The Astrodust+PAH Model: A Unified Description of the Extinction, Emission, and Polarization from Dust in the Diffuse Interstellar Medium

Hensley, Brandon; Draine, B. T.

doi:10.3847/1538-4357/acc4c2

“…This polarization level is much higher than the average level of the ISM of P ∼ 15% observed by Planck Collaboration et al (2020), but it is comparable to the observations toward the massive filament DR21 (Ching et al 2022) andG34.43 (Soam et al 2019). Previous modeling of Planck data concluded that a high alignment degree of grains is required to reproduce the observational data (Guillet et al 2018;Hensley & Draine 2023). Therefore, we expect that the higher polarization level observed in the outer regions of massive filaments can be achieved only if grains can be perfectly aligned.…”

Section: On the Role Of Magnetic Relaxation On Rat Alignmentmentioning

confidence: 82%

B-fields and Dust in Interstellar Filaments Using Dust Polarization (BALLAD-POL). I. The Massive Filament G11.11–0.12 Observed by SOFIA/HAWC+

Ngoc

¹

,

Diep

²

,

Hoang

³

et al. 2023

ApJ

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We report the first measurement of polarized thermal dust emission toward the entire infrared dark cloud G11.11−0.12 taken by the polarimeter SOFIA/HAWC+ at 214 μm. The obtained magnetic fields (B-fields) from the polarized emission of the early-stage and massive filament tend to be perpendicular to its spine. We produce a map of B-field strengths for the center region of the filament. The strengths vary in the range of 100–600 μG and are strongest along the filament's spine. The central region is sub-Alfvénic and mostly subcritical, meaning that B-fields dominate over turbulence and are strong enough to resist gravitational collapse. The alignment and properties of dust grains in the filament are studied using radiative torque (RAT) theory. We find the decrease of polarization degree P with emission intensity I, i.e., depolarization effect, of the form P ∝ I −α with α ∼ 0.8–0.9, implying a significant loss of grain alignment in the filament's spine. The depolarization can be explained by the decrease in RAT alignment efficiency toward the denser regions with weaker radiation field, which cannot be explained by B-field tangling. We study the effect of the enhanced magnetic relaxation by embedded iron inclusions on RAT alignment and find that the high polarization fraction P ∼ 20%–30% in the outer layer of the filament is potential evidence for the magnetically enhanced RAT alignment mechanism. This is the first time this effect is evaluated in a filament. Based on the polarization fraction and RAT alignment theory, we also find evidence for grain growth in the filament.

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“…G 0 = 1 is defined as the intensity of the radiation field integrated between 6 and 13.6 eV for the standard ISRF. Note the different nomenclature referring to the radiation heating the dust, where U ; 1.6G 0 by Hensley & Draine (2023). The dust models with high G 0 and U are particularly interesting in our work because it represents the ISM under a strong radiation field from star-forming regions.…”

Section: Polarization Spectrum Of Starburst Galaxiesmentioning

confidence: 99%

The Magnetic Fields of Starburst Galaxies. I. Identification and Characterization of the Thermal Polarization in the Galactic Disk and Outflow

Lopez-Rodriguez

2023

ApJ

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Far-infrared polarized emission by means of magnetically aligned dust grains is an excellent tracer of the magnetic fields (B-fields) in the cold phase of the galactic outflows of starburst galaxies. We present a comprehensive study of the B-fields in three nearby (3.5–17.2 Mpc) starbursts (M82, NGC 253, and NGC 2146) at 5 pc–1.5 kpc resolutions using publicly available 53–890 μm imaging polarimetric observations with Stratospheric Observatory for Infrared Astronomy/HAWC+, James Clerk Maxwell Telescope/POL-2, and ALMA. We find that the polarized spectral energy distributions (SEDs) of the full galaxies are dominated by the polarized SEDs of the outflows with dust temperatures of T d , outflow PI ∼ 45 K and an emissive index of β outflow PI ∼ 2.3 . The disks are characterized by low T d , disk PI = [ 24 , 31 ] K and β disk PI ∼ 1 . We show that disk- and outflow-dominated galaxies can be better distinguished by using polarized SEDs instead of total SEDs. We compute the 53–850 μm polarization spectrum of the disk and outflow, and find that dust models of the diffuse ISM can reproduce the fairly constant polarization spectrum of the disk, 〈 P disk〉 = 1.2% ± 0.5%. The dust models of heterogenous clouds and two-temperature components are required to explain the polarization spectrum of the outflow (2%–4% at 53 μm, ∼1% at 850 μm, and a minimum within 89–154 μm). We conclude that the polarized dust grains in the outflow arise from a dust population with higher dust temperature and emissivities than those from the total flux. The B-fields of the outflows have maximum extensions within 89–214 μm reaching heights of ∼4 kpc, and have flatter polarized fluxes than total fluxes. The extension of the B-field permeating the circumgalactic medium increases as the star formation rate increases.

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“…) is the number density of grains with sizes [a, a + da] (we do not assume any subresolution clumping: the density is the number of grains divided by the cell volume). We assume the efficiencies Q ext as computed by Draine & Lee (1984) and Laor & Draine (1993) for silicates and carbonaceous grains, respectively, though in future versions of this code we will implement the newly developed "astrodust" size-dependent extinction properties of dust grains (Hensley & Draine 2023). The wavelength-dependent extinction is:…”

Section: Modeling the Isrfmentioning

confidence: 99%

“…and is usually defined for PAHs containing <10 3 carbon atoms ). Hensley & Draine (2023) relate the number of carbon atoms to the PAH size via:…”

Section: Pah Masses and Q Pah Fractionsmentioning

confidence: 99%

“…While a full exploration of the impact of the threshold velocities on the PAH population is outside the scope of what is computationally feasible here, we note that a similar implementation of dust collisional processes in the SIMBA simulation byLi et al (2021) of Milky Way (MW)-like galaxies in a cosmological simulation results in grain-size distributions comparable to aMathis et al (1977) "MRN" size distribution, and extinction laws comparable to theCardelli et al (1989) Galactic constraints. We have therefore adopted the same threshold velocity here, without any tuning.13 We note that the more recent models byHensley & Draine (2023) and references therein have advanced a picture of "astrodust+PAHs," where large grains are aggregated in their properties into astrodust, while nanoparticle aromatic carbonaceous grains are considered PAHs. From the standpoint of the interface between our model and theDraine et al (2021) model, the treatment of the larger grains is less important than the PAHs themselves.…”

mentioning

confidence: 99%

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A Framework for Modeling Polycyclic Aromatic Hydrocarbon Emission in Galaxy Evolution Simulations

Narayanan,

Smith,

Hensley

et al. 2023

ApJ

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We present a new methodology for simulating mid-infrared emission from polycyclic aromatic hydrocarbons (PAHs) in galaxy evolution simulations. To do this, we combine theoretical models of PAH emission features as they respond to varying interstellar radiation fields, grain-size distributions, and ionization states with a new model for dust evolution in galaxy simulations. We apply these models to three idealized arepo galaxy evolution simulations within the smuggle physics framework. We use these simulations to develop numerical experiments investigating the buildup of PAH masses and luminosities in galaxies in idealized analogs of the Milky Way, a dwarf galaxy, and a starburst disk. Our main results are as follows. Galaxies with high specific star formation rates have increased feedback energy per unit mass, and are able to shatter grains efficiently, driving up the fraction of ultrasmall grains. At the same time, in our model large radiation fields per unit gas density convert aliphatic grains into aromatics. The fraction of dust grains in the form of PAHs (q PAH) can be understood as a consequence of these processes, and in our model PAHs form primarily from interstellar processing (shattering) of larger grains rather than from the growth of smaller grains. We find that the hardness of the radiation field plays a larger role than variations in the grain-size distribution in setting the total integrated PAH luminosities, though cosmological simulations are necessary to investigate fully the complex interplay of processes that drive PAH band luminosities in galaxies.

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The Astrodust+PAH Model: A Unified Description of the Extinction, Emission, and Polarization from Dust in the Diffuse Interstellar Medium

Cited by 47 publications

References 213 publications

B-fields and Dust in Interstellar Filaments Using Dust Polarization (BALLAD-POL). I. The Massive Filament G11.11–0.12 Observed by SOFIA/HAWC+

B-fields and Dust in Interstellar Filaments Using Dust Polarization (BALLAD-POL). I. The Massive Filament G11.11–0.12 Observed by SOFIA/HAWC+

The Magnetic Fields of Starburst Galaxies. I. Identification and Characterization of the Thermal Polarization in the Galactic Disk and Outflow

A Framework for Modeling Polycyclic Aromatic Hydrocarbon Emission in Galaxy Evolution Simulations

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