Atomic Hydrogen Column Density Research Articles

Comparison of terrestrial exospheric hydrogen 3D distributions at solar minimum and maximum using TWINS Lyman-α observations

Remote sensing observations of far-ultraviolet (FUV) emissions have been used to estimate 3D neutral hydrogen (H) density models of the terrestrial exosphere under solar minimum (2008) and solar maximum (2013 and 2015) conditions. Specifically, we used Lyman-alpha (Lyman-α, FUV at 121.6 nm) radiance data acquired by the Lyman-alpha detectors (LADs) onboard NASA’s TWINS satellites, collected over several days for each of the 3 years to provide sufficient coverage of the exospheric region. The datasets included Lyman-α measurements taken only above Earth radii (Re) of 3.75, assumed to be the optically thin region of the exosphere, where the measured Lyman-α intensity along a line of sight (LOS) is linearly proportional to the atomic hydrogen column density. Based on the calibration using multiple UV-bright stars, a significant decrease in TWINS1 LAD1/2 sensitivities was found for 2013 (LAD2 ∼1/2, LAD ∼1/3) and 2015 (LAD2 ∼1/3, LAD ∼1/7) compared to 2008. The calibration uncertainty was derived to be, on average, 5%. We estimated the 3D global hydrogen density distributions from these radiance data using two different tomographic inversion methods: first, a parametric fitting method based on a spherical harmonic function of order 3 and second, a high degree-of-freedom retrieval approach to validate our retrievals of H density. Both inversion methods consistently incorporate the effects of Lyman-α absorption within the exosphere and Lyman-α re-emission from Earth’s albedo. Our results reveal that H densities during solar maximum conditions are, on average, 30%–40% higher than those during solar minimum. All models showed a high concentration of atomic H on the dayside and nightside near the Sun–Earth line, which determines a nose/geotail structure consistent with theoretical effects from the solar radiation pressure. Furthermore, we identified that H-density enhancements during solar maximum with respect to solar minimum conditions occur at mid-to-high latitudes, particularly on the dusk side, while no significant enhancement seems to occur near the dayside nose.

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 $z<8$.

Photodissociation and photoionization of molecules of astronomical interest

Context. Vacuum-ultraviolet (VUV) photons are important drivers of chemical processes in space. Thus, it is important to accurately characterize and constrain photorates in different radiation fields, via the photodissociation and photoionization cross sections of individual atoms and molecules. These have been available in the Leiden VUV photodissocation and photoionization cross section database. Aims. Experimental and theoretical advances in the past decade or so have allowed multiple new cross sections to be obtained, particularly photoionization cross sections of radicals. The database is hereby updated by including these more recent cross sections and is also expanded with several astronomically relevant species. Methods. The cross sections have been used to calculate photodissociation and photoionization rates in several different radiation fields as well as from cosmic-ray-induced VUV fluxes. The reduction of rates in shielded regions was calculated as a function of dust, molecular and atomic hydrogen, atomic carbon, and self-shielding column densities. The relative importance of these shielding types is molecule and atom dependent, as well as the assumed dust absorbance. All the data are publicly available from the Leiden VUV cross section database. Results. The Leiden VUV cross section database has been updated with 14 new astrophysically relevant molecular species and 16 updates to previous entries. The database update is accompanied by a brief review of the basic physical processes, particularly photoionization processes which have not been reviewed in the context of previous database updates.

Direct observations of the atomic-molecular phase transition in the Milky Way’s nuclear wind

ABSTRACT Hundreds of high-velocity atomic gas clouds exist above and below the Galactic Centre, with some containing a molecular component. However, the origin of these clouds in the Milky Way’s wind is unclear. This paper presents new high-resolution MeerKAT observations of three atomic gas clouds and studies the relationship between the atomic and molecular phases at ∼1 pc scales. The clouds’ atomic hydrogen column densities, NH i, are less than a few × 1020 cm−2, but the two clouds closest to the Galactic Centre none the less have detectable CO emission. This implies the presence of H2 at levels of NH i at least a factor of ten lower than in the typical Galactic interstellar medium. For the cloud closest to the Galactic Centre, detectable CO coexists across the entire range of H i column densities. In contrast, for the intermediate cloud, detectable CO is heavily biased towards the highest values of NH i. The cloud most distant from the Galactic Centre has no detectable CO at similar NH i values. Moreover, we find that the two clouds with detectable CO are too molecule-rich to be in chemical equilibrium, given the depths of their atomic shielding layers, which suggests a scenario whereby these clouds consist of pre-existing molecular gas from the disc that the Galactic wind has swept up, and that is dissociating into atomic hydrogen as it flows away from the Galaxy. We estimate that entrained molecular material of this type has a ∼few − 10 Myr lifetime before photodissociating.

Evidence for Large-scale Excesses Associated with Low H i Column Densities in the Sky. I. Dust Excess

Where dust and gas are uniformly mixed, atomic hydrogen can be traced through the detection of far-infrared (FIR) or UV emission of dust. We considered, for the origin of discrepancies observed between various direct and indirect tracers of gas outside the Galactic plane, possible corrections to the zero levels of the Planck High Frequency Instrument (HFI) detectors. We set the zero levels of the Planck-HFI skymaps as well as the 100 μm map from COBE/DIRBE and IRAS from the correlation between FIR emission and atomic hydrogen column density excluding regions of lowest gas column density. A modified blackbody model fit to those new zero-subtracted maps led to significantly different maps of the opacity spectral index β and temperature T and an overall increase in the optical depth at 353 GHz τ 353 of 7.1 × 10−7 compared to the data release 2 Planck map. When comparing τ 353 and the H i column density, we observed a uniform spatial distribution of the opacity outside regions with dark neutral gas and CO except in various large-scale regions of low N H i that represent 25% of the sky. In those regions, we observed an average dust column density 45% higher than predictions based on N H i with a maximum of 250% toward the Lockman Hole region. From the average opacity σ e353 = (8.9 ± 0.1) × 10−27 cm2, we deduced a dust-to-gas mass ratio of 0.53 × 10−2. We did not see evidence of dust associated with a Reynolds layer of ionized hydrogen. We measured a far-ultraviolet isotropic intensity of 137 ± 15 photons s−1 cm−2 sr−1 Å−1 in agreement with extragalactic flux predictions and a near-ultraviolet isotropic intensity of 378 ± 45 photons s−1 cm−2 sr−1 Å−1 corresponding to twice the predicted flux.

Dynamical effects of the radiative stellar feedback on the H I-to-H2 transition

Context. The atomic-to-molecular hydrogen (H/H2) transition has been extensively studied as it controls the fraction of gas in a molecular state in an interstellar cloud. This fraction is linked to star-formation by the Schmidt–Kennicutt law. While theoretical estimates of the column density of the H I layer have been proposed for static photodissociation regions (PDRs), Herschel and well-resolved Atacama Large Millimeter Array observations have revealed dynamical effects in star forming regions, caused by the process of photoevaporation. Aims. We extend the analytic study of the H/H2 transition to include the effects of the propagation of the ionization front, in particular in the presence of photoevaporation at the walls of blister H II regions, and we find its consequences on the total atomic hydrogen column density at the surface of clouds in the presence of an ultraviolet field, and on the properties of the H/H2 transition. Methods. We solved semi-analytically the differential equation giving the H2 column density profile by taking into account H2 formation on grains, H2 photodissociation, and the ionization front propagation dynamics modeled as advection of the gas through the ionization front. Results. Taking this advection into account reduces the width of the atomic region compared to static models. The atomic region may disappear if the ionization front velocity exceeds a certain value, leading the H/H2 transition and the ionization front to merge. For both dissociated and merged configurations, we provide analytical expressions to determine the total H I column density. Our results take the metallicity into account. Finally, we compared our results to observations of PDRs illuminated by O-stars, for which we conclude that the dynamical effects are strong, especially for low-excitation PDRs.

Reconciling X-Ray and λ21 cm H I Absorption Gas Column Densities toward Obscured AGN

Abstract Hydrogen column densities inferred from X-ray absorption are typically 5–30 times larger than the neutral atomic hydrogen column densities derived from λ21 cm H I absorption toward radio-loud active galactic nuclei (AGN). Some part of the difference is ascribed to uncertainty in the spin temperature T sp = 100 K that is often used to convert λ21 cm H I absorption to N(H I). Here we propose another way to infer the gas column from H I absorption. In our Galaxy there is a nearly linear correlation between the inteferometrically measured integrated λ21 cm absorption ϒH I and reddening, ϒH I ∝ E(B–V)1.10 for ϒH I ≳ 0.7 km s−1 or E(B–V) ≳ 0.04 mag. Scaling E(B–V) then provides the total gas column density N(H) from the same dust column that is responsible for optical obscuration and X-ray absorption, without calculating N(H I). Values of N(H) so derived typically exceed N(H I) by a factor 4 because the ubiquitous Galactic λ21 cm H I absorption samples only a portion of the interstellar gas. If the well-studied case of Hydra-A is a guide, even very large disparities in X-ray and λ21 cm gas column densities can be explained by resolving the core radio continuum and inferring N(H) from λ21 cm absorption. Milky Way conditions are often invoked in discussion of obscured AGN, so the empirical relationship seen in the Milky Way should be a relevant benchmark.

FIFI-LS Observations of the Circumnuclear Ring— Probing the High-density Phase of the PDR

Abstract We investigate the nature of dense gas in the 3–10 pc circumnuclear ring (CNR) in the galactic center of the Milky Way, which is a structure that may be dynamically connecting the supermassive black hole Sgr A* with the central molecular zone at the 100 pc scale, and is the closest reservoir of molecular gas to the massive stars located within the central cluster. In the first of several papers addressing open issues with the CNR, we use far-infrared (FIR) diagnostic emission lines to probe the hot and dense phase of the photodissociation region (PDR) exposed to the radiation field of the central population of massive stars. We use the Far Infrared Field-Imaging Line Spectrometer (FIFI-LS) instrument on board the Stratospheric Observatory For Infrared Astronomy airborne observatory to obtain spatially resolved maps of FIR emission lines of the region with an angular resolution approximately 4 times higher than previous published data. We complement our data with archival continuum images at 19.7, 31.5 and 37.1 μm obtained with FORCAST and 70, 100 and 160 μm archival continuum images from PACS. We use the FIFI-LS emission line flux maps from ionized ([C ii] 157.7 μm), atomic ([O i] 63.2 μm, [O i] 145.5 μm), and molecular (CO J = 14–13 186.0 μm) species for a comparison with model predictions for PDRs. We present a method that dissects emission from the low and from the high excitation phase of the PDR and that also accounts for, e.g., absorption especially in the [O i] 63.2 μm transition. We present spatially resolved maps of dust temperature, atomic hydrogen column density, and FIR flux. The derived atomic hydrogen column density map is aligned with the galactic plane and extends spatially beyond previous near-infrared and radio based A v determinations. The atomic hydrogen column densities range from 1022.5 to 1023.1 cm−2 resulting in a total enclosed mass of the order of 103.5 M ⊙. We derive a [O i] 63.2 μm absorption map that is aligned with the galactic plane with no or little absorption in the northern lobe of the CNR but moderate absorption in the southern lobe of the CNR, which is consistent with the picture where the illuminated front surfaces of gas clouds in the northern lobe are directly visible to us, while in the southern lobe the illuminated surfaces are hidden by the clouds within the lobe itself. Local gas densities in the CNR are generally below the Roche limit.

Time-of-day-dependent behavior of surficial lunar hydroxyl/water: Observations and modeling

In this study we determine the depth of the absorption band around 3 µm wavelength, which indicates the presence of OH/H2O in a thin surficial layer of the lunar regolith, for 18 lunar highland regions observed by the Moon Mineralogy Mapper (M3) instrument at 4–8 different local times of day. For removing the thermal emission component, a physically motivated thermal equilibrium based method is used, which also takes into account the roughness of the regolith surface. We propose a continuity equation based model of the time-of-day-dependent column densities of surficial atomic hydrogen (H) and hydroxyl (OH). The considered source processes for H are implantation of solar wind protons and photolysis of OH, and for OH the reaction H+O→OH. Sink processes are diffusive loss for H and OH, and photolysis for OH. Sputtering of H and OH is found to be negligible in comparison to the other sink processes. Additionally, we suggest a similar differential equation based model to describe the time-of-day-dependent behavior of micrometeoroid-delivered OH and H2O. The observed 3 µm band depth values indicate that the surficial OH/H2O does not vanish even at local midday at low latitudes, while the model predicts nearly complete removal of surficial OH/H2O at midday. This apparent contradiction between model and observations is reconciled by adding to the model a region-specific “offset” OH component which is assumed to be stable against diffusive loss and photolysis and is therefore interpreted as a strongly bounded OH component. Meteoroid bombardment is found to be negligible in comparison with the solar wind source of OH. Fitting the model to the observed 3 µm band depth values allows for estimating the H activation energy, the OH photolysis time, region-specific values of the offset OH component, and the proportionality factor between OH column density and 3 µm band depth. The observed time-of-day-dependent behavior of the 3 µm band depth at low and high latitudes can be explained convincingly by the modeled source and sink processes. The best-fit OH photolysis time is much shorter than the lunar day and corresponds to 1–3 times the gas-phase value, which indicates that photolysis is a mechanism of high relevance for the behavior of solar wind induced surficial OH. The surface roughness assumed in the M3 data analysis does not have a major influence on the modeling results. The strongly bounded OH component is nearly latitude-independent for low latitudes but decreases sharply for high latitudes. As a possible mode of origin, we suggest slow diffusion of solar wind induced OH to depths inaccessible for ultraviolet photons and into binding states of higher energy, counteracted by sputtering.

Molecular gas and star formation in an absorption-selected galaxy: Hitting the bull’s eye atz≃ 2.46

We present the detection and detailed analysis of a diffuse molecular cloud atzabs= 2.4636 towards the quasar SDSS J 1513+0352 (zem≃ 2.68) observed with the X-shooter spectrograph at the Very Large Telescope. We measured very high column densities of atomic and molecular hydrogen with logN(H I, H2) ≃ 21.8, 21.3. This is the highest H2column density ever measured in an intervening damped Lyman-αsystem but we did not detect CO, implying logN(CO)/N(H2) < −7.8, which could be due to a low metallicity of the cloud. From the metal absorption lines, we derived the metallicity to beZ≃ 0.15Z⊙and determined the amount of dust by measuring the induced extinction of the background quasar light,AV≃ 0.4. We simultaneously detected Lyman-αemission at the same redshift with a centroid located at a most probable impact parameter of onlyρ≃ 1.4 kpc. We argue that the line of sight is therefore likely passing through the interstellar medium (ISM), as opposed to the circumgalactic medium (CGM), of a galaxy. The relation between the surface density of gas and that of star formation seems to follow the global empirical relation derived in the nearby Universe although our constraints on the star formation rate (SFR) and the galaxy extent remain too loose to be conclusive. We study the transition from atomic to molecular hydrogen using a theoretical description based on the microphysics of molecular hydrogen. We use the derived chemical properties of the cloud and physical conditions (Tk≃ 90 K andn≃ 250 cm−3) derived through the excitation of H2rotational levels and neutral carbon fine structure transitions to constrain the fundamental parameters that govern this transition. By comparing the theoretical and observed H Icolumn densities, we are able to bring an independent constraint on the incident ultra-violet (UV) flux, which we find to be in agreement with that estimated from the observed SFR.

Photodissociation and photoionisation of atoms and molecules of astrophysical interest

A new collection of photodissociation and photoionisation cross sections for 102 atoms and molecules of astrochemical interest has been assembled, along with a brief review of the basic processes involved. These have been used to calculate dissociation and ionisation rates, with uncertainties, in a standard ultraviolet interstellar radiation field (ISRF) and wavelength-dependent radiation fields. The new ISRF rates generally agree within 30% with our previous compilations, with a few notable exceptions. The reduction of rates in shielded regions was calculated as a function of dust, molecular and atomic hydrogen, atomic C, and self-shielding column densities. The relative importance of shielding types depends on the species in question and the dust optical properties. The new data are publicly available from the Leiden photodissociation and ionisation database. Sensitivity of rates to variation of temperature and isotope, and cross section uncertainties, are tested. Tests were conducted with an interstellar-cloud chemical model, and find general agreement (within a factor of two) with the previous iteration of the Leiden database for the ISRF, and order-of-magnitude variations assuming various kinds of stellar radiation. The newly parameterised dust-shielding factors makes a factor-of-two difference to many atomic and molecular abundances relative to parameters currently in the UDfA and KIDA astrochemical reaction databases. The newly-calculated cosmic-ray induced photodissociation and ionisation rates differ from current standard values up to a factor of 5. Under high temperature and cosmic-ray-flux conditions the new rates alter the equilibrium abundances of abundant dark cloud abundances by up to a factor of two. The partial cross sections for H2O and NH3 photodissociation forming OH, O, NH2 and NH are also evaluated and lead to radiation-field-dependent branching ratios.

Hydrogen in diffuse molecular clouds in the Milky Way

Recent submillimeter and far-infrared wavelength observations of absorption in the rotational ground-state lines of various simple molecules against distant Galactic continuum sources have opened the possibility of studying the chemistry of diffuse molecular clouds throughout the Milky Way. In order to calculate abundances, the column densities of molecular and atomic hydrogen, HI, must be known. We aim at determining the atomic hydrogen column densities for diffuse clouds located on the sight lines toward a sample of prominent high-mass star-forming regions that were intensely studied with the HIFI instrument onboard Herschel. Based on Jansky Very Large Array data, we employ the 21 cm HI absorption-line technique to construct profiles of the HI opacity versus radial velocity toward our target sources. These profiles are combined with lower resolution archival data of extended HI emission to calculate the HI column densities of the individual clouds along the sight lines. We employ Bayesian inference to estimate the uncertainties of the derived quantities. Our study delivers reliable estimates of the atomic hydrogen column density for a large number of diffuse molecular clouds at various Galactocentric distances. Together with column densities of molecular hydrogen derived from its surrogates observed with HIFI, the measurements can be used to characterize the clouds and investigate the dependence of their chemistry on the molecular fraction, for example.

HI4PI: a full-sky H i survey based on EBHIS and GASS

Measurement of the Galactic neutral atomic hydrogen (HI) column density, NHI, and brightness temperatures, Tb, is of high scientific value for a broad range of astrophysical disciplines. In the past two decades, one of the most-used legacy HI datasets has been the Leiden/Argentine/Bonn Survey (LAB). We release the HI 4$\pi$ survey (HI4PI), an all-sky database of Galactic HI, which supersedes the LAB survey. The HI4PI survey is based on data from the recently completed first coverage of the Effelsberg-Bonn HI Survey (EBHIS) and from the third revision of the Galactic All-Sky Survey (GASS). EBHIS and GASS share similar angular resolution and match well in sensitivity. Combined, they are ideally suited to be a successor to LAB. The new HI4PI survey outperforms the LAB in angular resolution (16.2', FWHM) and sensitivity (RMS: 43 mK). Moreover, it has full spatial sampling and thus overcomes a major drawback of LAB, which severely undersamples the sky. We publish all-sky column density maps of the neutral atomic hydrogen in the Milky Way, along with full spectroscopic data, in several map projections including HEALPix.

All-sky catalogs and analysis of interstellar clouds

AbstractAll available databases of molecular and gas-dust clouds in the Galaxy and other galaxies including their main properties such as position, angular and linear dimensions, distances, radial velocities, atomic and molecular hydrogen mass and column densities, temperatures, masses and others available are briefly described in the paper. An initial list of about 10 000 entries was condensed into a cross-identified all-sky catalogue containing molecular and gas-dust clouds. Some relationships were studied between main physical features of clouds. Finally, we prepared the complex observing program and address prospective work for gaining the gaps.

${\rm H\,\scriptsize{I}}$, CO, ANDPLANCK/IRASDUST PROPERTIES IN THE HIGH LATITUDE CLOUD COMPLEX, MBM 53, 54, 55 AND HLCG 92 – 35. POSSIBLE EVIDENCE FOR AN OPTICALLY THICK ${\rm H\,\scriptsize{I}}$ ENVELOPE AROUND THE CO CLOUDS

We present an analysis of the HI and CO gas in conjunction with the Planck/IRAS submillimeter/far-infrared dust properties toward the most outstanding high latitude clouds MBM 53, 54, 55 and HLCG 92-35 at b = -30 deg to -45 deg. The CO emission, dust opacity at 353 GHz (tau353), and dust temperature (Td) show generally good spatial correspondence. On the other hand, the correspondence between the HI emission and the dust properties is less clear than in CO. The integrated HI intensity WHI and tau353 show a large scatter with a correlation coefficient of ~0.6 for a Td range from 16 K to 22 K. We find, however, that WHI and tau353 show better correlation for smaller ranges of Td every 0.5 K, generally with a correlation coefficient of 0.7-0.9. We set up a hypothesis that the HI gas associated with the highest Td >= 21.5 K is optically thin, whereas the HI emission is generally optically thick for Td lower than 21.5 K. We have determined a relationship for the optically thin HI gas between atomic hydrogen column density and tau353, NHI (cm-2) = (1.5 x 10^26) x tau353, under the assumption that the dust properties are uniform and we have applied this to estimate NHI from tau353 for the whole cloud. NHI was then used to solve for Ts and tauHI over the region. The result shows that the HI is dominated by optically thick gas having a low spin temperature of 20-40 K and a density of 40-160 cm-3. The HI envelope has a total mass of ~1.2 x 10^4 Msol, an order of magnitude larger than that of the CO clouds. The HI envelope properties derived by this method do not rule out a mixture of HI and H2 in the dark gas, but we present indirect evidence that most of the gas mass is in the atomic state.

N(H I)/E(B–V)

We explore the relationship between dust-emission derived reddening \EBV\ and atomic hydrogen column density N(H I) derived from 21 cm emission surveys. We consider measurements at galactic latitude $|b| \ga 20$\degr\ and \EBV\ $\la$ 0.1 mag where the interstellar gas is predominantly neutral and atomic and opacity corrections to 21cm H I profiles are small. Over the Galaxy at large at lower resolution in H I, and on smaller scales at higher resolution, we find that the reddening is always much smaller than would be expected from the usually-quoted relation N(H) $ = 5.8\times 10^{21}\pcc$ \EBV\ based on stellar reddening and {\it uv} absorption toward early-type stars. On wide scales, we find N(H I) = $8.3 \times 10^{21}\pcc$ \EBV. We cite various precedents for such a large N(H I)/\EBV\ ratio whenever wide-field 21cm emission surveys are considered, including when reddening based on galaxy counts or colors is substituted for the dust-emission derived reddening measure.

A RELATION BETWEEN THE WARM NEUTRAL AND IONIZED MEDIA OBSERVED IN THE CANADIAN GALACTIC PLANE SURVEY

We report on a comparison between 21 cm rotation measure (RM) and the optically-thin atomic hydrogen column density (N_HI) measured towards unresolved extragalactic sources in the Galactic plane of the northern sky. HI column densities integrated to the Galactic edge are measured immediately surrounding each of nearly 2000 sources in 1-arcminute 21 cm line data, and are compared to RMs observed from polarized emission of each source. RM data are binned in column-density bins 4x10^20 cm^-2 wide, and one observes a strong relationship between the number of hydrogen atoms in a 1 cm^2 column through the plane and the mean RM along the same line-of-sight and path length. The relationship is linear over one order of magnitude (from 0.8-14x10^21 atoms cm^-2) of column densities, with a constant RM/N_HI -23.2+/-2.3 rad m^-2/10^21 atoms cm^-2, and a positive RM of 45.0+/-13.8 rad m^-2 in the presence of no atomic hydrogen. This slope is used to calculate a mean volume-averaged magnetic field in the 2nd quadrant of <B_||>~1.0+/-0.1 micro-Gauss directed away from the Sun, assuming an ionization fraction of 8% (consistent with the WNM). The remarkable consistency between this field and <B>=1.2 micro-Gauss found with the same RM sources and a Galactic model of dispersion measures suggests that electrons in the partially ionized WNM are mainly responsible for pulsar dispersion measures, and thus the partially-ionized WNM is the dominant form of the magneto-ionic interstellar medium.

Calibration of X-ray absorption in our Galaxy

Prediction of the soft X-ray absorption along lines of sight through our Galaxy is crucial for understanding the spectra of extragalactic sources, but requires a good estimate of the foreground column density of photoelectric absorbing species. Assuming uniform elemental abundances this reduces to having a good estimate of the total hydrogen column density, N(Htot)=N(HI)+2N(H2). The atomic component, N(HI), is reliably provided using the mapped 21 cm radio emission but estimating the molecular hydrogen column density, N(H2), expected for any particular direction, is difficult. The X-ray afterglows of GRBs are ideal sources to probe X-ray absorption in our Galaxy because they are extragalactic, numerous, bright, have simple spectra and occur randomly across the entire sky. We describe an empirical method, utilizing 493 afterglows detected by the Swift XRT, to determine N(Htot) through the Milky Way which provides an improved estimate of the X-ray absorption in our Galaxy and thereby leads to more reliable measurements of the intrinsic X-ray absorption and, potentially, other spectral parameters, for extragalactic X-ray sources. We derive a simple function, dependent on the product of the atomic hydrogen column density, N(HI), and dust extinction, E(B-V), which describes the variation of the molecular hydrogen column density, N(H2), of our Galaxy, over the sky. Using the resulting N(Htot) we show that the dust-to-hydrogen ratio is correlated with the carbon monoxide emission and use this ratio to estimate the fraction of material which forms interstellar dust grains. Our resulting recipe represents a significant revision in Galactic absorption compared to previous standard methods, particularly at low Galactic latitudes.

Broad, weak 21 cm absorption in an early-type galaxy: spectral line finding and parametrization for future surveys

We report conclusive verification of the detection of associated HI 21 cm absorption in the early-type host galaxy of the compact radio source PMNJ2054-4242. We estimate an effective spectral line velocity width of 418 +/- 20 km s^{-1} and observed peak optical depth of 2.5 +/- 0.2 per cent, making this one of the broadest and weakest 21 cm absorption lines yet detected. For T_{spin}/f > 100 K the atomic neutral hydrogen column density is N_{HI} > 2 x 10^{21} cm^{-2}. The observed spectral line profile is redshifted by 187 +/- 46 km s^{-1}, with respect to the optical spectroscopic measurement, perhaps indicating that the HI gas is infalling towards the central active galactic nucleus. Our initial tentative detection would likely have been dismissed by visual inspection, and hence its verification here is an excellent test of our spectral line detection technique, currently under development in anticipation of future next-generation 21 cm absorption-line surveys.

STUDIES OF THE DIFFUSE INTERSTELLAR BANDS. IV. THE NEARLY PERFECT CORRELATION BETWEEN λλ6196.0 AND 6613.6

In a sample of 114 diffuse cloud sightlines spanning a wide range of interstellar environments, we find the equivalent widths of the diffuse interstellar bands (DIBs) λ6196.0 and λ6613.6 to be extremely well correlated, with a correlation coefficient of 0.986. A maximum likelihood functional relationship analysis shows that the observations are consistent with a perfect correlation if the observational errors, which are dominated by continuum placement and other systematics such as interfering lines, have been underestimated by a factor of 2. The quality of this correlation far exceeds other previously studied correlations, such as that between the λ5780.5 DIB and either the color excess or the atomic hydrogen column density. The unusually tight correlation between these two DIBs would seem to suggest that they might represent the first pair of DIBs known to be due to the same molecular carrier. However, further theoretical work will be required to determine whether the different linewidths and band shapes of these two DIBs can be consistent with a common carrier. If the two DIBs do not in fact share the same molecular carrier, their two carriers must be chemically very closely related.