Hydrogen Density Distribution Research Articles

SRG/eROSITA 3D mapping of the interstellar medium using X-ray absorption spectroscopy

We present a detailed study of the hydrogen density distribution in the local interstellar medium (ISM) using the X-ray absorption technique. Hydrogen column densities were precisely measured by fitting X-ray spectra from coronal sources observed during the initial eROSITA all-sky survey (eRASS1). Accurate distance measurements were obtained through cross-matching Galactic sources with the third Gaia data release (DR3). Despite the absence of a discernible correlation between column densities and distances or Galactic longitude, a robust correlation with Galactic latitude was identified. This suggests a decrease in ISM material density in the vertical direction away from the Galactic plane. We have also investigated the relation between the optical extinction and the hydrogen column density. To do so, we employed multiple density laws to fit the measured column densities, revealing constraints on height scale values ($9 < h_ z < 14$ pc). Unfortunately, radial scales and the central density remain unconstrained due to the scarcity of sources near the Galactic center. Subsequently, a 3D density map of the ISM was computed using a Gaussian process approach, inferring hydrogen density distribution from hydrogen column densities. The results unveil the presence of multiple beams and clouds of various sizes, indicative of small-scale structures. High-density regions were identified at approximately 100 pc, consistent with findings in dust-reddening studies, and are potentially associated with the Galactic Perseus arm or the local bubble. Moreover, high-density regions were pinpointed in proximity to the Orion, Chameleon, and Coalsack molecular complex, enriching our understanding of the intricate structure of the local ISM.

A numerical study of fuel recirculation in ammonia-fueled solid oxide fuel cell stacks

Fuel recirculation is a system configuration designed to improve the fuel utilization of fuel cells by recirculating the unreacted fuels at the outlet to the fuel inflow stream. In this study, fuel recirculation of a solid oxide fuel cell (SOFC) operated with ammonia is investigated. Recirculation operation results in a notable elevation of nitrogen content within the cell, originating from the decomposition of ammonia within the cell and subsequently reintroduced into the inflow stream. Therefore, the fuel flow rate needs to be increased to maintain the fuel utilization level the same as the base operation, which is an ammonia-fueled operation without recirculation. A multiscale model of an SOFC stack is used to compare the base operation and saturated level of the recirculation operation cases based on their polarization curves and distributions of ammonia, hydrogen, current density, temperature, first principal stress, and nitriding in the fuel electrode. The results show that overall stack performance, as reflected in the polarization curves, remains comparable between the base and recirculation operation cases. The power densities exhibit a difference of less than 2% between the base and recirculation operation cases. Moreover, it is indicated that the recirculation operation cases lead to more uniform hydrogen and current density distributions, lower temperature gradients, and lower thermal stresses. The results demonstrate that the recirculation operation cases lead to a reduction of up to 38% in the maximum tensile stresses within the fuel support layer. Nevertheless, these recirculation operation cases also induce nitriding across a larger portion of the active area. But as shown here this is a challenge that can effectively be addressed by applying nickel coatings to the inlet headers.

Bond energies and hydrogen density distributions for hydrogen bond

Abstract The three-body problem for hydrogen bond (X–Y–X; Y=H or D) is discussed assuming symmetrical double minimum of the Morse potentials (X–Y). Two types for hydrogen bond energies are estimated by deriving the confocal elliptic differential equation, and the lowest energy for hydrogen bond may be estimated using the trial function of the linear combination of the lowest Morse wave functions by variation method. The repulsive potentials for X2 are determined by the expectation values E | z p | E $\langle E\vert {z}^{p}\vert E\rangle $ in continuum state for the Morse potential for X2. Figures of the potential curve for O–H–O and the hydrogen density distribution for O–D–O are displayed. The hydrogen bond energies for O–H–O, O–D–O, N–H–N, and N–D–N are obtained.

The Earth’s Outer Exospheric Density Distributions Derived From PROCYON/LAICA UV Observations

AbstractCurrent three‐dimensional, data‐based models for the terrestrial exosphere have been derived from measurements of optically thin Lyman‐alpha (Ly‐α) emissions scattered by neutral hydrogen atoms. Such models are only valid for the middle exospheric region (3–8 Earth radii geocentric distances) since the orbital paths of the space‐based platforms used to acquire Ly‐α radiance were located within the exosphere, thus precluding the proper detection of the faint outer exospheric emission. Notwithstanding, accurate specifications of density distributions beyond 8 RE are needed to support comprehensive studies of the solar‐terrestrial interactions. Two upcoming missions, the Solar wind Magnetosphere‐Ionosphere Link Explorer and the Lunar Environment Heliospheric X‐ray Imager, will image the Earth’s magnetosheath in soft X‐rays, and neutral densities are crucial to extract ion distributions through inversion of the acquired images. This work develops a technique to estimate the Earth’s outer exospheric density distributions using far‐ultraviolet wide‐field data acquired by the Lyman‐Alpha Imaging Camera (LAICA) onboard the Proximate Object Close Flyby with Optical Navigation mission. Our approach formulates an inverse problem based on the linearity between measurements of scattered Ly‐α flux and the local atomic hydrogen density, which is solved using the Bayesian approach known as Maximum a posteriori estimation. We use the LAICA image to derive global, 3‐D hydrogen density distributions at 6–20 RE geocentric distances. We find that the spatial structure of the outer exosphere agrees well with the predictions of radiation pressure theory. Further, we find that the hydrogen density at the 10 RE subsolar point is 26.51 atoms/cm3.

Solar Minimum Exospheric Neutral Density Near the Subsolar Magnetopause Estimated From the XMM Soft X‐Ray Observations on 12 November 2008

AbstractThe Earth's magnetosheath and cusps emit soft X‐rays due to the charge exchange between highly charged solar wind ions and exospheric hydrogen atoms. The Lunar Environment Heliospheric X‐ray Imager and Solar wind Magnetosphere Ionosphere Link Explorer missions are scheduled to image the Earth's dayside magnetosphere system in soft X‐rays to investigate global‐scale magnetopause reconnection modes under varying solar wind conditions. The exospheric neutral hydrogen density distribution, especially the value of this density at the subsolar magnetopause is of particular interest for understanding X‐ray emissions near this boundary. This paper estimates the exospheric density during solar minimum using the X‐ray Multimirror Mission (XMM) astrophysics observatory. We selected an event on 12 November 2008 from the XMM data archive, which detects soft X‐rays of magnetosheath origin while solar wind and interplanetary magnetic field conditions are relatively constant. During the event the location of the magnetopause was measured in situ by the THEMIS mission, thus the location of the solar wind ions responsible for the magnetosheath emission is well constrained by observation. We estimated the exospheric density using the Open Geospace Global Circulation Model (OpenGGCM) and a spherically symmetric exosphere model. The ratio of the magnetosheath plasma flux between the OpenGGCM model and the THEMIS, was nearly 1, which means the magnetohydrodynamic model reasonably reproduces the magnetosheath plasma conditions. The OpenGGCM magnetosheath parameters were used to deconvolve soft X‐rays of exospheric origin from the XMM signal. The lower‐limit of the exospheric density of this solar minimum event is 36.8 ± 11.7 cm−3 at 10 RE subsolar location.

Effects of Spatial Discretization in Lyα Line Radiation Transfer Simulations

Abstract We describe the addition of Lyα resonant line transfer to our dust continuum radiation transfer code SKIRT, verifying our implementation with published results for spherical problems and using some self-designed three-dimensional setups. We specifically test spatial discretization through various grid types, including hierarchical octree grids and unstructured Voronoi tessellations. We then use a radiation transfer postprocessing model for one of the spiral galaxies produced by the Auriga cosmological zoom simulations to investigate the effect of spatial discretization on the synthetic observations. We find that the calculated Lyα line profiles exhibit an extraordinarily strong dependence on the type and resolution of the spatial grid, rendering the results untrustworthy at best. We attribute this effect to the large gradients in the hydrogen density distribution over small distances, which remain significantly underresolved in the input model. We therefore argue that further research is needed to determine the required spatial resolution of a hydrodynamical simulation snapshot to enable meaningful Lyα line transfer postprocessing.

The effect of surface structure and arrangement on wettability of substrate surface

Molecular dynamics simulation was used to simulate the wetting of water droplets on surfaces patterned with different texture structures. Through the study of texture structures with different rough elements, we find that the wettability is closely related to the texture structure, and the molecular dynamics contact angles of different wetting modes are in good agreement with the predicted value by the Cassie-Baxter equation, in which the influence of gravity is negligible. In order to investigate this phenomenon in detail, we analyzed the wetting states, contact angles, density distributions of droplets on different surfaces, radial distribution functions of different hydration layers, grid distribution of hydrogen bonds and density of hydrogen bonds. The results showed the arrangement of water molecules in the first hydration layer and the distributions of hydrogen bond grid have a substantial impact on wettability of rough surfaces. The influence mechanism of the surface structure on the wettability is explained from the atomic level. Meanwhile, the consistency between molecular dynamics simulation and Cassie-Baxter equation show that the contact angles increase with the decrease of solid surface area fraction even if the surface texture structures are different.

Quantifying the impact of variable BLR diffuse continuum contributions on measured continuum interband delays

ABSTRACT We investigate the contribution of reprocessed continuum emission (1000–10 000 Å) originating in broad-line region (BLR) gas, the diffuse continuum (DC), to the wavelength-dependent continuum delays measured in AGN disc reverberation mapping experiments. Assuming a spherical BLR geometry, we adopt a Local Optimally emitting Cloud (LOC) model for the BLR that approximately reproduces the broad emission-line strengths of the strongest UV lines (Ly α and C iv) in NGC 5548. Within this LOC framework, we explore how assumptions about the gas hydrogen density and column density distributions influence flux and delay spectra of the DC. We find that: (i) models which match well measured emission-line luminosities and time delays also produce a significant DC component, (ii) increased $\rm {\mathit{ n}_H}$ and/or $\rm {\mathit{ N}_H}$, particularly at smaller BLR radii, result in larger DC luminosities and reduced DC delays, (iii) in a given continuum band the relative importance of the DC component to the measured interband delays is proportional (though not 1:1) to its fractional contribution to the total light in that band, (iv) the measured DC delays and DC variability amplitude depend also on the variability amplitude and characteristic variability time-scale of the driving continuum, (v) the DC radial surface emissivity distributions F(r) approximate power laws in radius with indices close to −2 (≈1:1 response to variations in the driving continuum flux), thus their physics is relatively simple and less sensitive to the unknown geometry and uncertainties in radiative transfer. Finally, we provide a simple recipe for estimating the DC contribution in disc reverberation mapping experiments.

3D mapping of the neutral X-ray absorption in the local interstellar medium: the Gaia and XMM-Newton synergy

We present a three-dimensional map of the hydrogen density distribution in the Galactic interstellar medium. The hydrogen equivalent column densities were obtained from the Exploring the X-ray Transient and variable Sky project ({\sc EXTraS}) which provides equivalent $N_{\rm H}$ values from X-ray spectral fits of observations within the {\it XMM-Newton} Data Release. {\sc EXTraS} include multiple fits for each source, allowing an accurate determination of the equivalent column densities, which depends on the continuum modeling of the spectra. A cross-correlation between the {\sc EXTraS} catalogue and the first {\it Gaia} Data Release was performed in order to obtain accurate parallax and distance measurements. We use a Bayesian method explained in \citet{rez17} in order to predict the most probable distribution of the density at any arbitrary point, even for lines of sight along which there are no initial observation. The resulting map shows small-scale density structures which can not been modeled by using analytic density profiles. In this paper we present a proof of concept of the kind of science possible with the synergy of these catalogs. However, given the systematic uncertainties connected to the source identification and to the dependence of $N_{\rm H}$ on the spectral model, the present maps should be considered qualitatively at this point.

Theoretical study on hydrogen storage capacity of expanded h-BN systems

In this work, the hydrogen storage capacity of the expanded hexagonal Boron Nitride (eh-BN) systems has been presented. We have employed a new equation of state (EOS) for hydrogen gas to figure out the hydrogen density distribution profiles in the eh-BN systems. In this regard, the environmental conditions (i.e., temperature and pressure) are considered in the prediction procedure using DFT single point calculations. The eh-BN systems with different layer spacings are studied by PBE method with consideration of the long range dispersion corrections. On account of the in-plane polar bonds, a series of adsorption positions are considered. Additionally, the adsorption energy and hydrogen density profiles are reported. Furthermore, the relation between uptakes and the interlayer spacings with the effects of the environmental conditions are also provided. The limit of the physical hydrogen storage capacity in a perfect eh-BN system at 243K and 10MPa is founded to be 2.96wt.%.

A numerical study of the effect of various reactions, pressure and gas mixture ratio on the density distribution of etchant species (H, Br, Br+, and HBr+) in HBr/He plasma

In this study, a fluid model has been used to study the effect of gas mixing ratio and pressure on the density distribution of important etchant species, i.e., hydrogen (H), bromine (Br), Br+, and HBr+ in HBr/He plasma. Our simulation results show that the densities of active etchant species H, Br, and HBr+ increase with the increase in pressure as well as the HBr fraction in HBr/He mixture. On the contrary, the density of Br+ decreases with the increase in He percentage in HBr/He mixture and with the increase in the pressure. Time averaged reaction rates (of the reactions involved in the production and consumption of these species) have been calculated to study the effect of these reactions on the density distribution of these species. The spatial distribution of these species is explained with the help of the time averaged reaction rates. Important reactions have been identified that contribute considerably to the production and consumption of these active species. The code has been optimized by identifying 26 reactions (out of 40 reactions which contribute in the production and consumption of these species) that have insignificant effect on the densities of H, Br, Br+, and HBr+. This shows that out of 40 reactions, only 14 reactions can be used to calculate the density and distribution of the important species in HBr/He plasma discharge.

Quantification of Hydrogen Concentrations in Surface and Interface Layers and Bulk Materials through Depth Profiling with Nuclear Reaction Analysis

Nuclear reaction analysis (NRA) via the resonant 1H(15N,αγ)12C reaction is a highly effective method of depth profiling that quantitatively and non-destructively reveals the hydrogen density distribution at surfaces, at interfaces, and in the volume of solid materials with high depth resolution. The technique applies a 15N ion beam of 6.385 MeV provided by an electrostatic accelerator and specifically detects the 1H isotope in depths up to about 2 μm from the target surface. Surface H coverages are measured with a sensitivity in the order of ~1013 cm-2 (~1% of a typical atomic monolayer density) and H volume concentrations with a detection limit of ~1018 cm-3 (~100 at. ppm). The near-surface depth resolution is 2-5 nm for surface-normal 15N ion incidence onto the target and can be enhanced to values below 1 nm for very flat targets by adopting a surface-grazing incidence geometry. The method is versatile and readily applied to any high vacuum compatible homogeneous material with a smooth surface (no pores). Electrically conductive targets usually tolerate the ion beam irradiation with negligible degradation. Hydrogen quantitation and correct depth analysis require knowledge of the elementary composition (besides hydrogen) and mass density of the target material. Especially in combination with ultra-high vacuum methods for in-situ target preparation and characterization, 1H(15N,αγ)12C NRA is ideally suited for hydrogen analysis at atomically controlled surfaces and nanostructured interfaces. We exemplarily demonstrate here the application of 15N NRA at the MALT Tandem accelerator facility of the University of Tokyo to (1) quantitatively measure the surface coverage and the bulk concentration of hydrogen in the near-surface region of a H2 exposed Pd(110) single crystal, and (2) to determine the depth location and layer density of hydrogen near the interfaces of thin SiO2 films on Si(100).

Quantification of Hydrogen Concentrations in Surface and Interface Layers and Bulk Materials through Depth Profiling with Nuclear Reaction Analysis.

Nuclear reaction analysis (NRA) via the resonant (1)H((15)N,αγ)(12)C reaction is a highly effective method of depth profiling that quantitatively and non-destructively reveals the hydrogen density distribution at surfaces, at interfaces, and in the volume of solid materials with high depth resolution. The technique applies a (15)N ion beam of 6.385 MeV provided by an electrostatic accelerator and specifically detects the (1)H isotope in depths up to about 2 μm from the target surface. Surface H coverages are measured with a sensitivity in the order of ~10(13) cm(-2) (~1% of a typical atomic monolayer density) and H volume concentrations with a detection limit of ~10(18) cm(-3) (~100 at. ppm). The near-surface depth resolution is 2-5 nm for surface-normal (15)N ion incidence onto the target and can be enhanced to values below 1 nm for very flat targets by adopting a surface-grazing incidence geometry. The method is versatile and readily applied to any high vacuum compatible homogeneous material with a smooth surface (no pores). Electrically conductive targets usually tolerate the ion beam irradiation with negligible degradation. Hydrogen quantitation and correct depth analysis require knowledge of the elementary composition (besides hydrogen) and mass density of the target material. Especially in combination with ultra-high vacuum methods for in-situ target preparation and characterization, (1)H((15)N,αγ)(12)C NRA is ideally suited for hydrogen analysis at atomically controlled surfaces and nanostructured interfaces. We exemplarily demonstrate here the application of (15)N NRA at the MALT Tandem accelerator facility of the University of Tokyo to (1) quantitatively measure the surface coverage and the bulk concentration of hydrogen in the near-surface region of a H2 exposed Pd(110) single crystal, and (2) to determine the depth location and layer density of hydrogen near the interfaces of thin SiO2 films on Si(100).

Experimental characterization of the Advanced Liquid Hydrogen Cold Neutron Source spectrum of the NBSR reactor at the NIST Center for Neutron Research

The recent expansion of the National Institute of Standards and Technology (NIST) Center for Neutron Research facility has offered a rare opportunity to perform an accurate measurement of the cold neutron spectrum at the exit of a newly-installed neutron guide. Using a combination of a neutron time-of-flight measurement, a gold foil activation measurement, and Monte Carlo simulation of the neutron guide transmission, we obtain the most reliable experimental characterization of the Advanced Liquid Hydrogen Cold Neutron Source brightness to date. Time-of-flight measurements were performed at three distinct fuel burnup intervals, including one immediately following reactor startup. Prior to the latter measurement, the hydrogen was maintained in a liquefied state for an extended period in an attempt to observe an initial radiation-induced increase of the ortho (o)-hydrogen fraction. Since para (p)-hydrogen has a small scattering cross-section for neutron energies below 15meV (neutron wavelengths greater than about 2.3Å), changes in the o- p hydrogen ratio and in the void distribution in the boiling hydrogen influence the spectral distribution. The nature of such changes is simulated with a continuous-energy, Monte Carlo radiation-transport code using 20K o and p hydrogen scattering kernels and an estimated hydrogen density distribution derived from an analysis of localized heat loads. A comparison of the transport calculations with the mean brightness function resulting from the three measurements suggests an overall o- p ratio of about 17.5(±1) % o- 82.5% p for neutron energies<15meV, a significantly lower ortho concentration than previously assumed.

Terrestrial exospheric hydrogen density distributions under solar minimum and solar maximum conditions observed by the TWINS stereo mission

Abstract. Circumterrestrial Lyman-α column brightness observations above 3 Earth radii (Re) have been used to derive separate 3-D neutral hydrogen density models of the Earth's exosphere for solar minimum (2008, 2010) and near-solar-maximum (2012) conditions. The data used were measured by Lyman-α detectors (LAD1/2) onboard each of the TWINS satellites from very different orbital positions with respect to the exosphere. Exospheric H atoms resonantly scatter the near-line-center solar Lyman-α flux at 121.6 nm. Assuming optically thin conditions above 3Re along a line of sight (LOS), the scattered LOS-column intensity is proportional to the LOS H-column density. We found significant differences in the density distribution of the terrestrial exosphere under different solar conditions. Under solar maximum conditions we found higher H densities and a larger spatial extension compared to solar minimum. After a continuous, 2-month decrease in (27 day averaged) solar activity, significantly lower densities were found. Differences in shape and orientation of the exosphere under different solar conditions exist. Above 3 Re, independent of solar activity, increased H densities appear on the Earth's nightside shifted towards dawn. With increasing distance (as measured at 8Re) this feature is shifted westward/duskward by between −4 and −5° with respect to midnight. Thus, at larger geocentric distance the exosphere seems to be aligned with the aberrated Earth–solar-wind line, defined by the solar wind velocity and the orbital velocity of the Earth. The results presented in this paper are valid for geocentric distances between 3 and 8Re.

Exospheric hydrogen density distributions for equinox and summer solstice observed with TWINS1/2 during solar minimum

Abstract. The Lyman-α Detectors (LAD) on board the two TWINS 1/2-satellites allow for the simultaneous stereo imaging of the resonant emission glow of the H-geocorona from very different orbital positions. Terrestrial exospheric atomic hydrogen (H) resonantly scatters solar Lyman-α (121.567 nm) radiation. During the past solar minimum, relevant solar parameters that influence these emissions were quite stable. Here, we use simultaneous LAD1/2-observations from TWINS1 and TWINS2 between June 2008 and June 2010 to study seasonal variations in the H-geocorona. Data are combined to produce two datasets containing (summer) solstice and (combined spring and fall) equinox emissions. In the range from 3 to 10 Earth radii (RE), a three-dimensional (3-D) mathematical model is used that allows for density asymmetries in longitude and latitude. At lower geocentric distances (&lt; 3 RE), a best fitting r-dependent (Chamberlain, 1963)-like model is adapted to enable extrapolation of our information to lower heights. We find that dawn and dusk H-geocoronal densities differ by up to a factor of 1.3 with higher densities on the dawn side. Also, noon densities are greater by up to a factor of 2 compared to the dawn and dusk densities. The density profiles are aligned well with the Earth–Sun line and there are clear density depletions over both poles that show additional seasonal effects. These solstice and equinox empirical fits can be used to determine H-geocoronal densities for any day of the year for solar minimum conditions.

Structural properties of water around uncharged and charged carbon nanotubes

Studying the structural properties of water molecules around the carbon nanotubes is very important in a wide variety of carbon nanotubes applications. We studied the number of hydrogen bonds, oxygen and hydrogen density distributions, and water orientation around carbon nanotubes. The water density distribution for all carbon nanotubes was observed to have the same feature. In water-carbon nanotubes interface, a high-density region of water molecules exists around carbon nanotubes. The results reveal that the water orientation around carbon nanotubes is roughly dependent on carbon nanotubes surface charge. The water molecules in close distances to carbon nanotubes were found to make an HOH plane nearly perpendicular to the water-carbon nanotubes interface for carbon nanotubes with negative surface charge. For uncharged carbon nanotubes and carbon nanotubes with positive surface charge, the HOH plane was in tangential orientation with water-carbon nanotubes interface. There was also a significant reduction in hydrogen bond of water region around carbon nanotubes as compared with hydrogen bond in bulk water. This reduction was very obvious for carbon nanotubes with positive surface charge. In addition, the calculation of dynamic properties of water molecules in water-CNT interface revealed that there is a direct relation between the number of Hbonds and selfdiffusion coefficient of water molecules.

Lattice Location and Density Distribution of Hydrogen in β1-V2H

The lattice location of hydrogen and its spatial density distribution in the tetragonal β 1 -V 2 H single crystal prepared under tensile stress and the monoclinic β 1 -V 2 H 1.1 single crystal prepared without application of tensile stress have been studied at room temperature by the channelling method utilizing a nuclear reaction 1 H( 11 B,α)αα with a 11 B beam of about 2 MeV. In these crystals, H atoms are located at octahedral ( O ) sites with different distributions over O x , O y , and O z sites. In the tetragonal β 1 -V 2 H, about 80% of H atoms are located at O z sites, about 10% of them at O y sites and about 10% of them at O x sites. In the monoclinic β 1 -V 2 H 1.1 , about 65% of H atoms are located at O z sites, about 20% of them at O y sites and about 15% of them at O x sites. These results support the theoretical prediction that hydrogen preferentially occupies O z sites under tensile stress along the z -axis ( c -axis). In both crystals, the HWHM of the spatial density distribution of hydrog...

The impact of geocoronal density on ring current development

Long-term ring current decay following a magnetic storm is mainly due to charge exchange collisions of ring current ions with geocoronal neutral atoms forming energetic neutral atoms (ENAs) that leave the ring current system. Therefore, the density distribution of these cold and tenuous neutral hydrogen atoms plays a key role in the ring current recovery. TWINS ENA images provide a direct measurement of these ENA losses and therefore insight into the dynamics of the ring current decay through interactions with the geocorona. To assess the influence of geocoronal neutrals on ring current decay, we compare the predicted ENA emission using five different geocoronal models and the HEIDI ring current model to simulate the July 22, 2009 storm.We show that for high energy H+(≥100keV), all geocoronal models predict similar decay rates of the ring current ions. However, for low energy ions (≤100keV), the decay rate varies significantly depending on the geocoronal density model. Comparison with TWINS ENA images shows that the location of the peak ENA enhancements is highly dependent on the distribution of geocoronal hydrogen density. The ring current topology depends greatly on the hydrogen model used, therefore knowing the H-distribution is very important in understanding how the ring current recovers following a magnetic storm.

Cold accretion flows and the nature of high column density H i absorption at redshift 3

Simulations predict that galaxies grow primarily through the accretion of gas that has not gone through an accretion shock near the virial radius and that this cold gas flows towards the central galaxy along dense filaments and streams. There is, however, little observational evidence for the existence of these cold flows. We use a large, cosmological, hydrodynamical simulation that has been post-processed with radiative transfer to study the contribution of cold flows to the observed z=3 column density distribution of neutral hydrogen, which our simulation reproduces. We find that nearly all of the HI absorption arises in gas that has remained colder than 10^5.5 K, at least while it was extragalactic. In addition, the majority of the HI is rapidly falling towards a nearby galaxy, with non-negligible contributions from outflowing and static gas. Above a column density of N_HI = 10^17 cm^-2, most of the absorbers reside inside haloes, but the interstellar medium only dominates for N_HI > 10^21 cm^-2. Haloes with total mass below 10^10 Msun dominate the absorption for 10^17<N_HI < 10^21 cm^-2, but the average halo mass increases sharply for higher column densities. Although very little of the HI in absorbers with N_HI <~ 10^20 cm^-2 resides inside galaxies, systems with N_HI > 10^17 cm^-2 are closely related to star formation: most of their HI either will become part of the interstellar medium before z=2 or has been ejected from a galaxy at z>3. Cold accretion flows are critical for the success of our simulation in reproducing the observed rate of incidence of damped Lyman-alpha and particularly that of Lyman limit systems. We therefore conclude that cold accretion flows exist and have already been detected in the form of high column density HI absorbers.