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Supernova ejecta and stellar winds are believed to produce interstellar dust grains with relatively large sizes. Smaller grains can be produced via the shattering of large grains that have been stochastically accelerated. To understand this stochastic acceleration, we have implemented novel magnetohydrodynamic(MHD)-particle-in-cell(PIC) methods into the astrophysical fluid code RAMSES. We treat dust grains as a set of massive ``superparticles'' that experience aerodynamic drag and Lorentz force. We subject our code to a range of numerical tests designed to validate our method in different physical conditions, as well as to illustrate possible mechanisms by which grains can be accelerated. As a final test as well as a foundation for future work, we present the results of decaying dusty MHD turbulence simulations with grain parameters chosen to resemble 1-2 $\mu$m grains in typical cold neutral medium conditions. We find that in these conditions, these grains can be effectively accelerated to well beyond their shattering velocities. This is true for both electrically charged and neutral grains. While the peak of the gas-grain relative drift velocity distribution is higher for neutral grains, the drift velocity distribution of charged grains exhibits an extended exponential tail out to much greater velocities. Even so, the shapes of the distributions are such that the extra gas-grain coupling provided by the Lorentz force offers grains relative protection from shattering. We also discuss the connection between our simulations and the relatively pristine ~$\mu$m sized presolar grains that do not appear to have undergone significant wear in their lifetimes., Comment: 23 pages, 13 figures, Accepted for publication in MNRAS

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 $\sim$0.02 $\mu$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., Comment: 34 pages, 19 figures. Data available at https://dataverse.harvard.edu/dataverse/astrodust

We present a new method for using the observed starlight polarization and polarized submm emission to constrain the shapes and porosities of interstellar grains. We present the modified picket fence approximation (MPFA), and verify that it is sufficiently accurate for modeling starlight polarization. We introduce the starlight polarization integral $\Pi_{\rm obs}$ as a measure of overall strength of the observed polarization of starlight, and the starlight polarization efficiency integral $\Phi$ to characterize the effectiveness of different grain types for producing polarization of starlight. The starlight polarization integral $\Pi_{\rm obs}$ determines the mass-weighted alignment $\langle f_{\rm align}\rangle$ of the grains. Approximating the aligned grains in the interstellar medium as spheroids, we use $\Pi_{\rm obs}/\Phi$ to show that the observed starlight polarization constrains the grains to have a minimum degree of asphericity. For porosity ${\cal P}=0$, the minimum axial ratio is $\sim$1.4 for oblate spheroids, or $\sim$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 submm wavelengths, but with further limits on shape and porosity. Porosities ${\cal P}>0.75$ are ruled out. If interstellar grains can be approximated by astrodust spheroids, we predict the ratio of 10$\mu{\rm m}$ polarization to starlight polarization $p_V$: $p(10\mu{\rm m})/p_V=0.222\pm0.026$. For Cyg OB2-12 we predict $p(10\mu{\rm m})=(2.1\pm0.3)\%$, which should be observable., Comment: submitted to ApJ

In the cold neutral medium, high out-of-equilibrium temperatures are created by intermittent dissipation processes, including shocks, viscous heating, and ambipolar diffusion. The high-temperature excursions are thought to explain the enhanced abundance of CH$^{+}$ observed along diffuse molecular sight-lines. Intermittent high temperatures should also have an impact on H$_2$ line luminosities. We carry out simulations of MHD turbulence in molecular clouds including heating and cooling, and post-process them to study H$_2$ line emission and hot-gas chemistry, particularly the formation of CH$^+$. We explore multiple magnetic field strengths and equations of state. We use a new H$_2$ cooling function for $n_{\rm H} \leq 10^5\,{\rm cm}^{-3}$, $T\leq 5000\,{\rm K}$, and variable H$_2$ fraction. We make two important simplifying assumptions: (i) the ${\rm H}_2/{\rm H}$ fraction is fixed everywhere, and (ii) we exclude from our analysis regions where the ion-neutral drift velocity is calculated to be greater than 5 km/s. Our models produce H$_2$ emission lines in accord with many observations, although extra excitation mechanisms are required in some clouds. For realistic r.m.s. magnetic field strengths ($\approx 10$ $\mu$G) and velocity dispersions, we reproduce observed CH$^+$ abundances. These findings contrast with those of Valdivia et al. (2017). Comparison of predicted dust polarization with observations by {\it Planck} suggests that the mean field $\gtrsim 5 \mu$G, so that the turbulence is sub-Alfv\'enic. We recommend future work treating ions and neutrals as separate fluids to more accurately capture the effects of ambipolar diffusion on CH$^+$ abundance., Comment: 20 pages, 18 figures, accepted for publication to MNRAS

The sightline toward the luminous blue hypergiant Cyg OB2-12 is widely used in studying interstellar dust on account of its large extinction ($A_V \simeq 10$ 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 ISO-SWS 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 $A_\lambda$ from 2.4--37 $\mu$m. In addition to the prominent 9.7 and 18 $\mu$m silicate features, we robustly detect absorption features associated with polycyclic aromatic hydrocarbons (PAHs), including the first identification of the 7.7 $\mu$m feature in absorption. The 3.3 $\mu$m aromatic feature is found to be much broader in absorption than is typically seen in emission. The 3.4 and 6.85 $\mu$m aliphatic hydrocarbon features are observed with relative strengths consistent with observation of these features on sightlines toward the Galactic Center. We identify and characterize more than sixty 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 $A_\lambda/A_{2.2 \mu m}$ 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., Comment: Submitted to ApJ, comments welcome

Dust and starlight are modeled for the KINGFISH project galaxies. With data from 3.6 micron to 500 micron, models are strongly constrained. For each pixel in each galaxy we estimate (1) dust surface density; (2) q_PAH, the dust mass fraction in PAHs; (3) distribution of starlight intensities heating the dust; (4) luminosity emitted by the dust; and (5) dust luminosity from regions with high starlight intensity. The models successfully reproduce both global and resolved spectral energy distributions. We provide well-resolved maps for the dust properties. As in previous studies, we find q_PAH to be an increasing function of metallicity, above a threshold Z/Z_sol approx 0.15. Dust masses are obtained by summing the dust mass over the map pixels; these "resolved" dust masses are consistent with the masses inferred from model fits to the global photometry. The global dust-to-gas ratios obtained from this study correlate with galaxy metallicities. Systems with Z/Z_sol > 0.5 have most of their refractory elements locked up in dust, whereas when Z/Z_sol < 0.3 most of these elements tend to remain in the gas phase. Within galaxies, we find that q_PAH is suppressed in regions with unusually warm dust with nu L_nu(70 um) > 0.4L_dust. With knowledge of one long-wavelength flux density ratio (e.g., f_{160}/f_{500}), the minimum starlight intensity heating the dust (U_min) can be estimated to within ~50%. For the adopted dust model, dust masses can be estimated to within ~0.07 dex accuracy using the 500 micron luminosity nu L_nu(500) alone. There are additional systematic errors arising from the choice of dust model, but these are hard to estimate. These calibrated prescriptions may be useful for studies of high-redshift galaxies., ApJ, accepted

Although interstellar grains are known to be aspherical, their actual shapes remain poorly constrained. We assess whether three continuous distributions of ellipsoidal shapes from the literature are suitable for describing the shapes of interstellar grains. Randomly-selected shapes from each distribution are shown as illustrations. The often-used Bohren-Huffman CDE includes a very large fraction of extreme shapes: fully 10% of random draws have axial ratio $a_3/a_1 > 19.7$, and 5% have $a_3/a_1 > 33$. The CDE2 distribution includes a much smaller fraction of extreme shapes, and appears to be more realistic. For each of the three CDEs considered, we derive shape-averaged cross sections for extinction and polarization in the Rayleigh limit. Finally, we describe a method for "synthesizing" a dielectric function for an assumed shape or shape distribution if the actual absorption cross sections per grain volume in the Rayleigh limit are known from observations. This synthetic dielectric function predicts the wavelength dependence of polarization, which can then be compared to observations to constrain the grain shape., 23 pages, 8 figures. ApJ, accepted

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$\mu$m. The detailed shape of the 10$\mu$m polarization profile depends on the assumed porosity and grain shape, but the 10$\mu$m spectropolarimetric data are not yet good enough to clearly favor one shape over another, nor to constrain the porosity. The expected 3.4$\mu$m feature polarization is consistent with existing upper limits, provided the 3.4$\mu$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$\mu$m polarization feature to be $(\Delta p)_{3.4\mu{\rm m}}/p(10\mu{\rm m})\approx 0.016$, just below current upper limits. Polarization by the same grains at submm wavelengths is also calculated., Comment: 32 pages, 10 figures. Submitted to Ap.J

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 sightlines. Representative solid phase abundances for those sightlines 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., Comment: 39 pages, 14 figures. Updated to accepted version

We present new Atacama Large Millimeter Array (ALMA) observations towards NGC 4725 B, a discrete, compact, optically-faint region within the star-forming disk of the nearby galaxy NGC 4725 that exhibits strong anomalous microwave emission (AME). These new ALMA data include continuum observations centered at 92, 133, 203, and 221 GHz accompanied by spectral observations of the $^{12}$CO ($J=2\rightarrow1$) line. NGC 4725 B is detected in the continuum at all frequencies, although the detection at 203 GHz is marginal. While molecular gas is not detected at the exact location of NGC 4725 B, there is molecular gas in the immediate vicinity (i.e., $\lesssim 100$ pc) along with associated diffuse 8 $\mu$m emission. When combined with existing Very Large Array continuum data at 1.5, 3, 5.5, 9, 14, 22, 33, and 44 GHz, the spectrum is best fit by a combination of AME, synchrotron, and free-free emission that is free-free absorbed below $\sim6$ GHz. Given the strength of the AME, there is surprisingly no indication of millimeter dust emission associated with NGC 4725 B on $\lesssim$6\arcsec~spatial scales at the sensitivity of the ALMA interferometric data. Based on the properties of the nearest molecular gas complex and the inferred star formation rate, NGC 4725 B is consistent with being an extremely young ($\sim 3-5$ Myr) massive ($\lesssim 10^{5} M_{\odot}$) cluster that is undergoing active cluster feedback. However, the lack of millimeter thermal dust emission is difficult to reconcile with a spinning dust origin of the 30 GHz emission. On the other hand, modeling NGC 4725 B as a new class of background radio galaxy is also unsatisfactory., Comment: 15 pages, 4 figures, accepted for publication in ApJ Letters

Some authors have proposed that electron energy distributions in H II regions and planetary nebulae may be significantly nonthermal, and kappa-distributions have been suggested as being appropriate. Here it is demonstrated that the electron energy distribution function is extremely close to a Maxwellian up to electron kinetic energies ~13 eV in HII regions, and up to ~16eV in planetary nebulae: kappa-distributions are inappropriate. The small departures from a Maxwellian have negligible effects on line ratios. When observed line ratios in H II regions deviate from models with a single electron temperature, it must arise from spatial variations in electron temperature, rather than local deviations from a Maxwellian., Comment: published in ApJ

Spinning dust grains in front of the bright Galactic synchrotron background can produce a weak absorption signal that could affect measurements of high redshift 21 cm absorption. At frequencies near 80 MHz where the EDGES experiment has reported 21\,cm absorption at $z \approx 17$, absorption could be produced by interstellar nanoparticles with radii $a \approx 50\AA$ in the cold interstellar medium at temperature $T \approx 50$ K. Atmospheric aerosols could contribute additional absorption. The strength of the absorption depends on the abundance of such grains and on their dipole moments, which are uncertain. The breadth of the absorption spectrum of spinning dust limits its possible impact on measurement of a relatively narrow 21 cm absorption feature., Comment: version published in ApJL

We present [C ii] 158 μm measurements from over 15,000 resolved regions within 54 nearby galaxies of the Kingfish program to investigate the so-called [C ii] "line-cooling deficit" long known to occur in galaxies with different luminosities. The [C ii]/TIR ratio ranges from above 1% to below 0.1% in the sample, with a mean value of 0.48 ± 0.21%. We find that the surface density of 24 μm emission dominates this trend, with [C ii]/TIR dropping as vIv (24 μm)increases. Deviations from this overall decline are correlated with changes in the gas-phase metal abundance, with higher metallicity associated with deeper deficits at a fixed surface brightness. We supplement the local sample with resolved [C ii] measurements from nearby luminous infrared galaxies and high-redshift sources from z = 1.8-6.4, and find that star formation rate density drives a continuous trend of deepening [C ii] deficit across six orders of magnitude in ΣSFR. The tightness of this correlation suggests that an approximate can be estimated directly from global measurements of [C ii]/TIR, and a relation is provided to do so. Several low-luminosity active galactic nucleus (AGN) hosts in the sample show additional and significant central suppression of [C ii]/TIR, but these deficit enhancements occur not in those AGNs with the highest X-ray luminosities, but instead those with the highest central starlight intensities. Taken together, these results demonstrate that the [C ii] line-cooling line deficit in galaxies likely arises from local physical phenomena in interstellar gas.

We present maps of the dust properties in the Small and Large Magellanic Clouds (SMC, LMC) from fitting Spitzer and Herschel observations with the \citet{DL07} dust model. We derive the abundance of the small carbonaceous grain (or polycyclic aromatic hydrocarbon; PAH) component. The global PAH fraction (q_pah, the fraction of the dust mass in the form of PAHs) is smaller in the SMC (1.0$^{+0.3}_{-0.3}$%) than in the LMC (3.3$^{+1.4}_{-1.3}$%). We measure the PAH fraction in different gas phases (H II regions, ionized gas outside of H II regions, molecular gas, and diffuse neutral gas). H II regions appear as distinctive holes in the spatial distribution of the PAH fraction. In both galaxies, the PAH fraction in the diffuse neutral medium is higher than in the ionized gas, but similar to the molecular gas. Even at equal radiation field intensity, the PAH fraction is lower in the ionized gas than in the diffuse neutral gas. We investigate the PAH life-cycle as a function of metallicity between the two galaxies. The PAH fraction in the diffuse neutral medium of the LMC is similar to that of the Milky Way ($\sim4.6$%), while it is significantly lower in the SMC. Plausible explanations for the higher PAH fraction in the diffuse neutral medium of the LMC compared to the SMC include: a more effective PAH production by fragmentation of large grains at higher metallicity, and/or the growth of PAHs in molecular gas., Comment: Accepted for publication in ApJ

The attenuation of starlight by dust in galactic environments is investigated through models of radiative transfer in a spherical, clumpy ISM. Extinction properties for MW, LMC, and SMC dust types are considered. It is illustrated that the attenuation curves are primarily determined by the wavelength dependence of absorption rather than by the underlying extinction (absorption+scattering) curve. Attenuation curves consistent with the "Calzetti attenuation curve" are found by assuming the silicate-carbonaceous dust model for the MW, but with the 2175A absorption bump suppressed or absent. The discrepancy between our results and previous work that claimed the SMC-type dust to be the most likely origin of the Calzetti curve is ascribed to the difference in adopted albedos; this study uses the theoretically calculated albedos whereas the previous ones adopted empirically derived albedos from observations of reflection nebulae. It is also found that the model attenuation curves calculated with the MW dust are well represented by a modified Calzetti curve with a varying slope and UV bump strength. The strong correlation between the slope and UV bump strength, with steeper curves having stronger bumps, as found in star-forming galaxies at 0.5, Comment: 28 pages, 30 figures, submited to ApJS

The quantization of energy levels in very nanoparticles suppresses dissipative processes that convert grain rotational kinetic energy into heat. For grains small enough to have GHz rotation rates, the suppression of dissipation can be extreme. As a result, alignment of such grains is suppressed. This applies both to alignment of the grain body with its angular momentum J, and to alignment of J with the local magnetic field B_0. If the anomalous microwave emission is rotational emission from spinning grains, it will be negligibly polarized at GHz frequencies, with P < 10^{-6} at frequencies above 10 GHz., submitted to ApJ

In light of recent observational results indicating an apparent lack of correlation between the Anomalous Microwave Emission (AME) and mid-infrared emission from polycyclic aromatic hydrocarbons (PAHs), we assess whether rotational emission from spinning silicate and/or iron nanoparticles could account for the observed AME without violating observational constraints on interstellar abundances, ultraviolet extinction, and infrared emission. By modifying the SpDust code to compute the rotational emission from these grains, we find that nanosilicate grains could account for the entirety of the observed AME, whereas iron grains could be responsible for only a fraction, even for extreme assumptions on the amount of interstellar iron concentrated in ultrasmall iron nanoparticles. Given the added complexity of contributions from multiple grain populations to the total spinning dust emission, as well as existing uncertainties due to the poorly-constrained grain size, charge, and dipole moment distributions, we discuss generic, carrier-independent predictions of spinning dust theory and observational tests that could help identify the AME carrier(s)., Comment: Submitted to ApJ, comments welcome

Laboratory measurements are used to constrain the dielectric tensor for graphite, from microwave to X-ray frequencies. The dielectric tensor is strongly anisotropic even at X-ray energies. The discrete dipole approximation is employed for accurate calculations of absorption and scattering by single-crystal graphite spheres and spheroids. For randomly-oriented single-crystal grains, the so-called 1/3 - 2/3 approximation for calculating absorption and scattering cross sections is exact in the limit a/lambda -> 0, provides better than ~10% accuracy in the optical and UV even when a/lambda is not small, but becomes increasingly inaccurate at infrared wavelengths, with errors as large as ~40% at lambda = 10 micron. For turbostratic graphite grains, the Bruggeman and Maxwell Garnett treatments yield similar cross sections in the optical and ultraviolet, but diverge in the infrared, with predicted cross sections differing by over an order of magnitude in the far-infrared. It is argued that the Maxwell Garnett estimate is likely to be more realistic, and is recommended. The out-of-plane lattice resonance of graphite near 11.5 micron may be observable in absorption with the MIRI spectrograph on JWST. Aligned graphite grains, if present in the ISM, could produce polarized X-ray absorption and polarized X-ray scattering near the carbon K edge., To appear in ApJ

To compute the SFR of galaxies from the rest-frame UV it is essential to take into account the obscuration by dust. To do so, one of the most popular methods consists in combining the UV with the emission from the dust itself in the IR. Yet, different studies have derived different estimators, showing that no such hybrid estimator is truly universal. In this paper we aim at understanding and quantifying what physical processes drive the variations between different hybrid estimators. Doing so, we aim at deriving new universal UV+IR hybrid estimators to correct the UV for dust attenuation, taking into account the intrinsic physical properties of galaxies. We use the CIGALE code to model the spatially-resolved FUV to FIR SED of eight nearby star-forming galaxies drawn from the KINGFISH sample. This allows us to determine their local physical properties, and in particular their UV attenuation, average SFR, average specific SFR (sSFR), and their stellar mass. We then examine how hybrid estimators depend on said properties. We find that hybrid UV+IR estimators strongly depend on the stellar mass surface density (in particular at 70 and 100 micron) and on the sSFR (in particular at 24 micron and the TIR). Consequently, the IR scaling coefficients for UV obscuration can vary by almost an order of magnitude. This result contrasts with other groups who found relatively constant coefficients with small deviations. We exploit these variations to construct a new class of hybrid estimators based on observed UV to near-IR colours and near-IR luminosity densities per unit area. We find that they can reliably be extended to entire galaxies. The new estimators provide better estimates of attenuation-corrected UV emission than classical hybrid estimators. Naturally taking into account the variable impact of dust heated by old stellar populations, they constitute a step towards universal estimators., Comment: 25 pages, 12 figures, 8 tables. Accepted for publication in A&A

We present the results of modeling dust SEDs across the SMC with the aim of mapping the distribution of PAHs in a low-metallicity environment. Using Spitzer Survey of the SMC (S3MC) photometry from 3.6-160 um over the main star-forming regions of the Wing and Bar along with spectral mapping from 5-38 um from the Spitzer Spectroscopic Survey of the SMC (S4MC) in selected regions, we model the dust SED and emission spectrum to determine the fraction of dust in PAHs across the SMC. We use the regions of overlapping photometry and spectroscopy to test the reliability of the PAH fraction as determined from SED fits alone. The PAH fraction in the SMC is low compared to the Milky Way and variable--with relatively high fractions (q_PAH~1-2%) in molecular clouds and low fractions in the diffuse ISM (=0.6%). We use the map of PAH fraction across the SMC to test a number of ideas regarding the production, destruction and processing of PAHs in the ISM. We find weak or no correlation between the PAH fraction and the distribution of carbon AGB stars, the location of supergiant H I shells and young SN remnants, or the turbulent Mach number. We find that the PAH fraction is correlated with CO intensity, peaks in the dust surface density and the molecular gas surface density as determined from 160 um emission. The PAH fraction is high in regions of active star-formation, as predicted by its correlation with molecular gas, but is suppressed in H II regions. Because the PAH fraction in the diffuse ISM is generally very low--in accordance with previous work on modeling the SED of the SMC--and the PAH fraction is relatively high in molecular regions, we suggest that PAHs are destroyed in the diffuse ISM of the SMC and/or PAHs are forming in molecular clouds. We discuss the implications of these observations for our understanding of the PAH life cycle, particularly in low-metallicity galaxies., Comment: accepted for publication in ApJ, abstract abridged

We employ an all-sky map of the anomalous microwave emission (AME) produced by component separation of the microwave sky to study correlations between the AME and Galactic dust properties. We find that while the AME is highly correlated with all tracers of dust emission, the best predictor of the AME strength is the dust radiance. Fluctuations in the AME intensity per dust radiance are uncorrelated with fluctuations in the emission from polycyclic aromatic hydrocarbons (PAHs), casting doubt on the association between AME and PAHs. The PAH abundance is strongly correlated with the dust optical depth and dust radiance, consistent with PAH destruction in low density regions. We find that the AME intensity increases with increasing radiation field strength, at variance with predictions from the spinning dust hypothesis. Finally, the temperature-dependence of the AME per dust radiance disfavors the interpretation of the AME as thermal emission. A reconsideration of other AME carriers, such as ultrasmall silicates, and other emission mechanisms, such as magnetic dipole emission, is warranted., Comment: Submitted to ApJ, comments welcome

We investigate the far infrared spectrum of NGC 1266, a S0 galaxy that contains a massive reservoir of highly excited molecular gas. Using the SPIRE-FTS, we detect the $^{12}$CO ladder up to J=(13-12), [C I] and [N II] lines, and also strong water lines more characteristic of UltraLuminous IR Galaxies (ULIRGs). The 12CO line emission is modeled with a combination of a low-velocity C-shock and a PDR. Shocks are required to produce the H2O and most of the high-J 12CO emission. Despite having an infrared luminosity thirty times less than a typical ULIRG, the spectral characteristics and physical conditions of the ISM of NGC 1266 closely resemble those of ULIRGs, which often harbor strong shocks and large-scale outflows., Accepted for publication in ApJL 11/15/2013

Interstellar shock waves can erode and destroy grains present in the shocked gas, primarily as the result of sputtering and grain-grain collisions. Uncertainties in current estimates of sputtering yields are reviewed. Results are presented for the simple case of sputtering of fast grains being stopped in cold gas. An upper limit is derived for sputtering of refractory grains in C-type MHD shocks: shock speeds $v_s \gtrsim 50 \kms$ are required for return of more than 30\% of the silicate to the gas phase. Sputtering can also be important for removing molecular ice mantles from grains in two-fluid MHD shock waves in molecular gas. Recent estimates of refractory grain lifetimes against destruction in shock waves are summarized, and the implications of these short lifetimes are discussed.

The unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 micrometer, commonly attributed to polycyclic aromatic hydrocarbon (PAH) molecules, have been recently ascribed to coal- or kerogen-like organic nanoparticles with a mixed aromatic-aliphatic structure. However, we show in this Letter that this hypothesis is inconsistent with observations. We estimate the aliphatic fraction of the UIE carriers based on the observed intensities of the 3.4 and 6.85 micrometer emission features by attributing them exclusively to aliphatic C-H stretch and aliphatic C-H deformation vibrational modes, respectively. We derive the fraction of carbon atoms in aliphatic form to be, 10 pages, 2 figures, accepted for publication in ApJ Letters

Stellar density and bar strength should affect the temperatures of the cool (T ~ 20-30 K) dust component in the inner regions of galaxies, which implies that the ratio of temperatures in the circumnuclear regions to the disk should depend on Hubble type. We investigate the differences between cool dust temperatures in the central 3 kpc and disk of 13 nearby galaxies by fitting models to measurements between 70 and 500 microns. We attempt to quantify temperature trends in nearby disk galaxies, with archival data from Spitzer/MIPS and new observations with Herschel/SPIRE, which were acquired during the first phases of the Herschel observations for the KINGFISH (key insights in nearby galaxies: a far-infrared survey with Herschel) sample. We fit single-temperature modified blackbodies to far-infrared and submillimeter measurements of the central and disk regions of galaxies to determine the temperature of the component(s) emitting at those wavelengths. We present the ratio of central-region-to-disk-temperatures of the cool dust component of 13 nearby galaxies as a function of morphological type. We find a significant temperature gradient in the cool dust component in all galaxies, with a mean center-to-disk temperature ratio of 1.15 +/- 0.03. The cool dust temperatures in the central ~3 kpc of nearby galaxies are 23(+/-3)% hotter for morphological types earlier than Sc, and only 9(+/-3)% hotter for later types. The temperature ratio is also correlated with bar strength, with only strongly barred galaxies having a ratio over 1.2. The strong radiation field in the high stellar density of a galactic bulge tends to heat the cool dust component to higher temperatures, at least in early-type spirals with relatively large bulges, especially when paired with a strong bar., Comment: Accepted for publication on the A&A Herschel Special Issue

Using photometry of NGC 1097 from the Herschel PACS (Photodetector Array Camera and Spectrometer) instrument, we study the resolved properties of thermal dust continuum emission from a circumnuclear starburst ring with a radius ~ 900 pc. These observations are the first to resolve the structure of a circumnuclear ring at wavelengths that probe the peak (i.e. lambda ~ 100 micron) of the dust spectral energy distribution. The ring dominates the far-infrared (far-IR) emission from the galaxy - the high angular resolution of PACS allows us to isolate the ring's contribution and we find it is responsible for 75, 60 and 55% of the total flux of NGC 1097 at 70, 100 and 160 micron, respectively. We compare the far-IR structure of the ring to what is seen at other wavelengths and identify a sequence of far-IR bright knots that correspond to those seen in radio and mid-IR images. The mid- and far-IR band ratios in the ring vary by less than +/- 20% azimuthally, indicating modest variation in the radiation field heating the dust on ~ 600 pc scales. We explore various explanations for the azimuthal uniformity in the far-IR colors of the ring including a lack of well-defined age gradients in the young stellar cluster population, a dominant contribution to the far-IR emission from dust heated by older (> 10 Myr) stars and/or a quick smoothing of local enhancements in dust temperature due to the short orbital period of the ring. Finally, we improve previous limits on the far-IR flux from the inner ~ 600 pc of NGC 1097 by an order of magnitude, providing a better estimate of the total bolometric emission arising from the active galactic nucleus and its associated central starburst., Accepted for publication in the A&A Herschel Special Edition

We present a detailed analysis of the radial distribution of dust properties in the SINGS sample, performed on a set of UV, IR and HI surface brightness profiles, combined with published molecular gas profiles and metallicity gradients. The internal extinction, derived from the TIR-to-FUV luminosity ratio, decreases with radius, and is larger in Sb-Sbc galaxies. The TIR-to-FUV ratio correlates with the UV spectral slope beta, following a sequence shifted to redder UV colors with respect to that of starbursts. The star formation history (SFH) is identified as the main driver of this departure. We have also derived radial profiles of the total dust mass surface density, the fraction of the dust mass contributed by PAHs, the fraction of the dust mass heated by very intense starlight and the intensity of the radiation field heating the grains. The dust profiles are exponential, their radial scale-length being constant from Sb to Sd galaxies (only ~10% larger than the stellar scale-length). Many S0/a-Sab galaxies have central depressions in their dust radial distributions. The PAH abundance increases with metallicity for 12+\log(O/H), 73 pages in preprint format, 21 figures, published in ApJ. The definitive publisher authenticated version is available online at http://dx.doi.org/10.1088/0004-637X/701/2/1965

We study the polarized infrared emission by Polycyclic Aromatic Hydrocarbons (PAHs), when anisotropically illuminated by UV photons. PAH molecules are modeled as planar disks with in-plane and out-of-plane vibrational dipoles. As first pointed out by Leger (1988), infrared emission features resulting from in-plane and out-of-plane modes should have orthogonal polarization directions. We show analytically how the degree of polarization depends on the viewing geometry and the molecule's internal alignment between principal axis of inertia and angular momentum, which gets worse after photon absorption. Longer wavelength features, emitted after better internal alignment is recovered, should be more strongly polarized. The degree of polarization for uni-directional illumination (e.g., by a star) is larger than for diffuse illumination (e.g., by a disk galaxy), all else being equal. For PAHs in the Cold Neutral Medium, the predicted polarization is probably too small to distinguish from the contribution of linear dichroism by aligned foreground dust. The level of polarization predicted for PAH emission from the Orion Bar is only ~0.06% at 3.3 microns; Sellgren et al. (1988) report a much larger value, 0.86+-0.28%, which suggests that the smallest PAHs may have moderately suprathermal rotation rates. Future observations of (or upper limits on) the degree of polarization for the Orion Bar or for dust above edge-on galaxies (e.g., NGC 891 or M82) may constrain the internal alignment of emitting PAHs, thus providing clues to their rotational dynamics., Comment: 9 pages, 4 figures, 1 table, submitted to ApJ

The Molecular Hydrogen Explorer, H2EX, was proposed in response to the ESA 2015 - 2025 Cosmic Vision Call as a medium class space mission with NASA and CSA participations. The mission, conceived to understand the formation of galaxies, stars and planets from molecular hydrogen, is designed to observe the first rotational lines of the H2 molecule (28.2, 17.0, 12.3 and 9.7 micron) over a wide field, and at high spectral resolution. H2EX can provide an inventory of warm (> 100 K) molecular gas in a broad variety of objects, including nearby young star clusters, galactic molecular clouds, active galactic nuclei, local and distant galaxies. The rich array of molecular, atomic and ionic lines, as well as solid state features available in the 8 to 29 micron spectral range brings additional science dimensions to H2EX. We present the optical and mechanical design of the H2EX payload based on an innovative Imaging Fourier Transform Spectrometer (IFTS) fed by a 1.2m telescope. The 20'x20' field of view is imaged on two 1024x1024 Si:As detectors. The maximum resolution of 0.032 cm^-1 (FWHM) means a velocity resolution of 10 km s^-1 for the 0-0 S(3) line at 9.7 micron. This instrument offers the large field of view necessary to survey extended emission in the Galaxy and local Universe galaxies as well as to perform unbiased extragalactic and circumstellar disks surveys. The high spectral resolution makes H2EX uniquely suited to study the dynamics of H2 in all these environments. The mission plan is made of seven wide-field spectro-imaging legacy programs, from the cosmic web to galactic young star clusters, within a nominal two years mission. The payload has been designed to re-use the Planck platform and passive cooling design., Accepted for publication in Experimental Astronomy special issue on Cosmic Vision Proposals, 19 pages and 9 figures

The fluid dynamics of MHD shock waves in magnetized molecular gas is reviewed. The different types of shock solutions, and the circumstances under which the different types occur, are delineated. Current theoretical work on C∗- and J-type shocks, and on the stability of C-type shocks, is briefly described. Observations of the line emission from MHD shocks in different regions appear to be in conflict with theoretical expectations for single, plane-parallel shocks. Replacement of plane-parallel shocks by bow shocks may help reconcile theory and observation, but it is also possible that the observed shocks may not be “steady”, or that theoretical models have omitted some important physics.

We present a sample of low-resolution 5-38um Spitzer IRS spectra of the inner few square kiloparsecs of 59 nearby galaxies spanning a large range of star formation properties. A robust method for decomposing mid-infrared galaxy spectra is described, and used to explore the behavior of PAH emission and the prevalence of silicate dust extinction. Evidence for silicate extinction is found in ~1/8 of the sample, at strengths which indicate most normal galaxies undergo A_V < ~3 magnitudes averaged over their centers. The contribution of PAH emission to the total infrared power is found to peak near 10% and extend up to ~20%, and is suppressed at metallicities Z < ~Z_sun/4, as well as in low-luminosity AGN environments. Strong inter-band PAH feature strength variations (2-5x) are observed, with the presence of a weak AGN and, to a lesser degree, increasing metallicity shifting power to the longer wavelength bands. A peculiar PAH emission spectrum with markedly diminished 5-8um features arises among the sample solely in systems with relatively hard radiation fields harboring low-luminosity AGN. The AGN may modify the emitting grain distribution and provide the direct excitation source of the unusual PAH emission, which cautions against using absolute PAH strength to estimate star formation rates in systems harboring active nuclei. Alternatively, the low star formation intensity often associated with weak AGN may affect the spectrum. The effect of variations in the mid-infrared spectrum on broadband infrared surveys is modeled, and points to more than a factor of two uncertainty in results which assume a fixed PAH emission spectrum, for redshifts z=0-2.5.

The quantity of dust in a spiral disk can be estimated using the dust's typical emission or the extinction of a known source. In this paper, we compare two techniques, one based on emission and one on absorption, applied on sections of fourteen disk galaxies. The two measurements reflect, respectively the average and apparent optical depth of a disk section. Hence, they depend differently on the average number and optical depth of ISM structures in the disk. The small scale geometry of the cold ISM is critical for accurate models of the overall energy budget of spiral disks. ISM geometry, relative contributions of different stellar populations and dust emissivity are all free parameters in galaxy Spectral Energy Distribution (SED) models; they are also sometimes degenerate, depending on wavelength coverage. Our aim is to constrain typical ISM geometry. The apparent optical depth measurement comes from the number of distant galaxies seen in HST images through the foreground disk. We measure the IR flux in images from the {\it Spitzer} Infrared Nearby Galaxy Survey in the same section of the disk that was covered by HST. A physical model of the dust is fit to the SED to estimate the dust surface density, mean temperature, and brightness in these disk sections. The surface density is subsequently converted into the average optical depth estimate. The two measurements generally agree. The ratios between the measured average and apparent optical depths of the disk sections imply optically thin clouds in these disks. Optically thick disks, are likely to have more than a single cloud along the line-of-sight., Comment: 31 pages, 5 figures, 4 tables, accepted for publication in AJ

The Spitzer Space Telescope Warm Mission capability, specifically broadband IRAC imaging at 3.6 and 4.5 μm, provides a unique opportunity to probe the content and structure of the Milky Way's disk, particularly along lines of sight that suffer significant interstellar obscuration. This white paper examines the factors that favor and constrain the application of a warm Spitzer to Galactic structure and interstellar medium science. Although the paper briefly discusses some specific survey choices, its primary purpose is to highlight the value of an extended Spitzer mission and outline the factors that might influence the construction of a large proposal focused on Galactic science in the future.

[abridged] We present observations of the dust and atomic gas phase in seven dwarf irregular galaxies of the M81 group from the SINGS and THINGS surveys. The Spitzer observations provide a first glimpse of the nature of the non-atomic ISM in these metal-poor (Z~0.1 Z_sun), quiescent (SFR~0.001-0.1 M_sun/yr) dwarf galaxies. Dust emission is detected in five out of the seven targets. Most detected dust emission is restricted to HI column densities >1x10^21 cm^-2. Spitzer spectroscopy of two regions in the brightest galaxies (IC 2574 and Holmberg II) show distinctly different spectral shapes. The spectrum of IC 2574 shows aromatic features that are less luminous (relative to the FIR luminosity) compared to an average SINGS spiral galaxy by a factor of \~7 . The aromatic features in Holmberg~II (which has only a slightly lower gas-phase metallicity) are fainter than in IC 2574 by an order of magnitude. This result emphazises that the strength of the aromatic features is not a simple linear function of metallicity. We estimate dust masses of ~10^4-10^6 M_sun for the M81 dwarf galaxies, resulting in an average dust--to--gas ratio (M_dust/M_HI) of ~3x10^-4 (1.5x10^-3 if only the HI that is associated with dust emission is considered); this is an order of magnitude lower than the typical value derived for the SINGS spirals. The dwarf galaxies are underluminous per unit star formation rate at 70um as compared to the more massive galaxies in SINGS by a factor of ~2. However, the average 70um/160um ratio in the sample dwarf galaxies is higher than what is found in the other galaxies of the SINGS sample. This can be explained by a combination of a lower dust content in conjunction with a higher dust temperature in the dwarfs., Accepted for publication in the Astrohysical Journal. High resolution version available at: http://www.mpia.de/homes/walter/preprint_walter_highres.pdf

Our current understanding of interstellar dust is summarized at an introductory level. Submicron-sized interstellar dust grains absorb and scatter light, and reradiate the absorbed energy in the infrared. The grain population spans a range of sizes, from molecules containing only tens of atoms, to particles containing 10^{10} atoms. Most of the grain mass appears to be due to two types of solid, in approximately equal amounts: amorphous silicate mineral, and carbonaceous material. Approximately 2/3 of the interstellar carbon in diffuse clouds is in solid form. Interstellar grains are important for many reasons: they obscure our view (in a wavelength-dependent manner); the infrared emission from dust provides a valuable probe of dense regions; and because dust grains play a central role in interstellar chemistry, gas dynamics, and heating and cooling of interstellar gas.

A gamma ray burst with strong optical-UV emission occuring in a molecular cloud will photodissociate H2, photoionize H2, H, and He, and destroy dust grains. We model these processes, including time-dependent radiative transfer in both continuum radiation and the resonance lines of H2. The UV will pump H2 into vibrationally-excited levels. We calculate the absorption spectrum imprinted on radiation from the GRB at various times. In addition to the strong absorption lines of H2(v=0) at lambda < 1110 Angstrom, due to cold ambient gas, we find that radiation reaching us from the GRB and its afterglow will show strong absorption lines due to vibrationally-excited H2 at 1110 < lambda < 1705 Angstrom. These absorption lines, if observed, would provide unequivocal evidence for association of the GRB with molecular gas. Low-resolution spectra will exhibit conspicuous features due to clustering of individual lines; a list of the strongest such absorption features is given for spectral resolution R approx 350 characteristic of the grism on the Swift UV-Optical Telescope., submitted to ApJ, 27 pages, 11 figures, Latex

We present results of mid-infrared spectroscopic mapping observations of six star-forming regions in the Small Magellanic Cloud from the Spitzer Spectroscopic Survey of the SMC (S4MC). We detect the mid-IR emission from polycyclic aromatic hydrocarbons (PAHs) in all of the mapped regions, greatly increasing the range of environments where PAHs have been spectroscopically detected in the SMC. We investigate the variations of the mid-IR bands in each region and compare our results to studies of the PAH bands in the SINGS sample and in a sample of low-metallicity starburst galaxies. PAH emission in the SMC is characterized by low ratios of the 6-9 micron features relative to the 11.3 micron feature and weak 8.6 and 17.0 micron features. Interpreting these band ratios in the light of laboratory and theoretical studies, we find that PAHs in the SMC tend to be smaller and less ionized than those in higher metallicity galaxies. Based on studies of PAH destruction, we argue that a size distribution shifted towards smaller PAHs cannot be the result of processing in the interstellar medium, but instead reflects differences in the formation of PAHs at low metallicity. Finally, we discuss the implications of our observations for our understanding of the PAH life-cycle in low-metallicity galaxies---namely that the observed deficit of PAHs may be a consequence of PAHs forming with smaller average sizes and therefore being more susceptible to destruction under typical interstellar medium conditions., Submitted to ApJ