Atomic Hydrogen Line Research Articles

The spin-resolved atomic velocity distribution and 21-cm line profile of dark-age gas

T The 21-cm hyper-fine line of atomic hydrogen (H I) is a promising probe of the cosmic dark ages. In past treatments of 21 -cm radiation it was assumed the hyperfine level populations on H I could be characterized by a velocity-independent 'spin temperature' T s determined by a competition between 21-cm radiative transitions, spin-changing collisions, and (at lower redshifts) Lyα scattering. However we show here that, if the collisional time is comparable to the radiative time, the spin temperature will depend on atomic velocity, T s = T s (υ), and one must replace the usual hyperfine level rate equations with a Boltzmann equation describing the spin and velocity dependence of the H I distribution function. We construct here the Boltzmann equation relevant to the cosmic dark ages and solve it using a basis-function method. Accounting for the actual spin-resolved atomic velocity distribution results in up to a ∼2 per cent suppression of the 21-cm emissivity, and a redshift and angular-projection-dependent suppression or enhancement of the linear power spectrum of 21-cm fluctuations of up to ∼5 per cent. The effect on the 21-cm line profile is more dramatic - its full width at half-maximum can be enhanced by up to ∼60 per cent relative to the velocity-independent calculation. We discuss the implications for 21-cm tomography of the dark ages.

Fine–scale structure of the neutral ISM in M81

AbstractWe present an analysis of the fine–scale structure of the neutral ISM as traced via the 21-cm line of atomic hydrogen (HI) in the nearby galaxy M 81. The data show a stunning amount of detail in the form of 330 expanding shells and holes in the neutral ISM of M 81. A comparison with similar structures found in two other spirals and two dwarf galaxies (M 31, M 33, IC 2574 and Holmberg II) reveals that the ISM in M 81 shares a lot of similarities with the two spirals, whereas the structure of its ISM is different to that in dwarf galaxies. The sizes of the HI holes in M 81 range from 80 pc (close to the resolution limit) to 600 pc; the expansion velocities can reach 20 km s−1; estimated ages are 2.5 to 35 Myrs and the energies involved range from 1050 to 3.5 x 1052 ergs. The amount of neutral gas involved is of order 104 to 106 solar masses.

Geocoronal hydrogen studies using Fabry–Perot interferometers, part 1: Instrumentation, observations, and analysis

Ground-based Fabry–Perot observations of the solar excited Balmer α emission line of neutral atomic hydrogen have become one of the primary means of studying the distribution and behavior of hydrogen in the thermosphere+exosphere. This first of two companion papers on “Geocoronal Hydrogen Studies Using Fabry–Perot Interferometers”, will outline the technique of Fabry–Perot spectroscopy as applied to geocoronal hydrogen. Representative high spectral resolution data sets will be discussed which will serve to highlight advances in instrumentation and in the interpretation of geocoronal Balmer α intensity and line profile observations. A second companion paper will address long-term observations of the Balmer α emission.

Multi-aperture photometry of extended IR sources with ISOPHOT

Context. ISOPHOT multi-aperture photometry is an efficient method to resolve compact sources or to detect extended emission down to relatively faint levels with single detectors in the wavelength range 3 to 100 μm. Aims. Using ISOPHOT multi-aperture photometry and complementary ISO spectra and IR spectral energy distributions we discuss the nature of the extended IR emission of the two PNe NGC 6543 and NGC 7008. Methods. In the on-line appendix we describe the data reduction, calibration and interpretation methods based on a simultaneous determination of the IR source and background contributions from the on-source multi-aperture sequences. Normalized profiles enable direct comparison with point source and flat-sky references. Modelling the intensity distribution offers a quantitative method to assess source extent and angular scales of the main structures and is helpful in reconstructing the total source flux, if the source extends beyond a radius of 1 arcmin. The photometric calibration is described and typical accuracies are derived. General uncertainty, quality and reliability issues are addressed, too. Transient fitting to non-stabilised signal time series, by means of combinations of exponential functions with different time constants, improves the actual average signals and reduces their uncertainty. Results. The emission of NGC 6543 in the 3.6μm band coincides with the core region of the optical nebula and is homogeneously distributed. It is comprised of 65% continuum and 35% atomic hydrogen line emission. In the 12μm band a resolved but compact double source is surrounded by a fainter ring structure with all emission confined to the optical core region. Strong line emission of [ArIII] at 8.99 μm and in particular [SIV] at 10.51μm shapes this spatial profile. The unresolved 60μm emission originates from dust. It is described by a modified (emissivity index β = 1.5) blackbody with a temperature of 85 K, suggesting that warm dust with a mass of 6.4 × 10 -4 M Θ is mixed with the ionised gas. The gas-to-dust mass ratio is about 220. The 25 μm emission of NGC 7008 is characterised by a FWHM of about 50 with an additional spot-like or ring-like enhancement at the bright rim of the optical nebula. The 60μm emission exhibits a similar shape, but is about twice as extended. Analysis of the spectral energy distribution suggests that the 25 μm emission is associated with 120 K warm dust, while the 60μm emission is dominated by a second dust component with 55 K. The dust mass associated with this latter component amounts to 1.2 x 10 -3 M Θ , significantly higher than previously derived. The gas-to-dust mass ratio is 59 which, compared to the average value of 160 for the Milky Way, hints at dust enrichment by this object.

A "Missing" Supernova Remnant Revealed by the 21 cm Line of Atomic Hydrogen

Although some 20,000-30,000 supernova remnants (SNRs) are expected to exist in the Milky Way, only about 230 are currently known. This implies that most SNRs are missing. Recently, we proposed that small (1°), faint, high-velocity features seen in large-scale 21 cm line surveys of atomic hydrogen (H I) in the Galactic plane could be examples of such missing old SNRs. Here we report on high-resolution H I observations of one such candidate, FVW 190.2+1.1, which is revealed to be a rapidly expanding (~80 km s-1) shell. The parameters of this shell seem only consistent with FVW 190.2+1.1 being the remnant of a SN explosion that occurred in the outermost fringes of the Galaxy some ~3 × 105 yr ago. This shell is not seen in any other wave band, suggesting that it represents the oldest type of SNR, that which is essentially invisible except via its H I line emission. FVW 190.2+1.1 is one of a hundred forbidden-velocity wings (FVWs) recently identified in the Galactic plane, and our discovery suggests that many of these are likely to be among the oldest SNRs. We discuss the possible link between FVWs and fast-moving atomic clouds in the Galaxy.

The existence and detection of optically dark galaxies by 21-cm surveys

One explanation for the disparity between cold dark matter (CDM) predictions of galaxy numbers and observations could be that there are numerous dark galaxies in the Universe. These galaxies may still contain baryons, but no stars, and may be detectable in the 21-cm line of atomic hydrogen. The results of surveys for such objects, and simulations that do/do not predict their existence, are controversial. In this paper, we use an analytical model of galaxy formation, consistent with CDM, to first show that dark galaxies are certainly a prediction of the model. Secondly, we show that objects like VIRGOHI21, a dark galaxy candidate recently discovered by us, while rare are predicted by the model. Thirdly, we show that previous ‘blind’ H i surveys have placed few constraints on the existence of dark galaxies. This is because they have either lacked the sensitivity and/or velocity resolution or have not had the required detailed optical follow up. We look forward to new 21-cm blind surveys [Arecibo Legacy Fast ALFA (ALFALFA) survey and Arecibo Galactic Environments Survey (AGES)] using the Arecibo multibeam instrument which should find large numbers of dark galaxies if they exist.

Electron-impact broadening of radio recombination lines of atomic hydrogen for different Δn

In 1967 Griem gave a general formula for the electron-impact broadening of radio recombination lines, and an approximate expression for the width of the Δn = 1 lines. Here his general formula is calculated numerically for higher values of Δn, and an approximate expression is fitted to the results for Δn up to 70. It is shown that, for the lines with different n and Δn near a given frequency ν, the calculated widths increase monotonically up to n > 300. This result does not agree with the maximum found near n ≈ 200 by Bell et al (2000 Publ. Astron. Soc. Pac. 112 1236) for the astronomical sources W51 and Orion A at 6 GHz. Thus this mystery is not resolved by the present calculations.

Plasma 2D modeling and diagnostics of DLC deposition on PET

The possibility to apply two dimensional (2D) emission spectra of short-lived excited species for estimating the uniformity of production of ions and radicals that have an important role in the deposition of diamond-like coatings on polymeric substrates was investigated. Images of the α-balmer line of atomic hydrogen in CH 4/H 2 discharges were recorded under various total gas pressures and substrate bias voltages to reveal that the increase of the total gas pressure enhances radial non-uniformities of the electron population with energy higher than 16 eV. A comparison between these images and the results of a 2D simulator of CH 4/H 2 discharges shows that the most abundant ions (H 3 +, CH 5 +) and radicals with high reactivity in the gas phase (CH 2, CH, C) present similar axial and radial distribution with these emission maps.

Evidence of an energy transfer reaction between atomic hydrogen and argon II or helium II as the source of excessively hot H atoms in radio-frequency plasmas

From the width of the emitted 656.3 nm Balmer $\alpha$ line, it was found that low-pressure (0.5 torr) capacitively coupled radio-frequency (RF) ${\rm He}/{\rm H}_2$ (95/5%) as well as ${\rm Ar}/{\rm H}_2$ (95/5%) plasmas showed excess broadening throughout the volume (13.5 cm ID $\times$ 38 cm length) of a General Electronics Conference (GEC)-type cell; not merely in the vicinity of the electrodes of ${\rm Ar}/{\rm H}_2$ plasmas as reported by several groups. About 50% of the hydrogen of the ${\rm He}/{\rm H}_2$ was ‘hot’ with an average hydrogen atom energy of 40–50 eV, compared to $\sim$1 eV for pure hydrogen, whereas ${\rm Ar}/{\rm H}_2$ showed a single fast or ‘hot’ 40–50 eV population. The broadening was undiminished at 15 cm from the powered electrode, independent of power over a substantial range, but dependent on the hydrogen concentration. In contrast to the atomic hydrogen lines, no broadening was observed in helium or argon lines. Also, in ‘control’ ${\rm Xe}/{\rm H}_2$ plasmas run in the same cell at similar pressures and absorbed power, no significant broadening of atomic hydrogen, ${\rm Xe}$, or any other lines was observed. Stark broadening or acceleration of charged species due to high electric fields cannot explain the results since: (i) the electron density was insufficient by orders of magnitude, (ii) the RF field was essentially confined to the cathode fall region in contrast to the broadening that was independent of position, and (iii) only the atomic hydrogen lines were broadened. Rather, the data are consistent with a model wherein ${\rm He}^+$ and ${\rm Ar}^+$ act catalytically through a resonant energy transfer mechanism to create ‘hot’ hydrogen atoms in plasmas.

H i signal from re-ionization epoch

We investigate the all-sky signal in redshifted atomic hydrogen (H I) line from the re-ionization epoch. We model the phase of re-ionization as multiple point sources that carve out spherical Stromgren spheres. We study ionization histories compatible with Wilkinson Microwave Anisotropy Probe (WMAP) observation. The Lyman a and soft X-ray emission from these sources are taken into account for studying the H i signal. H i can be observed in both emission and absorption depending on the ratio of Lyman α to ionizing flux and the spectrum of the radiation in soft X-ray. We also compute the signal from pre-reionization epoch and show that within the uncertainty in cosmological parameters, it is fairly robust. The main features of H i signal can be summarized as follows. (i) The pre-ionized H I can be seen in absorption for v ≃ 10-40 MHz; the maximum signal strength is ≃70-100 mK. (ii) A sharp absorption feature of width?5 MHz might be observed in the frequency range ≃50-100 MHz, depending on the re-ionization history. The strength of the signal is proportional to the ratio of the Lyman a and the hydrogen-ionizing flux and the spectral index of the radiation field in soft X-ray. (iii) At larger frequencies, HI is seen in emission with peak frequency between 60 and 100 MHz, depending on the ionization history of the Universe; the peak strength of this signal is ≃50 mK. From Fisher matrix analysis, we compute the precision with which the parameters of the model can be estimated from a future experiment as follows: (i) the pre-reionization signal can constrain a region in the Ω b h 2 -Ω m h 2 plane (ii) H I observed in emission can be used to give precise,?1 per cent, measurement of the evolution of the ionization fraction in the Universe and (iii) the transition region from absorption to emission can be used as a probe of the spectrum of ionizing sources; in particular, the Hi signal in this regime can give a reasonably precise measurement of the fraction of the Universe heated by soft X-ray photons.

Electron density and temperature determination using the concept of particle confinement time uniqueness

The use of atomic hydrogen line emission to determine the particle confinement time τp of a tokamak plasma is a well-known diagnostic technique. Using such a method, for any one of the emission lines, be it from Lyman, Balmer, or Paschen series, the same (i.e., unique) value of τp must be obtained. Furthermore, this measurement is directly related to the local values of the electron temperature and density. We have developed a method based on the Hα, Hβ, and Hγ hydrogen line emissions and on the concept of τp uniqueness for a tokamak plasma, to determine the local electron density and temperature. The technique has been applied to plasma discharges generated in the NOVA-UNICAMP tokamak. The results show good agreement with measurements from multichannel Thomson scattering and Langmuir probe. A procedure to simulate the Hα emissivity radial profile using the obtained results is also discussed.

H/sub /spl alpha// emission as a feedback control sensor for reactive sputter deposition of nano-structured, diamond-like carbon coatings

This paper discusses the potential use of the hydrogen atomic line emission at 656.3 nm (H/sub /spl alpha//) as an effective in-situ sensor for a closed-loop control system to improve the reproducibility of reactive sputter deposition of nano-structured, metal-containing, hydrogenated, diamond-like carbon (Me:DLC:H) coatings. The paper includes experimental results showing a good correlation between H/sub /spl alpha// emission in the process plasma and the formation of metal-carbide, an important component of these coatings. The first attempts at actual feedback-control of the process showed that this sensor can be effective, at least over the stage of the deposition when mostly carbides are formed in the coating. A spectrally resolved analysis of the H/sub /spl alpha// emission for the various stages of the deposition have shown that the emission profile is dominated by a "hot" component (/spl sim/10-20 eV), which can be attributed to dissociative excitation of molecular hydrogen (H/sub 2/). The molecular hydrogen is understood to evolve from the coating as a result of carbon incorporation from the reactive gas (C/sub 2/H/sub 2/) and is particularly sensitive to metal carbide formation in the film, when most of the hydrogen is released from the surface in molecular form.

Neutral hydrogen in the Milky Way

Since its prediction and first detection just over 50 years ago, the 21-cm line of neutral atomic hydrogen has been a key probe of the structure and dynamics of the Milky Way Galaxy and of the astrophysics of the Galactic interstellar medium (ISM). Emission from Galactic atomic hydrogen is seen in all directions and the structures revealed reflect the processes relevant to the evolution of the Galaxy and phase transitions of the ISM. As in other areas of astronomy, our knowledge of the neutral ISM from HI studies has progressed in lock step with radio telescope technology. Over the last several years interferometric surveys have provided arcminute-scale HI images that allow the atomic hydrogen component of the ISM to be viewed in the context of the other major ISM components and reveal dynamic structures in HI related to the interaction of ISM with the stellar component of the Galaxy. The International Galactic Plane Survey, a collaboration to combined interferometric surveys from Canada, the US and Australia, is imaging over 90% of the stellar disk of the Milky Way in HI at arcminute resolution. When it is built in the post 2010 era, the prodigious survey speed and angular resolution of the Square Kilometre Array will affect another seminal advance in Galactic HI studies.

Reference Wavelengths for Strong Lines of Atomic Hydrogen and Deuterium

Wavelengths of the individual fine-structure components of the n = 1-2 (Ly(alpha)), n = 1-3 (Ly(beta)), n = 1-4 (Ly(gamma)), n = 1-5 (Ly(delta)), n = 1-6 (Ly(epsilon)), n = 1-7 (Ly(zeta)), n = 2-3 (H(alpha)), n = 2-4 (H(beta)), n = 2-5 (H(gamma)), n = 2-6 (H(delta)), and n = 2-7 (H(epsilon)) transitions of H and D are determined from theoretical values for the binding energies. Theoretical line strengths are used to obtain recommended values for the peaks of unresolved blends of these components as likely to be observed with discharge light sources and spectrometers with low to moderate resolution.

Diagnostics of APG discharge in neon with a small admixture of hydrogen

The aim of this work is to study the atmospheric pressure glow discharge (APG discharge) in neon. The discharge was generated between two metal electrodes, which were covered with an alumina layer. A small admixture of hydrogen could be added to neon. The type of the discharge (as concerning filamentary or glow discharge) was mainly derived from electrical measurements. The light emitted by the discharge was studied by means of optical emission spectroscopy (OES) with a temporal and spatial resolution. The results confirm that in neon the same type of diffuse dielectric-barrier discharge can be generated as in helium, namely a transient sub-normal glow discharge. This regime was also observed if up to 1% of hydrogen was added to neon. In general the recorded spectra consisted of atomic spectral lines of neon, as well as hydrogen atomic lines and molecular bands of N2 and OH due to external admixture or impurities in the carrier gas. When the amount of impurities was low enough, a continuous spectrum probably belonging to bremsstrahlung of electrons on neutral atoms was observed.

Prevalence of galaxy-galaxy interactions in AGN hosts

Studies in optical starlight have failed to reach a consensus on the importance of either galaxy interactions, bars, or nuclear spirals in triggering luminous active galactic nuclei (AGNs). Here, we present the first systematic imaging study of Seyfert (disk) galaxies in the 21-cm line of neutral atomic hydrogen (HI) gas. HI is the most sensitive and enduring tracer of galaxy interactions, and can reveal tidal features not otherwise visible in optical starlight. Our sample comprises all twenty-eight galaxies in the Véron-Cetty & Véron (1998) catalog with nuclear magnitudes −19 ≥ MB > −23 (including Seyfert, LINER, and HII galaxies) at 0.015 ≤ z ≤ 0.017 in the northern hemisphere, and a matched control sample of twenty-seven inactive galaxies at z≈0.008. We have detected nearly all the galaxies observed, and find a much higher incidence of tidal interactions — usually not seen in optical starlight — among the Seyfert galaxies by comparison with the matched control sample. Those Seyferts with uncertain or no clear tidal features show disturbed HI morphologies and/or kinematics, as well as HI companion galaxies, more frequently than the control sample. Our study suggests that the undisturbed optical appearence of active galaxies may be deceptive, and imply that galaxy-galaxy interactions trigger a significant fraction luminous AGNs at low redshifts. The majority of the Seyfert galaxies in our sample appear to be at a relatively early stage of an encounter rather than late in a merger.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Abundance differences between the neutral and the ionized gas of the dwarf galaxy IZw 36

We present a FUSE spectroscopic study of the nearby gas-rich, metal-deficient blue compact dwarf (BCD) galaxy IZw 36. Atomic hydrogen and many metal lines are observed in absorption against the stellar continuum of young, massive stars embedded in the ionized region. Profile fitting of absorption lines allowed us to determine abundances and investigate the chemical composition of the neutral gas. This study presents strong evidence that nitrogen is 16 +7 -8 times less abundant in the neutral gas than in the ionized gas (all uncertainties are 2σ). Similarly, the oxygen abundance estimated using phosphorus as a tracer is lower in the neutral gas by a factor of 8 +17 -5 . We also find that argon is underabundant by a factor of 32 +8 -7 and that log (Ar I/OI) < -3.0 which is inconsistent with the Ar/O ratio -2.1 ± 0.1 (Lodders 2003), implying that argon is likely ionized into Ar II in the neutral medium.

Hαemission in the presence of dust inan Ar-C2H2radio-frequency discharge

We present the results of time resolved spectroscopymeasurements of the Hα line of atomic hydrogen in an Ar-C2H2 radiofrequency plasma. The presenceof the fast (high-energy) and slow (low-energy) components ofthe hydrogen atoms can be deduced from the Doppler broadeningof the Hα line. With the appearance of the dust particles, the Hα profile became narrower, indicating reduction ofthe fast component. We discuss several mechanisms for theformation of the fast hydrogen atoms in our plasma. The maincause for the reduction of the hydrogen atom fast component isthe plasma change from electropositive to electronegative,decreasing the sheath's width and voltage. The change of plasma conditions coincides with the end of the dustcoagulation/agglomeration when the dust particles becomemultiply negatively charged. We propose here a new experimentaltechnique for detection of the dust coagulation/agglomeration phase.

Ab initio calculation of electronic transition moments for singlet excited states of the H 2 molecule

Ab initio CI electronic dipole transition moments have been calculated for the transitions between singlet states of the hydrogen molecule correlating asymptotically with H( nl)+H(1 s) ( n=1,2,3). The investigated singlet–singlet transitions include the 30 ( n=3) inter-Rydberg transitions and the 32 transitions which may contribute to absorption in the far wings of the Balmer α line of atomic hydrogen perturbed by another hydrogen atom in its ground state. Results are presented for internuclear distances 1.0 a 0⩽ R⩽12 a 0. The present results compare well with the previous theoretical calculations available for about half of the transitions treated in the present work. Thirty eight new transitions are presented. Adiabatic potential energies for the 1 1 Δ u and 3 1 Π g and improved energies for the 3 1 Π u and 4 1 Π u states are reported as well.

Extremely metal-poor Lyman limit system at $\mathsf{{\vec z}_{abs} = 2.917}$ toward the quasar HE 0940–1050

We report on detailed Monte Carlo inversion analysis of the Lyman limit system observed at z a b s = 2.917 in the VLT/UVES spectrum of the quasar HE 0940-1050. Metal absorption lines of carbon and silicon in three ionization stages and numerous atomic hydrogen lines have been analyzed simultaneously. It is found that in order to match the observations, the shape of the ultraviolet background ionizing spectrum of Haardt & Madau (1996) should be modified: a spectrum with a higher intensity of the emission feature at 3 Ryd is required. It is also found that synthetic galactic spectra (or different mixtures of them with power law spectra) cannot reproduce the observations, indicating that the stellar contribution to the ionizing background is negligible at z ∼ 3. For the first time a very low carbon abundance of [C/H] = -2.93 ′ 0.13 and the abundance ratio [Si/C] = 0.35 ′ 0.15 are directly measured in the Lyman limit system with N(HI) = 3.2 x 10 1 7 cm - 2 . If the absorber at z a b s = 2.917 provides an example of a pristine gas enriched by the nucleosynthetic products of early generations of stars, then the measured value of [Si/C] seems to indicate that the initial mass functions for these stellar populations are constrained to intermediate masses, M u p ≤ 25 M O ..