We present a synthesis of the astronomical observations constraining the wavelength-dependent extinction, emission, and polarization from interstellar dust from UV to microwave wavelengths on diffuse ...Galactic sight lines. Representative solid-phase abundances for those sight lines are also derived. Given the sensitive new observations of polarized dust emission provided by the Planck satellite, we place particular emphasis on dust polarimetry, including continuum polarized extinction, polarization in the carbonaceous and silicate spectroscopic features, the wavelength-dependent polarization fraction of the dust emission, and the connection between optical polarized extinction and far-infrared polarized emission. Together, these constitute a set of constraints that should be reproduced by models of dust in the diffuse interstellar medium.
Abstract
We present a new model of interstellar dust in which large grains are a single composite material, “astrodust,” and nanoparticle-sized grains come in distinct varieties including polycyclic ...aromatic hydrocarbons (PAHs). We argue that a single-composition model for grains larger than ∼0.02
μ
m most naturally explains the lack of frequency dependence in the far-infrared (FIR) polarization fraction and the characteristic ratio of optical to FIR polarization. We derive a size distribution and alignment function for 1.4:1 oblate astrodust grains that, with PAHs, reproduce the mean wavelength dependence and polarization of Galactic extinction and emission from the diffuse interstellar medium while respecting constraints on solid-phase abundances. All model data and Python-based interfaces are made publicly available.
We present a new map of interstellar reddening, covering the 39% of the sky with low H i column densities ( N H I < 4 × 10 20 cm−2 or E ( B − V ) 45 mmag) at 16 1 resolution, based on all-sky ...observations of Galactic H i emission by the HI4PI Survey. In this low-column-density regime, we derive a characteristic value of N H I E ( B − V ) = 8.8 × 10 21 cm 2 mag − 1 for gas with v LSR < 90 km s−1 and find no significant reddening associated with gas at higher velocities. We compare our H i-based reddening map with the Schlegel et al. (SFD) reddening map and find them consistent to within a scatter of 5 mmag. Further, the differences between our map and the SFD map are in excellent agreement with the low-resolution ( 4 5 ) corrections to the SFD map derived by Peek and Graves based on observed reddening toward passive galaxies. We therefore argue that our H i-based map provides the most accurate interstellar reddening estimates in the low-column-density regime to date. Our reddening map is made publicly available at doi.org/10.7910/DVN/AFJNWJ.
Polarized Galactic foregrounds are one of the primary sources of systematic error in measurements of the B-mode polarization of the cosmic microwave background (CMB). Experiments are becoming ...increasingly sensitive to complexities in the foreground frequency spectra that are not captured by standard parametric models, potentially affecting our ability to efficiently separate out these components. Employing a suite of dust models encompassing a variety of physical effects, we simulate observations of a future seven-band CMB experiment to assess the impact of these complexities on parametric component separation. We identify configurations of frequency bands that minimize the "model errors" caused by fitting simple parametric models to more complex "true" foreground spectra, which bias the inferred CMB signal. We find that: (a) fits employing a simple two-parameter modified blackbody (MBB) dust model tend to produce significant bias in the recovered polarized CMB signal in the presence of physically realistic dust foregrounds; (b) generalized MBB models with three additional parameters reduce this bias in most cases, but non-negligible biases can remain, and can be hard to detect; (c) line-of-sight effects, which give rise to frequency decorrelation, and the presence of iron grains are the most problematic complexities in the dust emission for recovering the true CMB signal. More sophisticated simulations will be needed to demonstrate that future CMB experiments can successfully mitigate these more physically realistic dust foregrounds.
Abstract
We have developed a new method for using the observed starlight polarization and polarized submillimeter emission to constrain the shapes and porosities of interstellar grains. We first ...present the modified picket-fence approximation and verify that it is sufficiently accurate for modeling starlight polarization. We then introduce the observed starlight polarization integral Π
obs
as a measure of the overall strength of the observed polarization of starlight, and the starlight polarization efficiency integral Φ to characterize the effectiveness of different grain types for producing polarization of starlight. The ratio Π
obs
/Φ determines the mass-weighted alignment 〈
f
align
〉 of the grains. Approximating the aligned grains in the ISM as spheroids, we use Π
obs
/Φ to show that the observed starlight polarization constrains the grains to have a minimum degree of asphericity. For porosity
=
0
, the minimum axial ratio is ∼1.4 for oblate spheroids, or ∼1.8 for prolate spheroids. If the grains are porous, more extreme axial ratios are required. The same grains that produce the starlight polarization are able to provide the observed polarized emission at submillimeter wavelengths but with further limits on shape and porosity. Porosities
≳
0.75
are ruled out. If interstellar grains can be approximated by “astrodust” spheroids, we predict the ratio of 10
μ
m polarization to starlight polarization
p
V
:
p
(10
μ
m)/
p
V
= 0.219 ± 0.029. For Cyg OB2-12, we predict
p
(10
μ
m) = (2.1 ± 0.3)%, which should be observable.
Recent analyses of 21 cm neutral hydrogen (H i) emission have demonstrated that H i gas is organized into linear filamentary structures that are preferentially aligned with the local magnetic field, ...and that the coherence of these structures in velocity space traces line-of-sight magnetic field tangling. On this basis, we introduce a paradigm for modeling the properties of magnetized, dusty regions of the interstellar medium (ISM), using the orientation of H i structure at different velocities to map "magnetically coherent" regions of space. We construct three-dimensional (position-position-velocity) Stokes parameter maps using H i4PI full-sky spectroscopic H i data. We compare these maps, integrated over the velocity dimension, to Planck maps of the polarized dust emission at 353 GHz. Without any free parameters governing the relation between H i intensity and dust emission, we find that our Q and U maps are highly correlated (r > 0.75) with the 353 GHz Q and U maps of polarized dust emission observed by Planck and reproduce many of its large-scale features. The E/B ratio of the dust emission maps agrees well with the H i-derived maps at large angular scales ( 120), supporting the interpretation that this asymmetry arises from the coupling of linear density structures to the Galactic magnetic field. We demonstrate that our 3D Stokes parameter maps constrain the 3D structure of the Galactic ISM and the orientation of the interstellar magnetic field.
The sight line toward the luminous blue hypergiant Cyg OB2-12 is widely used to study interstellar dust on account of its large extinction (AV 10 mag) and the fact that this extinction appears to be ...dominated by dust typical of the diffuse interstellar medium. We present a new analysis of archival Infrared Space Observatory Short Wavelength Spectrometer and Spitzer IRS observations of Cyg OB2-12 using a model of the emission from the star and its stellar wind to determine the total extinction Aλ from 2.4 to 37 m. In addition to the prominent 9.7 and 18 m silicate features, we robustly detect absorption features associated with polycyclic aromatic hydrocarbons, including the first identification of the 7.7 m feature in absorption. The 3.3 m aromatic feature is found to be much broader in absorption than is typically seen in emission. The 3.4 and 6.85 m aliphatic hydrocarbon features are observed with relative strengths that are consistent with observations of these features on sight lines toward the Galactic center. We identify and characterize more than 60 spectral lines in this wavelength range, which may be useful in constraining models of the star and its stellar wind. Based on this analysis, we present an extinction curve that extrapolates smoothly to determinations of the mean Galactic extinction curve at shorter wavelengths and to dust opacities inferred from emission at longer wavelengths, providing a new constraint on models of interstellar dust in the mid-infrared.
Line intensity mapping (LIM) is a promising approach to study star formation and the interstellar medium (ISM) in galaxies by measuring the aggregate line emission from the entire galaxy population. ...In this work, we develop a simple yet physically motivated framework for modeling the line emission as would be observed in LIM experiments. It is done by building on analytic models of the cosmic infrared background that connect total infrared luminosity of galaxies to their host dark matter halos. We present models of the 21 cm, CO (1−0), 158 m, and 122 and 205 m lines consistent with current observational constraints. With four case studies of various combinations of these lines that probe different ISM phases, we demonstrate the potential for reliably extracting physical properties of the ISM, and the evolution of these properties with cosmic time, from auto- and cross-correlation analysis of these lines as measured by future LIM experiments.
ABSTRACT
While Galactic dust emission is often accounted for in cosmic microwave background (CMB) analyses by fitting a two-parameter modified blackbody (MBB) model in each pixel, typically a number ...of such clouds are found along each line of sight and within each angular pixel, resulting in a superposition of their spectra. We study the effects of this superposition on pixel-based foreground fitting strategies by modeling the spectral energy distribution (SED) in each pixel as the integral of individual MBB spectra over various physically motivated statistical distributions of dust cloud properties. We show that fitting these SEDs with the two-parameter MBB model generally results in unbiased estimates of the CMB Stokes Q and U amplitudes per pixel, unless there are significant changes in both the dust SED and polarization angle along the line of sight, in which case significant (>10σ) biases are observed in an illustrative model. We find that the best-fitting values of the dust temperature, Td, and spectral index, βd, are significantly biased from the mean/median of the corresponding statistical distributions when the distributions are broad, suggesting that MBB model fits can give an unrepresentative picture of the physical properties of dust at microwave wavelengths if not interpreted carefully. Using Fisher matrix analysis, we determine the experimental sensitivity required to recover the parameters of the Td and βd distributions by fitting a probabilistic MBB model, finding that only the parameters of broad distributions can be measured by SED fitting on a single line of sight.
Abstract
The dielectric function of interstellar dust material is modeled using observations of extinction and polarization in the infrared, together with estimates for the mass of interstellar dust. ...The “astrodust” material is assumed to be a mix of amorphous silicates and other materials, including hydrocarbons producing an absorption feature at 3.4
μ
m. The detailed shape of the 10
μ
m polarization profile depends on the assumed porosity and grain shape, but the 10
μ
m spectropolarimetric data are not yet good enough to clearly favor one shape over another, nor to constrain the porosity. The expected 3.4
μ
m feature polarization is consistent with existing upper limits, provided the 3.4
μ
m absorption is preferentially located in grain surface layers; a separate population of non-aligned carbonaceous grains is not required. We predict the 3.4
μ
m polarization feature to be (Δ
p
)
3.4
μ
m
/
p
(10
μ
m) ≈ 0.016, just below current upper limits. Polarization by the same grains at submillimeter wavelengths is also calculated.
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