Diffuse Interstellar Radiation Research Articles

Turbulent Diffuse Molecular Media with Nonideal Magnetohydrodynamics and Consistent Thermochemistry: Numerical Simulations and Dynamic Characteristics

Turbulent diffuse molecular clouds can exhibit complicated morphologies caused by the interactions among radiation, chemistry, fluids, and fields. We performed full 3D simulations for turbulent diffuse molecular interstellar media, featuring time-dependent nonequilibrium thermochemistry coevolved with magnetohydrodynamics (MHD). Simulation results exhibit the relative abundances of key chemical species (e.g., C, CO, OH) vary by more than one order of magnitude for the “premature” epoch of chemical evolution (t ≲ 2 × 105 yr). Various simulations are also conducted to study the impacts of physical parameters. Nonideal MHD effects are essential in shaping the behavior of gases, and strong magnetic fields (∼10 μG) tend to inhibit vigorous compressions and thus reduce the fraction of warm gases (T ≳ 102 K). Thermodynamical and chemical conditions of the gas are sensitive to modulation by dynamic conditions, especially the energy injection by turbulence. Chemical features, including ionization (cosmic ray and diffuse interstellar radiation), would not directly affect the turbulence power spectra. Nonetheless, their effects are prominent in the distribution profiles of temperatures and gas densities. Comprehensive observations are necessary and useful to eliminate the degeneracies of physical parameters and constrain the properties of diffuse molecular clouds with confidence.

A Two-Component Probability Distribution Function Describes the Mid-IR Emission from the Disks of Star-forming Galaxies

High-resolution JWST-MIRI images of nearby spiral galaxies reveal emission with complex substructures that trace dust heated both by massive young stars and the diffuse interstellar radiation field. We present high angular (0.″85) and physical resolution (20–80 pc) measurements of the probability distribution function (PDF) of mid-infrared (mid-IR) emission (7.7–21 μm) from 19 nearby star-forming galaxies from the PHANGS-JWST Cycle 1 Treasury. The PDFs of mid-IR emission from the disks of all 19 galaxies consistently show two distinct components: an approximately lognormal distribution at lower intensities and a high-intensity power law component. These two components only emerge once individual star-forming regions are resolved. Comparing with locations of H ii regions identified from Very Large Telescope/MUSE Hα mapping, we infer that the power-law component arises from star-forming regions and thus primarily traces dust heated by young stars. In the continuum-dominated 21 μm band, the power law is more prominent and contains roughly half of the total flux. At 7.7–11.3 μm, the power law is suppressed by the destruction of small grains (including PAHs) close to H ii regions, while the lognormal component tracing the dust column in diffuse regions appears more prominent. The width and shape of the lognormal diffuse emission PDFs in galactic disks remain consistent across our sample, implying a lognormal gas column density N(H) ≈ 1021 cm−2 shaped by supersonic turbulence with typical (isothermal) turbulent Mach numbers ≈5−15. Finally, we describe how the PDFs of galactic disks are assembled from dusty H ii regions and diffuse gas and discuss how the measured PDF parameters correlate with global properties such as star formation rate and gas surface density.

PHANGS–JWST First Results: Tracing the Diffuse Interstellar Medium with JWST Imaging of Polycyclic Aromatic Hydrocarbon Emission in Nearby Galaxies

JWST observations of polycyclic aromatic hydrocarbon (PAH) emission provide some of the deepest and highest resolution views of the cold interstellar medium (ISM) in nearby galaxies. If PAHs are well mixed with the atomic and molecular gas and illuminated by the average diffuse interstellar radiation field, PAH emission may provide an approximately linear, high-resolution, high-sensitivity tracer of diffuse gas surface density. We present a pilot study that explores using PAH emission in this way based on Mid-Infrared Instrument observations of IC 5332, NGC 628, NGC 1365, and NGC 7496 from the Physics at High Angular resolution in Nearby GalaxieS-JWST Treasury. Using scaling relationships calibrated in Leroy et al., scaled F1130W provides 10–40 pc resolution and 3σ sensitivity of Σgas ∼ 2 M ⊙ pc−2. We characterize the surface densities of structures seen at <7 M ⊙ pc−2 in our targets, where we expect the gas to be H i-dominated. We highlight the existence of filaments, interarm emission, and holes in the diffuse ISM at these low surface densities. Below ∼10 M ⊙ pc−2 for NGC 628, NGC 1365, and NGC 7496 the gas distribution shows a “Swiss cheese”-like topology due to holes and bubbles pervading the relatively smooth distribution of the diffuse ISM. Comparing to recent galaxy simulations, we observe similar topology for the low-surface-density gas, though with notable variations between simulations with different setups and resolution. Such a comparison of high-resolution, low-surface-density gas with simulations is not possible with existing atomic and molecular gas maps, highlighting the unique power of JWST maps of PAH emission.

PHANGS–JWST First Results: Mid-infrared Emission Traces Both Gas Column Density and Heating at 100 pc Scales

We compare mid-infrared (mid-IR), extinction-corrected Hα, and CO (2–1) emission at 70–160 pc resolution in the first four PHANGS–JWST targets. We report correlation strengths, intensity ratios, and power-law fits relating emission in JWST’s F770W, F1000W, F1130W, and F2100W bands to CO and Hα. At these scales, CO and Hα each correlate strongly with mid-IR emission, and these correlations are each stronger than the one relating CO to Hα emission. This reflects that mid-IR emission simultaneously acts as a dust column density tracer, leading to a good match with the molecular-gas-tracing CO, and as a heating tracer, leading to a good match with the Hα. By combining mid-IR, CO, and Hα at scales where the overall correlation between cold gas and star formation begins to break down, we are able to separate these two effects. We model the mid-IR above I ν = 0.5 MJy sr−1 at F770W, a cut designed to select regions where the molecular gas dominates the interstellar medium (ISM) mass. This bright emission can be described to first order by a model that combines a CO-tracing component and an Hα-tracing component. The best-fitting models imply that ∼50% of the mid-IR flux arises from molecular gas heated by the diffuse interstellar radiation field, with the remaining ∼50% associated with bright, dusty star-forming regions. We discuss differences between the F770W, F1000W, and F1130W bands and the continuum-dominated F2100W band and suggest next steps for using the mid-IR as an ISM tracer.

Modelling grain alignment by radiative torques and hydrogen formation torques in reflection nebula

Reflection nebulae--dense cores--illuminated by surrounding stars offer a unique opportunity to directly test our quantitative model of grain alignment based on radiative torques (RATs) and to explore new effects arising from additional torques. In this paper, we first perform detailed modeling of grain alignment by RATs for the IC 63 reflection nebula illuminated both by a nearby $\gamma$ Cas star and the diffuse interstellar radiation field. We calculate linear polarization $p_{\lambda}$ of background stars by radiatively aligned grains and explore the variation of fractional polarization ($p_{\lambda}/A_V$) with visual extinction $A_{V}$ across the cloud. Our results show that the variation of $p_{V}/A_{V}$ versus $A_{V}$ from the dayside of IC 63 to its center can be represented by a power-law ($p_{V}/A_{V}\propto A_{V}^{\eta}$) with different slopes depending on $A_{V}$. We find a shallow slope $\eta \sim- 0.1$ for $A_{V}< 3$ and a very steep slope $\eta\sim -2$ for $A_{V}> 4$. We then consider the effects of additional torques due to H$_{2}$ formation and model grain alignment by joint action of RATs and H$_2$ torques. We find that $p_{V}/A_{V}$ tends to increase with an increasing magnitude of H$_{2}$ torques. In particular, the theoretical predictions obtained for $p_{V}/A_{V}$ and peak wavelength $\lambda_{\max}$ in this case show an improved agreement with the observational data. Our results reinforce the predictive power of the RAT alignment mechanism in a broad range of environmental conditions and show the effect of pinwheel torques in environments with efficient H$_2$ formation. Physical parameters involved in H$_2$ formation may be constrained using detailed modeling of grain alignment combined with observational data. In addition, we discuss implications of our modeling for interpreting latest observational data by {\it Planck} and other ground-based instruments.

The relationship between polycyclic aromatic hydrocarbon emission and far-infrared dust emission from NGC 2403 and M83

We examine the relation between polycyclic aromatic hydrocarbon (PAH) emission at 8 microns and far-infrared emission from hot dust grains at 24 microns and from large dust grains at 160 and 250 microns in the nearby spiral galaxies NGC 2403 and M83 using data from the Spitzer Space Telescope and Herschel Space Observatory. We find that the PAH emission in NGC 2403 is better correlated with emission at 250 microns from dust heated by the diffuse interstellar radiation field (ISRF) and that the 8/250 micron surface brightness ratio is well-correlated with the stellar surface brightness as measured at 3.6 microns. This implies that the PAHs in NGC 2403 are intermixed with cold large dust grains in the diffuse interstellar medium (ISM) and that the PAHs are excited by the diffuse ISRF. In M83, the PAH emission appears more strongly correlated with 160 micron emission originating from large dust grains heated by star forming regions. However, the PAH emission in M83 is low where the 24 micron emission peaks within star forming regions, and enhancements in the 8/160 micron surface brightness ratios appear offset relative to the dust and the star forming regions within the spiral arms. This suggests that the PAHs observed in the 8 micron band are not excited locally within star forming regions but either by light escaping non-axisymmetrically from star forming regions or locally by young, non-photoionising stars that have migrated downstream from the spiral density waves. The results from just these two galaxies show that PAHs may be excited by different stellar populations in different spiral galaxies.

THE HEATING OF MID-INFRARED DUST IN THE NEARBY GALAXY M33: A TESTBED FOR TRACING GALAXY EVOLUTION

Because the 8 {\mu}m polycyclic aromatic hydrocarbon (PAH) emission has been found to correlate with other well-known star formation tracers, it has widely been used as a star formation rate (SFR) tracer. There are, however, studies that challenge the accuracy and reliability of the 8 {\mu}m emission as a SFR tracer. Our study, part of the Herschel M33 Extended Survey (HERM33ES) open time key program, aims at addressing this issue by analyzing the infrared emission from the nearby spiral galaxy M33 at the high spatial scale of 75 pc. Combining data from the Herschel Space Observatory and the Spitzer Space Telescope we find that the 8 {\mu}m emission is better correlated with the 250 {\mu}m emission, which traces cold interstellar gas, than with the 24 {\mu}m emission. The L(8)/L(24) ratio is highly depressed in 24 {\mu}m luminous regions, which correlate with known HII regions. We also compare our results with the dust emission models by Draine & Li (2007). We confirm that the depression of 8 {\mu}m PAH emission near star-forming regions is higher than what is predicted by models; this is possibly an effect of increased stellar radiation from young stars destroying the dust grains responsible for the 8 {\mu}m emission as already suggested by other authors. We find that the majority of the 8 {\mu}m emission is fully consistent with heating by the diffuse interstellar medium, similar to what recently determined for the dust emission in M31 by Draine at al. (2013). We also find that the fraction of 8 {\mu}m emission associated with the diffuse interstellar radiation field ranges between 60% and 80% and is 40% larger than the diffuse fraction at 24 {\mu}m.

The 617 MHz–λ 850 μm correlation (cosmic rays and cold dust) in NGC 3044 and NGC 4157

We present the first maps of NGC 3044 and NGC 4157 at $\lambda\,450 \mu$m and $\lambda\,850 \mu$m from the JCMT as well as the first maps at 617 MHz from the GMRT. High latitude emission has been detected in both the radio continuum and sub-mm for NGC 3044 and in the radio continuum for NGC 4157, including several new features. The dust spectrum at long wavelengths required fitting with a two-temperature model for both galaxies, implying the presence of cold dust. Dust masses are $M_d\,=\,1.6\,\times\,10^8 M_\odot$ and $M_d\,=\,2.1\,\times\,10^7 M_\odot$ for NGC 3044 and NGC 4157, respectively, and are dominated by the cold component. There is a clear correlation between the 617 MHz and $\lambda\,850 \mu$m emission in the two galaxies. In the case of NGC 3044, this implies a relation between the non-thermal synchrotron emission and cold dust. The 617 MHz component represents an integration of massive star formation over the past $10^{7-8}$ yr and the $\lambda \,850 \mu$m emission represents heating from the diffuse interstellar radiation field (ISRF). The 617 MHz -- $\lambda\,850 \mu$m correlation improves when a smoothing kernel is applied to the $\lambda\,850 \mu$m data to account for differences between the CR electron diffusion scale and the mean free path of an ISRF photon to dust. The best-fit relation is $L_{617_{\rm MHz}}\,\propto\,{L_{850 \mu{\rm m}}}^{2.1\,\pm\,0.2}$ for NGC 3044. If variations in the cold dust emissivity are dominated by variations in dust density, and the synchrotron emission depends on magnetic field strength (a function of gas density) as well as CR electron generation (a function of massive star formation rate and therefore density via the Schmidt law) then the expected correlation for NGC 3044 is $L_{617_{\rm MHz}}\,\propto\,{L_{850 \mu{\rm m}}}^{2.2}$, in agreement with the observed correlation.

GAMA/H-ATLAS: THE DUST OPACITY–STELLAR MASS SURFACE DENSITY RELATION FOR SPIRAL GALAXIES

We report the discovery of a well-defined correlation between B-band face-on central optical depth due to dust, \tau^f_B, and the stellar mass surface density, \mu_{*}, of nearby (z < 0.13) spiral galaxies: log(\tau^f_B) = 1.12(+-0.11)log(\mu_{*}/M_sol kpc^2)-8.6(+-0.8). This relation was derived from a sample of spiral galaxies taken from the Galaxy and Mass Assembly (GAMA) survey and detected in the FIR/submm in the Herschel-ATLAS survey. Using a quantitative analysis of the NUV attenuation-inclination relation for complete samples of GAMA spirals categorized according to \mu_{*} we demonstrate that this correlation can be used to statistically correct for dust attenuation purely on the basis of optical photometry and S'ersic-profile morphological fits. Considered together with previously established empirical relationships between stellar mass, metallicity and gas mass, the near linearity and high constant of proportionality of the \tau^f_B-\mu_{*} relation disfavors a stellar origin for the bulk of refractory grains in spiral galaxies, instead being consistent with the existence of a ubiquitous and very rapid mechanism for the growth of dust in the ISM. We use the \tau^f_B-\mu_{*} relation in conjunction with the radiation transfer model for spiral galaxies of Popescu & Tuffs (2011) to derive intrinsic scaling relations between specific star formation rate (sSFR), stellar mass, and \mu_{*}, in which the attenuation of the UV light used to measure the SFR is corrected on an object-to-object basis. A marked reduction in scatter in these relations is achieved which is demonstrably due to correction of both the inclination-dependent and face-on components of attenuation. Our results are consistent with a picture of spiral galaxies in which most of the submm emission originates from grains residing in translucent structures, exposed to UV in the diffuse interstellar radiation field.

LIFECYCLE OF THE INTERSTELLAR DUST GRAINS IN OUR GALAXY VIEWED WITH AKARI/MIR ALL-SKY SURVEY

The interstellar dust grains are formed and supplied to interstellar space from asymptotic giant branch (AGB) stars or supernova remnants, and become constituents of the star- and planetformation processes that lead to the next generation of stars. Both a qualitative, and a compositional study of this cycle are essential to understanding the origin of the pre-solar grains, the missing sources of the interstellar material, and the chemical evolution of our Galaxy. The AKARI/MIR all-sky survey was performed with two mid-infrared photometric bands centered at 9 and 18 ¹m. These data have advantages in detecting carbonaceous and silicate circumstellar dust of AGB stars, and the interstellar polycyclic aromatic hydrocarbons separately from large grains of amorphous silicate. By using the AKARI/MIR All-Sky point source catalogue, we surveyed C-rich and O-rich AGB stars in our Galaxy, which are the dominant suppliers of carbonaceous and silicate grains, respectively. The C-rich stars are uniformly distributed across the Galactic disk, whereas O-rich stars are concentrated toward the Galactic center, following the metallicity gradient of the interstellar medium, and are presumably affected by the environment of their birth place. We will compare the distributions of the dust suppliers with the distributions of the interstellar grains themselves by using the AKARI/MIR All-Sky diffuse maps. To enable discussions on the faint diffuse interstellar radiation, we are developing an accurate AKARI/MIR All-Sky diffuse map by correcting artifacts such as the ionising radiation effects, scattered light from the moon, and stray light from bright sources.

Spatial distribution of far-infrared emission in spiral galaxies. II. Heating sources and gas-to-dust ratio.

We study the radial distribution of the temperature of the warm dust and gas-to-dust mass ratios in a sample of 22 spiral galaxies. The heating capabilities of the diffuse interstellar radiation field (ISRF), based on Desert et al. model, are investigated in 13 of the sample galaxies. In general, the temperature of the warm dust decreases away from the center, reaches a minimum value at the mid-disk and increases again in the outer parts of galaxies. Heating a mixture of small and big grains by the ISRF is able to explain the observed behavior qualitatively. However, ultraviolet photons from recent star formation events are necessary for a detailed matching of the warm dust temperature profiles. Very small grains contribute typically more than 50% to the observed flux at 60 micron beyond half the disk radius in galaxies. Optical depth profiles, derived from the observed 60 micron and warm dust temperature profiles, peak at or close to the galactic center. In 13 of the galaxies, where dust temperature profiles are modeled, we obtain gas-to-dust mass ratio profiles, after correction for the contaminating effects of very small grains. The gas-to-dust mass ratio decreases by a factor of 8 from the center to the optical isophotal radius, where the value approaches the local galactic value. We demonstrate that the observed steep gradient is a result of the over-estimation of the molecular mass, and can be flattened out to within a factor of 2, if the molecular hydrogen mass (H2) is recomputed assuming a metallicity dependent conversion factor from CO intensity to H2 column density. The flattened radial profiles indicate a global gas-to-dust ratio of around 300, which is within a factor of two of the local galactic value.

A search for the He I lambda 5876 recombination line from the diffuse interstellar medium

view Abstract Citations (91) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Search for the He i lambda 5876 Recombination Line from the Diffuse Interstellar Medium Reynolds, R. J. ; Tufte, S. L. Abstract The He I lambda 5876/hydrogen alpha line intensity ratio in the warm ionized component of the interstellar medium is significantly less than that observed in traditional H II regions surrounding O stars. The observations, carried out in two directions at low Galactic latitude, reveal that the helium is primarily neutral, n(He II)/n(Hetotal approximately less than 0.27, within warm, low-density regions in which the hydrogen is primarily ionized. This implies that the spectrum of the diffuse interstellar radiation field that ionizes the hydrogen is softer than that from the Galactic O star population in the solar neighborhood. Specifically, the ratio of helium ionizing to hydrogen ionizing photons Q(He0)/Q(H0) approximately less than 0.030, which corresponds to a spectral type of O8 or later and, if representative of the interstellar background, challenges the O star ionization models. Publication: The Astrophysical Journal Pub Date: January 1995 DOI: 10.1086/187734 Bibcode: 1995ApJ...439L..17R Keywords: Emission Spectra; Fine Structure; H Ii Regions; Helium; Interstellar Matter; Line Spectra; H Alpha Line; Hydrogen Ions; O Stars; Astronomy; ISM: H II REGIONS; ISM: GENERAL full text sources ADS | data products SIMBAD (9)

Diffuse radiation in the Galaxy

view Abstract Citations (1532) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Diffuse radiation in the Galaxy. Henyey, L. G. ; Greenstein, J. L. Abstract Observations have been obtained to verify the existence of diffuse interstellar radiation. A Fabry photometer, attached to the 40-inch refractor at the Yerkes Observatory, was used to measure the brightness of regions over a wide range of galactic latitude. The intensities in the photographic region of the spectrum were calibrated by means of the Polar Sequence stars. The mean of four such runs across the Milky Way, on circles of nearly constant longitude, 1 = 40°, shows a maximum of brightness of 8o stars of the tenth magnitude per square degree for the diffuse extra-terrestrial radiation. The mean of three runs near 1 = 1400 shows a maximum of 35 in the same units. It is shown that the observed intensity of diffuse light may be explained as scal~tered stellar radiation if the phase function governing the scattering of starlight by the interstellar matter is strongly forward- throwing. The concentration of the diffuse light toward the galactic circle is also in agreement with this property of the phase function. The observations also indicate that the scattering efficiency, or albedo, of the particles is greater than 0.3 Publication: The Astrophysical Journal Pub Date: January 1941 DOI: 10.1086/144246 Bibcode: 1941ApJ....93...70H full text sources ADS | data products SIMBAD (8)