Molecular Hydrogen Fraction Research Articles

A Reexamination of Phosphorus and Chlorine Depletions in the Diffuse Interstellar Medium* * Based on observations made with the NASA/ESA Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer mission, obtained from the MAST data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.

We present a comprehensive examination of interstellar P and Cl abundances based on an analysis of archival spectra acquired with the Space Telescope Imaging Spectrograph of the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer. Column densities of P ii, Cl i, and Cl ii are determined for a combined sample of 107 sight lines probing diffuse atomic and molecular gas in the local Galactic interstellar medium. We reevaluate the nearly linear relationship between the column densities of Cl i and H2, which arises from the rapid conversion of Cl+ to Cl0 in regions where H2 is abundant. Using the observed total gas-phase P and Cl abundances, we derive depletion parameters for these elements, adopting the methodology of Jenkins. We find that both P and Cl are essentially undepleted along sight lines showing the lowest overall depletions. Increasingly severe depletions of P are seen along molecule-rich sight lines. In contrast, gas-phase Cl abundances show no systematic variation with molecular hydrogen fraction. However, enhanced Cl (and P) depletion rates are found for a subset of sight lines showing elevated levels of Cl ionization. An analysis of neutral chlorine fractions yields estimates for the amount of atomic hydrogen associated with the H2-bearing gas in each direction. These results indicate that the molecular fraction in the H2-bearing gas is at least 10% for all sight lines with and that the gas is essentially fully molecular at .

Evidence that 1I/2017 U1 (‘Oumuamua) was Composed of Molecular Hydrogen Ice

Abstract ‘Oumuamua (I1 2017) was the first macroscopic (l ∼ 100 m) body observed to traverse the inner solar system on an unbound hyperbolic orbit. Its light curve displayed strong periodic variation, and it showed no hint of a coma or emission from molecular outgassing. Astrometric measurements indicate that ‘Oumuamua experienced nongravitational acceleration on its outbound trajectory, but energy balance arguments indicate this acceleration is inconsistent with a water ice sublimation-driven jet of the type exhibited by solar system comets. We show that all of ‘Oumaumua’s observed properties can be explained if it contained a significant fraction of molecular hydrogen (H2) ice. H2 sublimation at a rate proportional to the incident solar flux generates a surface-covering jet that reproduces the observed acceleration. Mass wasting from sublimation leads to monotonic increase in the body axis ratio, explaining ‘Oumuamua’s shape. Back-tracing ‘Oumuamua’s trajectory through the solar system permits calculation of its mass and aspect ratio prior to encountering the Sun. We show that H2-rich bodies plausibly form in the coldest dense cores of giant molecular clouds, where number densities are of order n ∼ 105, and temperatures approach the T = 3 K background. Post-formation exposure to galactic cosmic rays implies a τ ∼ 100 Myr age, explaining the kinematics of ‘Oumuamua’s inbound trajectory.

Chlorine-bearing molecules in molecular absorbers at intermediate redshifts

We use observations of chlorine-bearing species in molecular absorbers at intermediate redshifts to investigate chemical properties and 35Cl/37Cl isotopic ratios in the absorbing sightlines. Chloronium (H2Cl+) is detected along three independent lines of sight in the z = 0.89 and z = 0.68 molecular absorbers located in front of the lensed quasars PKS 1830−211 and B 0218+357, respectively. Hydrogen chloride (HCl) was observed only toward PKS 1830−211, and is found to behave differently from H2Cl+. It is detected in one line of sight with an abundance ratio [H2Cl+] / [HCl] ∼1, but remains undetected in the other, more diffuse, line of sight, with a ratio [H2Cl+] / [HCl] > 17. The absorption profiles of these two chlorine-bearing species are compared to other species and discussed in terms of the physical properties of the absorbing gas. Our findings are consistent with the picture emerging from chemical models where different species trace gas with different molecular hydrogen fraction. The 35Cl/37Cl isotopic ratios are measured in the different lines of sight and are discussed in terms of stellar nucleosynthesis.

The Spatially Resolved Dust-to-metals Ratio in M101

Abstract The dust-to-metals ratio describes the fraction of heavy elements contained in dust grains, and its variation provides key insights into the life cycle of dust. We measure the dust-to-metals ratio in M101, a nearby galaxy with a radial metallicity (Z) gradient spanning ∼1 dex. We fit the spectral energy distribution of dust from 100 to 500 μm with five variants of the modified blackbody dust emission model in which we vary the temperature distribution and how emissivity depends on wavelength. Among them, the model with a single-temperature blackbody modified by a broken power-law emissivity gives the statistically best fit and physically most plausible results. Using these results, we show that the dust-to-gas ratio is proportional to . This implies that the dust-to-metals ratio is not constant in M101, but decreases as a function of radius, which is equivalent to a lower fraction of metals trapped in dust at low metallicity (large radius). The dust-to-metals ratio in M101 remains at or above what would be predicted by the minimum depletion level of metals observed in the Milky Way. Our current knowledge of the metallicity-dependent CO-to-H2 conversion factor suggests that variations in the conversion factor cannot be responsible for the trends in dust-to-metals ratio we observe. This change of dust-to-metals ratio is significantly correlated with the mass fraction of molecular hydrogen, which suggests that the accretion of gas-phase metals onto existing dust grains could contribute to a variable dust-to-metals ratio.

Velocity-resolved [ ] Emission from Cold Diffuse Clouds in the Interstellar Medium

Abstract We have combined emission from the 158 μm fine structure transition of C+ observed with the GREAT and upGREAT instruments on SOFIA with 21 cm absorption spectra and visual extinction to characterize the diffuse interstellar clouds found along the lines of sight. The weak [C ii] emission is consistent in velocity and line width with the strongest H i component produced by the cold neutral medium. The H i column density and kinetic temperature are known from the 21 cm data and, assuming a fractional abundance of ionized carbon, we calculate the volume density and thermal pressure of each source, which vary considerably, with 27 cm−3 considering only the atomic hydrogen along the lines of sight to be responsible for the C+, while 13 cm−3 including the hydrogen in both forms. The thermal pressure varies widely with 1970 cm−3 K 10,440 cm−3 K for H0 alone and 750 cm−3 K ≤ P th/k ≤ 9360 cm−3 K including both H0 and H2. The molecular hydrogen fraction varies between 0.10 and 0.67. Photoelectric heating is the dominant heating source, supplemented by a moderately enhanced cosmic ray ionization rate, constrained by the relatively low 45 K to 73 K gas temperatures of the clouds. The resulting thermal balance for the two lower-density clouds is satisfactory, but for the two higher-density clouds, the combined heating rate is insufficient to balance the observed C+ cooling.

The molecular chemistry of diffuse and translucent clouds in the line-of-sight to Sgr B2: Absorption by simple organic and inorganic molecules in the GBT PRIMOS survey

The 1–50 GHz PRebiotic Interstellar MOlecular Survey (PRIMOS) contains ~50 molecular absorption lines observed in clouds located in the line-of-sight to Sgr B2(N). The line-of-sight material is associated with diffuse and translucent clouds located in the Galactic center, bar, and spiral arms in the disk. We measured the column densities and estimate abundances, relative to H2, of 11 molecules and additional isotopologues observed in this material. We used absorption by optically thin transitions of c-C3H2 to estimate the molecular hydrogen columns, and argue that this method is preferable to more commonly used methods. We discuss the kinematic structure and abundance patterns of small molecules including the sulfur-bearing species CS, SO, CCS, H2CS, and HCS+; oxygen-bearing molecules OH, SiO, and H2CO; and simple hydrocarbon molecules c-C3H2, l-C3H, and l-C3H+. Finally, we discuss the implications of the observed chemistry for the structure of the gas and dust in the ISM. Highlighted results include the following. First, whereas gas in the disk has a molecular hydrogen fraction of 0.65, clouds on the outer edge of the Galactic bar and in or near the Galactic center have molecular fractions of 0.85 and >0.9, respectively. Second, we observe trends in isotope ratios with Galactocentric distance; while carbon and silicon show enhancement of the rare isotopes at low Galactocentric distances, sulfur exhibits no trend with Galactocentric distance. We also determine that the ratio of c-C3H2/c-H13CCCH provides a good estimate of the 12C/13C ratio, whereas H2CO/H213CO exhibits fractionation. Third, we report the presence of l-C3H+ in diffuse clouds for the first time. Finally, we suggest that CS has an enhanced abundance within higher density clumps of material in the disk, and therefore may be diagnostic of cloud conditions. If this holds, the diffuse clouds in the Galactic disk contain multiple embedded hyperdensities in a clumpy structure, and the density profile is not a simple function of AV.

Properties of H i discs in the Auriga cosmological simulations

We analyse the properties of the HI gas distribution in the Auriga project, a set of magnetohydrodynamic cosmological simulations performed with the moving-mesh code AREPO and a physics model for galaxy formation that succeeds in forming realistic late-type galaxies in the 30 Milky Way-sized haloes simulated in this project. We use a simple approach to estimate the neutral hydrogen fraction in our simulation set, which treats low-density and star-forming gas separately, and we explore two different prescriptions to subtract the contribution of molecular hydrogen from the total HI content. The HI gas in the vast majority of the systems forms extended discs although more disturbed morphologies are present. Notwithstanding the general good agreement with observed HI properties -- such as radial profiles and the mass-diameter relation -- the Auriga galaxies are systematically larger and more gas-rich than typical nearby galaxies. Interestingly, the amount of HI gas outside the disc plane correlates with the star formation rate, consistent with a picture where most of this extra-planar HI gas originates from a fountain-like flow. Our findings are robust with respect to the different assumptions adopted for computing the molecular hydrogen fraction and do not vary significantly over a wide range of numerical resolution. The HI modelling introduced in this paper can be used in future work to build artificial interferometric HI data cubes, allowing an even closer comparison of the gas dynamics in simulated galaxies with observations.

OH+and H2O+absorption toward PKS 1830–211

We report the detection of OH+ and H2O+ in the z=0.89 absorber toward the lensed quasar PKS1830-211. The abundance ratio of OH+ and H2O+ is used to quantify the molecular hydrogen fraction (fH2) and the cosmic-ray ionization rate of atomic hydrogen (zH) along two lines of sight, located at ~2 kpc and ~4 kpc to either side of the absorber's center. The molecular fraction decreases outwards, from ~0.04 to ~0.02, comparable to values measured in the Milky Way at similar galactocentric radii. For zH, we find values of ~2x10^-14 s^-1 and ~3x10^-15 s^-1, respectively, which are slightly higher than in the Milky Way at comparable galactocentric radii, possibly due to a higher average star formation activity in the z=0.89 absorber. The ALMA observations of OH+, H2O+, and other hydrides toward PKS1830-211 reveal the multi-phase composition of the absorbing gas. Taking the column density ratios along the southwest and northeast lines of sight as a proxy of molecular fraction, we classify the species ArH+, OH+, H2Cl+, H2O+, CH, and HF as tracing gases increasingly more molecular. Incidentally, our data allow us to improve the accuracy of H2O+ rest frequencies and thus refine the spectroscopic parameters.

Far-infrared study of tracers of oxygen chemistry in diffuse clouds

Context. The chemistry of the diffuse interstellar medium rests upon three pillars: exothermic ion-neutral reactions (" cold chemistry "), endothermic neutral-neutral reactions with significant activation barriers (" warm chemistry "), and reactions on the surfaces of dust grains. While warm chemistry becomes important in the shocks associated with turbulent dissipation regions, the main path for the formation of interstellar OH and H2O is that of cold chemistry. Aims. The aim of this study is to observationally confirm the association of atomic oxygen with both atomic and molecular gas phases, and to understand the measured abundances of OH and OH + as a function of the available reservoir of H2. Methods. We obtained absorption spectra of the ground states of OH, OH+ and OI with high-velocity resolution, with GREAT on-board SOFIA, and with the THz receiver at the APEX. We analyzed them along with ancillary spectra of HF and CH from HIFI. To deconvolve them from the hyperfine structure and to separate the blend that is due to various velocity components on the sightline, we fit model spectra consisting of an appropriate number of Gaussian profiles using a method combining simulated annealing with downhill simplex minimization. Together with HF and/or CH as a surrogate for H2, and HI $\lambda$21 cm data, the molecular hydrogen fraction f^N\_H2 = N(H 2)/(N(H) + 2N(H 2)) can be determined. We then investigated abundance ratios as a function of f^N\_H2. Results. The column density of OI is correlated at a high significance with the amount of available molecular and atomic hydrogen, with an atomic oxygen abundance of $3 \times 10 ^{-4}$ relative to H nuclei. While the velocities of the absorption features of OH and OH+ are loosely correlated and reflect the spiral arm crossings on the sightline, upon closer inspection they display an anticorrespondence. The arm-to-interarm density contrast is found to be higher in OH than in OH+. While both species can coexist, with a higher abundance in OH than in OH+, the latter is found less frequently in absence of OH than the other way around, which is a direct consequence of the rapid destruction of OH+ by dissociative recombination when not enough H2 is available. This conjecture has been substantiated by a comparison between the OH/OH+ ratio with f^N\_H2, showing a clear correlation. The hydrogen abstraction reaction chain OH+ (H2,H) H2O+ (H2,H)H3O+ is confirmed as the pathway for the production of OH and H 2 O. Our estimate of the branching ratio of the dissociative recombination of H3O+ to OH and H2O is confined within the interval of 84 to 91%, which matches laboratory measurements (74 to 83%). -- A correlation between the linewidths and column densities of OH+ features is found to be significant with a false-alarm probability below 5%. Such a correlation is predicted by models of interstellar MHD turbulence. For OH the same correlation is found to be insignificant because there are more narrow absorption features. Conclusions. While it is difficult to assess the contributions of warm neutral-neutral chemistry to the observed abundances, it seems fair to conclude that the predictions of cold ion-neutral chemistry match the abundance patterns we observed.

The inferred evolution of the cold gas properties of CANDELS galaxies at 0.5 <z< 3.0

We derive the total cold gas, atomic hydrogen, and molecular gas masses of approximately 24 000 galaxies covering four decades in stellar mass at redshifts 0.5 < z < 3.0, taken from the CANDELS survey. Our inferences are based on the inversion of a molecular hydrogen based star formation law, coupled with a prescription to separate atomic and molecular gas. We find that: 1) there is an increasing trend between the inferred cold gas (HI and H2), HI, and H2 mass and the stellar mass of galaxies down to stellar masses of 10^8 Msun already in place at z = 3; 2) the molecular fractions of cold gas increase with increasing stellar mass and look-back time; 3) there is hardly any evolution in the mean HI content of galaxies at fixed stellar mass; 4) the cold gas fraction and relative amount of molecular hydrogen in galaxies decrease at a relatively constant rate with time, independent of stellar mass; 5) there is a large population of low-stellar mass galaxies dominated by atomic gas. These galaxies are very gas rich, but only a minor fraction of their gas is molecular; 6) the ratio between star-formation rate (SFR) and inferred total cold gas mass (HI + H2) of galaxies (i.e., star-formation efficiency; SFE) increases with star-formation at fixed stellar masses. Due to its simplicity, the presented approach is valuable to assess the impact of selection biases on small samples of directly-observed gas masses and to extend scaling relations down to stellar mass ranges and redshifts that are currently difficult to probe with direct measurements of gas content.

The geodynamic setting in the northwest circumference of the Pacific Ocean: Effects on the dynamics of subsurface radon and on the gas composition of the heat carrier at the Mutnovskii geoelectric power station

This paper provides information on the behavior of the radon activity time series for the period from 2000 through 2015 in the water-saturated zone of the Paratunka Geothermal Field and that of the time series that recorded the volumetric fraction of molecular hydrogen in the gaseous heat carrier at well 016 in the Mutnovskii Field, as well as their relationships with the seismicity in the northwestern circumference of the Pacific Ocean. It was concluded that long-term trends in radon activity and the high volumetric fraction of molecular hydrogen in 2014 are caused by a change in the stress field that acts in the subduction zone of the northwest flank of the Pacific Ocean. It is hypothesized that an M > 7.5 earthquake is likely to occur during the next 1.5 years. In the opinion of Academician S.A. Fedotov, the most likely location for such an event is between the Shipunskii Peninsula and Shiashkotan Island, Middle Kuril islands.

HERSCHELSURVEY OF GALACTIC OH+, H2O+, AND H3O+: PROBING THE MOLECULAR HYDROGEN FRACTION AND COSMIC-RAY IONIZATION RATE

In diffuse interstellar clouds the chemistry that leads to the formation of the oxygen bearing ions OH+, H2O+, and H3O+ begins with the ionization of atomic hydrogen by cosmic rays, and continues through subsequent hydrogen abstraction reactions involving H2. Given these reaction pathways, the observed abundances of these molecules are useful in constraining both the total cosmic-ray ionization rate of atomic hydrogen (zeta_H) and molecular hydrogen fraction, f(H2). We present observations targeting transitions of OH+, H2O+, and H3O+ made with the Herschel Space Observatory along 20 Galactic sight lines toward bright submillimeter continuum sources. Both OH+ and H2O+ are detected in absorption in multiple velocity components along every sight line, but H3O+ is only detected along 7 sight lines. From the molecular abundances we compute f(H2) in multiple distinct components along each line of sight, and find a Gaussian distribution with mean and standard deviation 0.042+-0.018. This confirms previous findings that OH+ and H2O+ primarily reside in gas with low H2 fractions. We also infer zeta_H throughout our sample, and find a log-normal distribution with mean log(zeta_H)=-15.75, (zeta_H=1.78x10^-16 s^-1), and standard deviation 0.29 for gas within the Galactic disk, but outside of the Galactic center. This is in good agreement with the mean and distribution of cosmic-ray ionization rates previously inferred from H3+ observations. Ionization rates in the Galactic center tend to be 10--100 times larger than found in the Galactic disk, also in accord with prior studies.

Ci](1–0) observations in M33 A study of CO dark H2gas in M33

Based on [Ci] (1–0) observations and complementary Hi, 12/13 CO (1–0), 12 CO (2–1) and [Cii] data we estimated the column density and fraction of molecular hydrogen in GMCs along the major axis of the galaxy M33. We found that on average 75%± 14% of the hydrogen is in molecular form. Roughly 40–45% of the H 2 is both traced by [Cii] and CO, while ∼ 15% by the [Ci] lines. The found CO-dark H 2 gas fractions are within the predictions by Wolfire et al. (2010) for a half solar metallicity, Z = 0.5 Z ⊙ , as in M33.

LINE OVERLAP AND SELF-SHIELDING OF MOLECULAR HYDROGEN IN GALAXIES

The effect of line overlap in the Lyman and Werner bands, often ignored in galactic studies of the atomic-to-molecular transition, greatly enhances molecular hydrogen self-shielding in low metallicity environments, and dominates over dust shielding for metallicities below about 10% solar. We implement that effect in cosmological hydrodynamics simulations with an empirical model, calibrated against the observational data, and provide fitting formulae for the molecular hydrogen fraction as a function of gas density on various spatial scales and in environments with varied dust abundance and interstellar radiation field. We find that line overlap, while important for detailed radiative transfer in the Lyman and Werner bands, has only a minor effect on star formation on galactic scales, which, to a much larger degree, is regulated by stellar feedback.

Influence of excited state spatial distributions on plasma diagnostics: Atmospheric pressure laser-induced He-H2 plasma

Atmospheric pressure plasmas in helium-hydrogen mixtures with H2 molar concentrations ranging from 0.13% to 19.7% were investigated at times from 1 to 25 μs after formation by a Q-switched Nd:YAG laser. Spatially integrated electron density values are obtained using time resolved optical emission spectroscopic techniques. Depending on mixture concentration and delay time, electron densities vary from almost 1017 cm−3 to about 1014 cm−3. Helium based results agree reasonably well with each other, as do values extracted from the Hα and Hβ emission lines. However, in particular for delays up to about 7 μs and in mixtures with less than 1% hydrogen, large discrepancies are observed between results obtained from the two species. Differences decrease with increasing hydrogen partial pressure and/or increasing delay time. In mixtures with molecular hydrogen fraction of 7% or more, all methods yield electron densities that are in good agreement. These findings seemingly contradict the well-established idea that addition of small amounts of hydrogen for diagnostic purposes does not perturb the plasma. Using Abel inversion analysis of the experimental data and a semi-empirical numerical model, we demonstrate that the major part of the detected discrepancies can be traced to differences in the spatial distributions of excited helium and hydrogen neutrals. The model yields spatially resolved emission intensities and electron density profiles that are in qualitative agreement with experiment. For the test case of a 1% H2 mixture at 5 μs delay, our model suggests that high electron temperatures cause an elevated degree of ionization and thus a reduction of excited hydrogen concentration relative to that of helium near the plasma center. As a result, spatially integrated analysis of hydrogen emission lines leads to oversampling of the plasma perimeter and thus to lower electron density values compared to those obtained from helium lines.

CHEMICAL ANALYSIS OF A DIFFUSE CLOUD ALONG A LINE OF SIGHT TOWARD W51: MOLECULAR FRACTION AND COSMIC-RAY IONIZATION RATE

Absorption lines from the molecules OH+, H2O+, and H3+ have been observed in a diffuse molecular cloud along a line of sight near W51 IRS2. We present the first chemical analysis that combines the information provided by all three of these species. Together, OH+ and H2O+ are used to determine the molecular hydrogen fraction in the outskirts of the observed cloud, as well as the cosmic-ray ionization rate of atomic hydrogen. H3+ is used to infer the cosmic-ray ionization rate of H2 in the molecular interior of the cloud, which we find to be zeta_2=(4.8+-3.4)x10^-16 per second. Combining the results from all three species we find an efficiency factor---defined as the ratio of the formation rate of OH+ to the cosmic-ray ionization rate of H---of epsilon=0.07+-0.04, much lower than predicted by chemical models. This is an important step in the future use of OH+ and H2O+ on their own as tracers of the cosmic-ray ionization rate.

Diffuse interstellar bands in Upper Scorpius: probing variations in the DIB spectrum due to changing environmental conditions

We study the effects of local environmental conditions affecting the diffuse interstellar band (DIB) carriers within the Upper Scorpius subgroup of the Sco OB2 association. We obtained optical spectra with FEROS, and measured the equivalent widths of five DIBs (at 5780, 5797, 6196, 6379, and 6613 Angstrom) as well as those of absorption lines of di-atomic molecules (CH, CH+, CN) and atoms (KI, CaI) towards 89 targets in the direction of Upper Scorpius. We construct a simple radiative transfer and chemical network model of the diffuse interstellar medium (ISM) sheet in front of Upp Sco to infer the effective radiation field. Both the interstellar radiation field strength, I_UV, and the molecular hydrogen fraction, f_H2, have been derived for 55 sightlines probing the Upp Sco ISM. We discuss the relations between DIB strengths, CH and CH+ line strengths, E(B-V), I_UV, and f_H2. The ratio between the 5780 and 5797 DIBs reveals a (spatial) dependence on the local environment in terms of cloud density and exposure to the interstellar radiation field, reflecting the molecular nature of these DIB carriers.

CONSTRAINING THE ENVIRONMENT OF CH+FORMATION WITH CH+3OBSERVATIONS

The formation of CH+ in the interstellar medium has long been an outstanding problem in chemical models. In order to probe the physical conditions of the ISM in which CH+ forms, we propose the use of CH3+ observations. The pathway to forming CH3+ begins with CH+, and a steady state analysis of CH3+ and the reaction intermediary CH2+ results in a relationship between the CH+ and CH3+ abundances. This relationship depends on the molecular hydrogen fraction, f_H2, and gas temperature, T, so observations of CH+ and CH3+ can be used to infer the properties of the gas in which both species reside. We present observations of both molecules along the diffuse cloud sight line toward Cyg OB2 No. 12. Using our computed column densities and upper limits, we put constraints on the f_H2 vs. T parameter space in which CH+ and CH3+ form. We find that average, static, diffuse molecular cloud conditions (i.e. f_H2>0.2, T~60 K) are excluded by our analysis. However, current theory suggests that non-equilibrium effects drive the reaction C+ + H_2 --> CH+ + H, endothermic by 4640 K. If we consider a higher effective temperature due to collisions between neutrals and accelerated ions, the CH3+ partition function predicts that the overall population will be spread out into several excited rotational levels. As a result, observations of more CH3+ transitions with higher signal-to-noise ratios are necessary to place any constraints on models where magnetic acceleration of ions drives the formation of CH+.

Primordial magnetic fields and formation of molecular hydrogen

We study the implications of primordial magnetic fields for the thermal and ionization history of the post-recombination era. In particular, we compute the effects of dissipation of primordial magnetic fields owing to ambipolar diffusion and decaying turbulence in the intergalactic medium (IGM) and the collapsing haloes, and compute the effects of the altered thermal and ionization history on the formation of molecular hydrogen. We show that, for magnetic field strengths in the range 2 x 10 -10 ≤ B 0 ≤ 2 x 10 -9 G, the molecular hydrogen fraction in IGM and collapsing halo can increase by a factor of 5 to 1000 over the case with no magnetic fields. We discuss the implication of the increased molecular hydrogen fraction on the radiative transfer of ultraviolet photons and the formation of first structures in the universe.

Outflowing atomic and molecular gas at z∼ 0.67 towards 1504 + 377

Abstract We report the detection of OH 1667-MHz and wide H i 21-cm absorption at z∼ 0.67 towards the red quasar 1504 + 377, with the Green Bank Telescope and the Giant Metrewave Radio Telescope. The H i 21-cm absorption extends over a velocity range of ∼600 km s−1 blueward of the quasar redshift (z= 0.674), with the new OH 1667-MHz absorption component at ∼ −430 km s−1, nearly coincident with earlier detections of millimetre-wave absorption at z∼ 0.6715. The atomic and molecular absorption appear to arise from a fast gas outflow from the quasar, with a mass outflow rate and a molecular hydrogen fraction . The radio structure of 1504 + 377 is consistent with the outflow arising as a result of a jet–cloud interaction, followed by rapid cooling of the cloud material. The observed ratio of HCO+ and OH column densities is ∼20 times higher than typical values in Galactic and high-z absorbers. This could arise because of small-scale structure in the outflowing gas on sub-parsec scales, which would also explain the observed variability in the H i 21-cm line.