < 5.0. In agreement with previous studies, no evolution of Omega_g is seen from low-redshift to high-redshift, suggesting that star formation solely cannot explain this non-evolution and replenishment of gas and/or recombination of ionized gas is needed. (Abridged)

Design of non-traditional dynamic vibration absorber for damped linear structures

The Voigt-type of dynamic vibration absorber is a classical model and has attracted considerable attention in many years because of its simple design, high reliability and useful applications in the fields of civil and mechanical engineering. Recently, a non-traditional type of dynamic vibration absorber was proposed. Unlike the traditional damped absorber configuration, the non-traditional absorber has a linear viscous damper connecting the absorber mass directly to the ground instead of the main mass. There have been some studies on the design of the non-traditional dynamic vibration absorber in the case of undamped primary structures. Those studies have shown that the non-traditional dynamic vibration absorber has better performance than the traditional dynamic vibration absorber. However, when damping is present at the primary system, there are very few studies on the design of non-traditional dynamic vibration absorber. This article presents a simple approach to determine the approximate analytical solutions for the [Formula: see text] optimization of the non-traditional dynamic vibration absorber attached to the damped primary structure subjected to force excitation. The main idea of the study is based on the dual criterion suggested by Anh in order to replace approximately the original damped structure by an equivalent undamped structure. Then the approximate analytical solution of dynamic vibration absorber’s parameters is given by using known results for undamped structure obtained. The comparisons have been done to verify the effectiveness of the obtained results.

Mass–metallicity relation from z = 5 to the present: evidence for a transition in the mode of galaxy growth at z = 2.6 due to the end of sustained primordial gas infall

We analyze the redshift evolution of the mass-metallicity relation in a sample of 110 Damped Ly$\alpha$ absorbers spanning the redshift range $z=0.11-5.06$ and find that the zero-point of the correlation changes significantly with redshift. The evolution is such that the zero-point is constant at the early phases of galaxy growth (i.e. no evolution) but then features a sharp break at $z=2.6\pm 0.2$ with a rapid incline towards lower redshifts such that damped absorbers of identical masses are more metal rich at later times than earlier. The slope of this mass metallicity correlation evolution is $0.35 \pm 0.07$ dex per unit redshift. We compare this result to similar studies of the redshift evolution of emission selected galaxy samples and find a remarkable agreement with the slope of the evolution of galaxies of stellar mass log$(M_{*}/M_\odot) \approx 8.5$. This allows us to form an observational tie between damped absorbers and galaxies seen in emission. We use results from simulations to infer the virial mass of the dark matter halo of a typical DLA galaxy and find a ratio $(M_{vir}/M_{*}) \approx 30$. We compare our results to those of several other studies that have reported strong transition-like events at redshifts around $z=2.5-2.6$ and argue that all those observations can be understood as the consequence of a transition from a situation where galaxies were fed more unprocessed infalling gas than they could easily consume to one where they suddenly become infall starved and turn to mainly processing, or re-processing, of previously acquired gas.

Enforcing Nodes to Suppress Vibration Along a Harmonically Forced Damped Euler-Bernoulli Beam

In this paper, properly tuned damped absorbers are used to suppress excess vibration anywhere along an arbitrarily supported, damped Euler-Bernoulli beam during forced harmonic excitations. This vibration suppression is achieved by enforcing distinct nodes, or points of zero vibration, at desired locations along the beam. Instead of directly solving for the absorber parameter, which is highly computationally intensive, an efficient method is developed whereby the restoring forces exerted by the damped absorbers are first determined using Gaussian elimination. These restoring forces are then used to tune the parameters of the damped oscillators. Numerical experiments show that by inducing nodes at the appropriate locations, a region of nearly zero vibration amplitudes can be enforced, effectively quenching vibration in that segment of the beam.

Rest-frame ultraviolet spectrum of the gravitationally lensed galaxy “the 8 o'clock arc”: stellar and interstellar medium properties

We present the first detailed analysis of the rest-frame ultraviolet spectrum of the gravitationally lensed Lyman break galaxy (LBG), the “8 o'clock arc”, obtained with the intermediate-resolution X-shooter spectrograph recently commissioned on the ESO Very Large Telescope. Besides MS 1512–cB58, the Cosmic Horseshoe, and the Cosmic Eye, three other lensed LBGs at comparable redshifts, this is the fourth of such a study, usually unfeasible at high redshifts. The spectrum of the 8 o'clock arc is rich in stellar and interstellar features, and presents several similarities to the well-known MS 1512–cB58 LBG. The stellar photospheric absorption lines allowed us to constrain the systemic redshift, <i>±<i/> <i>0.0003<i/>, of the galaxy, and derive its stellar metallicity, , which is in excellent agreement with the metallicity determined from nebular emission lines. With a total stellar mass of , the 8 o'clock arc agrees with the mass-metallicity relation found for <i>z>2<i/> star-forming galaxies, although being located near the upper end of the distribution given its high mass and high metallicity. Broad HeII <i>λ<i/>1640 emission is found, indicative of the presence of Wolf-Rayet stars formed in an intense period of star formation. The 31 interstellar absorption lines detected led to the abundance measurements of 9 elements. The metallicity of the interstellar medium (ISM), (Si), is very comparable to the metallicity of stars and ionized gas, and suggests that the ISM of the 8 o'clock arc has been rapidly polluted and enriched by ejecta of OB stars. The ISM lines extend over a very large velocity range, km s<sup>-1<sup/>, from about -800 to <i>+300<i/> km s<sup>-1<sup/> relative to the systemic redshift, and have their peak optical depth blueshifted relative to the stars, implying gas outflows of km s<sup>-1<sup/>. The zero residual intensity in the strongest lines indicates a nearly complete coverage of the UV continuum by the ISM. The Ly<i>α<i/> line is dominated by a damped absorption profile on top of which is superposed a weak emission, redshifted relative to the ISM lines by about +690 km s<sup>-1<sup/> and resulting from multiply backscattered Ly<i>α<i/> photons emitted in the HII region surrounded by the cold, expanding ISM shell. A homogeneous spherical shell model with a constant outflow velocity, determined by the observations, is able to reproduce the observed Ly<i>α<i/> line profile. Furthermore, the required dust content, , is in good agreement with the attenuation measured from the Balmer decrement. These results obtained from the radiation transfer modeling of the Ly<i>α<i/> line in the 8 o'clock arc fully support the scenario proposed earlier, where the diversity of Ly<i>α<i/> line profiles in Lyman break galaxies and Ly<i>α<i/> emitters, from absorption to emission, is mostly due to variations of HI column density and dust content.

Design of a non-traditional dynamic vibration absorber

A non-traditional dynamic vibration absorber is proposed for the minimization of maximum vibration velocity response of a vibrating structure. Unlike the traditional damped absorber configuration, the proposed absorber has a linear viscous damper connecting the absorber mass directly to the ground instead of the main mass. Optimum parameters of the proposed absorber are derived based on the fixed-point theory for minimizing the maximum vibration velocity response of a single-degree-of-freedom system under harmonic excitation. The extent of reduction in maximum vibration velocity response of the primary system when using the traditional dynamic absorber is compared with that using the proposed one. Under the optimum tuning condition of the absorbers, it is proved analytically that the proposed absorber provides a greater reduction in maximum vibration velocity response of the primary system than the traditional absorber.

Origin of Lyα absorption in nearby starbursts and implications for other galaxies

<i>Context. <i/>Despite the privileged position that Lyman-<i>α<i/> (Ly<i>α<i/>) emission line holds in the exploration of the distant universe and modern observational cosmology, the origin of the observed diversity of Ly<i>α<i/> profiles still needs to be thoroughly explained. Observations of nearby star forming galaxies bring their batch of apparent contradictions between Ly<i>α<i/> emission and their physical parameters, and call for a detailed understanding of the physical processes at work. One of the most metal-poor galaxies known, IZw 18, is of particular interest in this context.<i>Aims. <i/>We fit the Ly<i>α<i/> spectrum of IZw 18 to understand the origin of the damped absorption profile and its spatial variations across the NW region, and establish a general picture of the physical parameters governing the Ly<i>α<i/> strength and profile both in local and in high-<i>z<i/> galaxies.<i>Methods. <i/>We used a 3D Ly<i>α<i/> radiation transfer code to model Hubble Space Telescope (HST) observations of IZw 18. Observational constraints of relevant parameters such as dust or Hi column density were derived from previous studies and from the present analysis. Different geometrical configurations of the source and the neutral gas were explored.<i>Results. <i/>The integrated Ly<i>α<i/> profile of NW region of IZw 18 is reproduced using the observed small amount of dust ( <i>≈<i/> 0.05) and a spherical Hi shell with <i>N<i/><sub>HI<sub/> = 6.5 10<sup>21<sup/> cm<sup>-2<sup/>. Such a high column density makes it possible to transform a strong Ly<i>α<i/> emission (<i>EW<i/><sub>Ly\alpha<sub/> = 60 Å) into a damped absorption even with low extinction. When a slab geometry is applied and a given line of sight is chosen, the Ly<i>α<i/> profile can be successfully reproduced with no dust at all and <i>N<i/><sub>HI<sub/> = 3 10<sup>21<sup/> cm<sup>-2<sup/>. The spatial variations of the profile shape are naturally explained by radiation transfer effects, i.e. by scattering of Ly<i>α<i/> photons, when the observed surface brightness profile of the source is taken into account. In the case of outflowing inter stellar medium (ISM), as commonly observed in Lyman break galaxies (LBGs), a high <i>N<i/><sub>HI<sub/> and dust content are required to observe Ly<i>α<i/> in absorption. For nearly static neutral gas as observed in IZw 18 and other low-luminosity galaxies, only a small amount of dust is required provided a sufficiently high <i>N<i/><sub>HI<sub/> covers the galaxy. We also show how geometrical and apertures effects affect the Ly<i>α<i/> profile.

Intergalactic HeiiAbsorption in the Spectra of Quasars at Redshifts 3.5 and 3.8, Observed with theHSTACS Prism

We present the UV spectra of two quasars obtained with the Hubble Space Telescope Advanced Camera for Surveys (HST ACS) prism (R ~ 100). One quasar, at redshift z = 3.5, shows an absorption edge consistent either with nearly fully ionized helium with an intergalactic He II Lyα optical depth at 304 A of τ304 > 3.1 (90% confidence), or a He III region with edge 5.4 Mpc from the QSO. The spectrum of the second quasar, at z = 3.8, has a broad absorption profile extending across the He II emission line of the quasar consistent with a damped absorption profile from an intervening He II system at z 3.7 with a column density of approximately 1022 cm−2. In this case, the extremely high He II column density and absence of strong H I absorption suggests that the radiation field is quite weak at 4 ryd. The combined He II and H I signatures are consistent with photoionization by a compact (1-100 pc) star-forming region located approximately 10 Mpc from the quasar.