We investigate the physical properties of structures seen in channel map observations of 21 cm neutral hydrogen (H i) emission. H i intensity maps display prominent linear structures that are well ...aligned with the ambient magnetic field in the diffuse interstellar medium (ISM). Some literature hold that these structures are "velocity caustics," fluctuations imprinted by the turbulent velocity field, and are not three-dimensional density structures in the ISM. We test this hypothesis by stacking probes of the density field-broadband far-infrared (FIR) emission and the integrated H i column density ( )-at the locations of linear H i intensity structures. We find that the H i intensity features are real density structures and not velocity caustics. We generalize the investigation to all small-scale structure in H i channel maps and analyze this correlation as a function of velocity channel width, finding no measurable contribution from velocity caustics to the H i channel map emission. Further, we find that small-scale H i channel map structures have elevated FIR/ , implying that this emission originates from a colder, denser phase of the ISM than the surrounding material. The data are consistent with a multiphase diffuse ISM in which small-scale structures in narrow H i channel maps are preferentially cold neutral medium (CNM) that is anisotropically distributed and aligned with the local magnetic field. The shallow spatial power spectrum (SPS) of narrow H i channels is often attributed to velocity caustics. We conjecture instead that the small-scale structure and narrow line widths typical of CNM explain the observed relationship between the SPS and channel width.
We describe a morphological imprint of magnetization found when considering the relative orientation of the magnetic field direction with respect to the density structures in simulated turbulent ...molecular clouds. This imprint was found using the Histogram of Relative Orientations (HRO), a new technique that utilizes the gradient to characterize the directionality of density and column density structures on multiple scales. We present results of the HRO analysis in three models of molecular clouds in which the initial magnetic field strength is varied, but an identical initial turbulent velocity field is introduced, which subsequently decays. The HRO analysis was applied to the simulated data cubes and mock-observations of the simulations produced by integrating the data cube along particular lines of sight. In the three-dimensional analysis we describe the relative orientation of the magnetic field B with respect to the density structures, showing that: (1) the magnetic field shows a preferential orientation parallel to most of the density structures in the three simulated cubes, (2) the relative orientation changes from parallel to perpendicular in regions with density over a critical density nT in the highest magnetization case, and (3) the change of relative orientation is largest for the highest magnetization and decreases in lower magnetization cases. This change in the relative orientation is also present in the projected maps. In conjunction with simulations, HROs can be used to establish a link between the observed morphology in polarization maps and the physics included in simulations of molecular clouds.
Within four nearby (d < 160 pc) molecular clouds, we statistically evaluated the structure of the interstellar magnetic field, projected on the plane of the sky and integrated along the line of ...sight, as inferred from the polarized thermal emission of Galactic dust observed by Planck at 353 GHz and from the optical and near-infrared polarization of background starlight. We compared the dispersion of the field orientation directly in vicinities with an area equivalent to that subtended by the Planck effective beam at 353 GHz (10′) and using the second-order structure functions of the field orientation angles. We found that the average dispersion of the starlight-inferred field orientations within 10′-diameter vicinities is less than 20°, and that at these scales the mean field orientation is on average within 5° of that inferred from the submillimetre polarization observations in the considered regions. We also found that the dispersion of starlight polarization orientations and the polarization fractions within these vicinities are well reproduced by a Gaussian model of the turbulent structure of the magnetic field, in agreement with the findings reported by the Planck Collaboration at scales ℓ > 10′ and for comparable column densities. At scales ℓ > 10′, we found differences of up to 14.̊7 between the second-order structure functions obtained from starlight and submillimetre polarization observations in the same positions in the plane of the sky, but comparison with a Gaussian model of the turbulent structure of the magnetic field indicates that these differences are small and are consistent with the difference in angular resolution between both techniques. The differences between the second-order structure functions calculated with each technique suggests that the increase in the angular resolution obtained with the starlight polarization observations does not introduce significant corrections to the dispersion of polarization orientations used in the calculation of the molecular-cloud-scale magnetic field strengths reported in previous studies by the Planck Collaboration.
Context. Current dust models are challenged by the dust properties inferred from the analysis of Planck observations in total and polarized emission. Aims. We propose new dust models compatible with ...polarized and unpolarized data in extinction and emission for translucent lines of sight (0.5 < AV < 2.5). Methods. We amended the DustEM tool to model polarized extinction and emission. We fit the spectral dependence of the mean extinction, polarized extinction, total and polarized spectral energy distributions (SEDs) with polycyclic aromatic hydrocarbons, astrosilicate and amorphous carbon (a-C) grains. The astrosilicate population is aligned along the magnetic field lines, while the a-C population may be aligned or not. Results. With their current optical properties, oblate astrosilicate grains are not emissive enough to reproduce the emission to extinction polarization ratio P353∕pV derived with Planck data. Successful models are those using prolate astrosilicate grains with an elongation a∕b = 3 and an inclusion of 20% porosity. The spectral dependence of the polarized SED is steeper in our models than in the data. Models perform slightly better when a-C grains are aligned. A small (6%) volume inclusion of a-C in the astrosilicate matrix removes the need for porosity and perfect grain alignment, and improves the fit to the polarized SED. Conclusions. Dust models based on astrosilicates can be reconciled with Planck data by adapting the shape of grains and adding inclusions of porosity or a-C in the astrosilicate matrix.
We report Arecibo 21 cm absorption-emission observations to characterize the physical properties of neutral hydrogen (H i) in the proximity of five giant molecular clouds (GMCs): Taurus, California, ...Rosette, Mon OB1, and NGC 2264. Strong H i absorption was detected toward all 79 background-continuum sources in the ∼60 × 20 square degree region. Gaussian decompositions were performed to estimate temperatures, optical depths, and column densities of the cold and warm neutral medium (CNM and WNM). The properties of individual CNM components are similar to those previously observed along random Galactic sightlines and in the vicinity of molecular clouds, suggesting a universality of cold H i properties. The CNM spin temperature (Ts) histogram peaks at ∼50 K. The turbulent Mach numbers of CNM components vary widely, with a typical value of ∼4, indicating that their motions are supersonic. About 60% of the total H i gas is WNM, and nearly 40% of the WNM lies in thermally unstable regime 500-5000 K. The observed CNM fraction is higher around GMCs than in diffuse regions, and increases with increasing column density ( ) to a maximum of ∼75%. On average, the optically thin approximation ( ) underestimates the total column density by ∼21%, but we find large regional differences in the relationship between and the required correction factor, f = . We examine two different methods (linear fit of f versus log10( ) and uniform Ts) to correct for opacity effects using emission data from the GALFA-H i survey. We prefer the uniform Ts method because the linear relationship does not produce convincing fits for all subregions.
ABSTRACT This paper introduces and describes the data cubes from GHIGLS, deep Green Bank Telescope (GBT) surveys of the 21 cm line emission of H i in 37 targeted fields at intermediate Galactic ...latitude. The GHIGLS fields together cover over 1000 deg2 at spatial resolution. The H i spectra have an effective velocity resolution of about 1.0 km s−1 and cover at least km s−1, extending to km s−1 for most fields. As illustrated with various visualizations of the H i data cubes, GHIGLS highlights that even at intermediate Galactic latitude the interstellar medium is very complex. Spatial structure of the H i is quantified through power spectra of maps of the integrated line emission or column density, . For our featured representative field, centered on the north ecliptic pole, the scaling exponents in power-law representations of the power spectra of maps for low-, intermediate-, and high-velocity gas components (LVC, IVC, and HVC) are , , and , respectively. After Gaussian decomposition of the line profiles, maps were also made corresponding to the narrow-line and broad-line components in the LVC range; for the narrow-line map the exponent is , reflecting more small-scale structure in the cold neutral medium (CNM). There is evidence that filamentary structure in the H i CNM is oriented parallel to the Galactic magnetic field. The power spectrum analysis also offers insight into the various contributions to uncertainty in the data, yielding values close to those obtained using diagnostics developed in our earlier independent analysis. The effect of 21 cm line opacity on the GHIGLS maps is estimated. Comparisons of the GBT data in a few of the GHIGLS fields with data from the EBHIS and GASS surveys explore potential issues in data reduction and calibration and reveal good agreement. The high quality of the GHIGLS data enables a variety of studies in directions of low Galactic column density, as already demonstrated by the Planck Collaboration. Fully reduced GHIGLS H i data cubes and other data products are available at www.cita.utoronto.ca/GHIGLS.
Observations of interstellar dust are often used as a proxy for total gas column density NH. By comparing Planck thermal dust data (Release 1.2) and new dust reddening maps from Pan-STARRS 1 and ...2MASS, with accurate (opacity-corrected) H i column densities and newly published OH data from the Arecibo Millennium survey and 21-SPONGE, we confirm linear correlations between dust optical depth τ353, reddening E(B − V), and the total proton column density NH in the range (1-30) × 1020 cm−2, along sightlines with no molecular gas detections in emission. We derive an NH/E(B − V) ratio of (9.4 1.6) × 1021 cm−2 mag−1 for purely atomic sightlines at b > 5 ° , which is 60% higher than the canonical value of Bohlin et al. We report a ∼40% increase in opacity 353 = τ353/NH, when moving from the low column density (NH < 5 × 1020 cm−2) to the moderate column density (NH > 5 × 1020 cm−2) regime, and suggest that this rise is due to the evolution of dust grains in the atomic interstellar medium. Failure to account for H i opacity can cause an additional apparent rise in 353 of the order of a further ∼20%. We estimate molecular hydrogen column densities N H 2 from our derived linear relations, and hence derive the OH/H2 abundance ratio of XOH ∼ 1 × 10−7 for all molecular sightlines. Our results show no evidence of systematic trends in OH abundance with N H 2 in the range N H 2 ∼ (0.1−10) × 1021 cm−2. This suggests that OH may be used as a reliable proxy for H2 in this range, which includes sightlines with both CO-dark and CO-bright gas.
Abstract
We present a comparison of the presence and properties of dust in two distinct phases of the Milky Way’s interstellar medium: the warm neutral medium (WNM) and the warm ionized medium (WIM). ...Using distant pulsars at high Galactic latitudes and vertical distance (∣
b
∣ > 40°,
D
sin
∣
b
∣
>
2
kpc
) as probes, we measure their dispersion measures and the neutral hydrogen component of the warm neutral medium (WNM
H I
) using H
i
column density. Together with dust intensity along these same sightlines, we separate the respective dust contributions of each ISM phase in order to determine whether the ionized component contributes to the dust signal. We measure the temperature (
T
), spectral index (
β
), and dust opacity (
τ
/
N
H
) in both phases. We find
T
(
WNM
H
I
)
=
20
−
2
+
3
K,
β
(WNM
H I
) = 1.5 ± 0.4, and
τ
353
/
N
H
(WNM
H I
) = (1.0 ± 0.1) × 10
−26
cm
2
. Assuming that the temperature and spectral index are the same in both the WNM
H I
and WIM, and given our simple model that widely separated lines of sight can be fit together, we find evidence that there is a dust signal associated with the ionized gas and
τ
353
/
N
H
(
WIM
)
=
(
0.3
±
0.3
)
×
10
−
26
, which is about 3 times smaller than
τ
353
/
N
H
(WNM
H I
). We are 80% confident that
τ
353
/
N
H
(
WIM
)
is at least 2 times smaller than
τ
353
/
N
H
(WNM
H I
).
Our PACS and SPIRE images of the Aquila Rift and part of the Polaris Flare regions, taken during the science demonstration phase of Herschel discovered fascinating, omnipresent filamentary structures ...that appear to be physically related to compact cores. We briefly describe a new multi-scale, multi-wavelength source extraction method used to detect objects and measure their parameters in our Herschel images. All of the extracted starless cores (541 in Aquila and 302 in Polaris) appear to form in the long and very narrow filaments. With its combination of the far-IR resolution and sensitivity, Herschel directly reveals the filaments in which the dense cores are embedded; the filaments are resolved and have deconvolved widths of ~35” in Aquila and ~59” in Polaris (~9000 AU in both regions). Our first results of observations with Herschel enable us to suggest that in general dense cores may originate in a process of fragmentation of complex networks of long, thin filaments, likely formed as a result of an interplay between gravity, interstellar turbulence, and magnetic fields. To unravel the roles of the processes, one has to obtain additional kinematic and polarization information; these follow-up observations are planned.
Context. Apart from its contribution to cosmology, the WMAP data brings new information on the Galactic interstellar medium. In particular the polarization data provide constraints on the Galactic ...magnetic field and the synchrotron emission, while the intensity data can be used to study the anomalous microwave emission. Aims. The main goals of this study is to use the information from both WMAP intensity and polarization data to do a separation of the Galactic components, with a focus on the synchrotron and anomalous emissions. Methods. Our analysis is made at 23 GHz where the signal-to-noise ratio is the highest and the estimate of the CMB map is less critical. Our estimate of the synchrotron intensity is based on an extrapolation of the Haslam 408 MHz data with a spatially varying spectral index constrained by the WMAP 23 GHz polarization data and a bi-symmetrical spiral model of the galactic magnetic field with a turbulent part following a –5/3 power law spectrum. Results. The 23 GHz polarization data are found to be compatible with a magnetic field with a pitch angle $p=-8.5^\circ$ and an amplitude of the turbulent part of the magnetic field 0.57 times the local value of the field, in agreement with what is found using rotation measures of pulsars and polarized extinction by dust. The synchrotron spectral index between 408 MHz and 23 GHz obtained from polarization data and our model of the magnetic field has a mean value of $\beta_{ s}=-3.00$ with a limited spatial variation with a standard deviation of 0.06. When thermal dust, free-free and synchrotron are removed from the WMAP intensity data, the residual anomalous emission is highly correlated with thermal dust emission with a spectrum in agreement with spinning dust models. Conclusions. Considering a classical model of the large scale Galactic magnetic field, we show that the polarization data of WMAP are in favor of a soft synchrotron intensity highly correlated with the 408 MHz data. Furthermore the combination of the WMAP polarization and intensity data brings strong evidence for the presence of unpolarized spinning dust emission in the 20–60 GHz range. In preparation for the Planck mission this joint analysis of polarization and intensity data opens new perspective on the study of the Galactic interstellar medium and on the component separation exercise.
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