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1. High-energy Radiation and Ion Acceleration in Three-dimensional Relativistic Magnetic Reconnection with Strong Synchrotron Cooling

   https://doi.org/10.3847/1538-4357  

   Chernoglazov, Alexander ; Hakobyan, Hayk ; Philippov, Alexander ( December 2023 , The Astrophysical Journal)

   We present the results of 3D particle-in-cell simulations that explore relativistic magnetic reconnection in pair plasma with strong synchrotron cooling and a small mass fraction of nonradiating ions. Our results demonstrate that the structure of the current sheet is highly sensitive to the dynamic efficiency of radiative cooling. Specifically, stronger cooling leads to more significant compression of the plasma and magnetic field within the plasmoids. We demonstrate that ions can be efficiently accelerated to energies exceeding the plasma magnetization parameter, ≫σ, and form a hard power-law energy distribution,f_i∝γ⁻¹. This conclusion implies a highly efficient proton acceleration in the magnetospheres of young pulsars. Conversely, the energies of pairs are limited to eitherσin the strong cooling regime or the radiation burnoff limit,γ_syn, when cooling is weak. We find that the high-energy radiation from pairs above the synchrotron burnoff limit,ε_c≈ 16 MeV, is only efficiently produced in the strong cooling regime,γ_syn<σ. In this regime, we find that the spectral cutoff scales asε_cut≈ε_c(σ/γ_syn) and the highest energy photons are beamed along the direction of the upstream magnetic field, consistent with the phenomenological models of gamma-ray emission from young pulsars. Furthermore, our results place constraints on the reconnection-driven models of gamma-ray flares in the Crab Nebula.

    

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2. High-energy Radiation and Ion Acceleration in Three-dimensional Relativistic Magnetic Reconnection with Strong Synchrotron Cooling

   https://doi.org/10.3847/1538-4357/acffc6  

   Chernoglazov, Alexander ; Hakobyan, Hayk ; Philippov, Alexander ( December 2023 , The Astrophysical Journal)

   We present the results of 3D particle-in-cell simulations that explore relativistic magnetic reconnection in pair plasma with strong synchrotron cooling and a small mass fraction of nonradiating ions. Our results demonstrate that the structure of the current sheet is highly sensitive to the dynamic efficiency of radiative cooling. Specifically, stronger cooling leads to more significant compression of the plasma and magnetic field within the plasmoids. We demonstrate that ions can be efficiently accelerated to energies exceeding the plasma magnetization parameter, ≫σ, and form a hard power-law energy distribution,f_i∝γ⁻¹. This conclusion implies a highly efficient proton acceleration in the magnetospheres of young pulsars. Conversely, the energies of pairs are limited to eitherσin the strong cooling regime or the radiation burnoff limit,γ_syn, when cooling is weak. We find that the high-energy radiation from pairs above the synchrotron burnoff limit,ε_c≈ 16 MeV, is only efficiently produced in the strong cooling regime,γ_syn<σ. In this regime, we find that the spectral cutoff scales asε_cut≈ε_c(σ/γ_syn) and the highest energy photons are beamed along the direction of the upstream magnetic field, consistent with the phenomenological models of gamma-ray emission from young pulsars. Furthermore, our results place constraints on the reconnection-driven models of gamma-ray flares in the Crab Nebula.

    

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3. The Cool Circumgalactic Medium of Low-redshift Star-forming Galaxies. I. Empirical Model and Mean Properties

   https://doi.org/10.3847/1538-4357/acf217  

   Faerman, Yakov ; Werk, Jessica K ( October 2023 , The Astrophysical Journal)

   We present an analytic model for the cool,T∼ 10⁴K, circumgalactic medium (CGM), describing the gas distribution, and thermal and ionization states. Our model assumes (total) pressure equilibrium with the ambient warm/hot CGM, photoionization by the metagalactic radiation, and allows for nonthermal pressure support, parameterized by the ratio of thermal pressures,η=P_hot,th/P_cool,th. We apply the model to the COS-Halos measurements and find that a nominal model withη= 3, gas distribution out tor≈ 0.6R_vir, andM_cool= 3 × 10⁹M_⊙, corresponding to a volume filling fraction off_V,cool≈ 1%, reproduces the Hiand low/intermediate metal ions (Cii, Ciii, Siii, Siiii, and Mgii) mean column densities. Variation of ±0.5 dex inηorM_coolencompasses ∼2/3 of the scatter between objects. Our nominal model underproduces the measured Civand Siivcolumns, and these can be reproduced with (i) a cool phase withM_cool∼ 10¹⁰M_⊙andη≈ 5, or (ii) cooling or mixing gas at intermediate temperatures, withM∼ 1.5 × 10¹⁰M_⊙and occupying ∼1/2 of the total CGM volume. For cool gas withf_V,cool≈ 1%, we estimate an upper limit on the cloud sizes,R_cl≲ 0.5 kpc. Our results suggest that for the average galaxy CGM, the mass and nonthermal support in the cool phase are lower than previously estimated, and extreme scenarios are not necessary. We estimate the rates of cool gas depletion and replenishment, and find accretion onto the galaxy can be offset, allowing${\dot{M}}_{\mathrm{cool}}\approx 0$over long timescales.

    

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4. Nitrogen-enriched, Highly Pressurized Nebular Clouds Surrounding a Super Star Cluster at Cosmic Noon

   https://doi.org/10.3847/1538-4357/acf75c  

   Pascale, Massimo ; Dai, Liang ; McKee, Christopher_F ; Tsang, Benny_T-H ( November 2023 , The Astrophysical Journal)

   Strong lensing offers a precious opportunity for studying the formation and early evolution of super star clusters that are rare in our cosmic backyard. The Sunburst Arc, a lensed Cosmic Noon galaxy, hosts a young super star cluster with escaping Lyman continuum radiation. Analyzing archival Hubble Space Telescope images and emission line data from Very Large Telescope/MUSE and X-shooter, we construct a physical model for the cluster and its surrounding photoionized nebula. We confirm that the cluster is ≲4 Myr old, is extremely massiveM_⋆∼ 10⁷M_⊙, and yet has a central component as compact as several parsecs, and we find a gas-phase metallicityZ= (0.22 ± 0.03)Z_⊙. The cluster is surrounded by ≳10⁵M_⊙of dense clouds that have been pressurized toP∼ 10⁹K cm⁻³by perhaps stellar radiation at within 10 pc. These should have large neutral columnsN_HI> 10^22.8cm⁻²to survive rapid ejection by radiation pressure. The clouds are likely dusty as they show gas-phase depletion of silicon, and may be conducive to secondary star formation ifN_HI> 10²⁴cm⁻²or if they sink farther toward the cluster center. Detecting strong [Niii]λλ1750,1752, we infer heavy nitrogen enrichment$\log (\mathrm{N}/\mathrm{O})=-{0.21}_{-0.11}^{+0.10}$. This requires efficiently retaining ≳500M_⊙of nitrogen in the high-pressure clouds from massive stars heavier than 60M_⊙up to 4 Myr. We suggest a physical origin of the high-pressure clouds from partial or complete condensation of slow massive star ejecta, which may have an important implication for the puzzle of multiple stellar populations in globular clusters.

    

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5. Physics and Chemistry of Radiation Driven Cloud Evolution. [C ii] Kinematics of IC 59, and IC 63

   https://doi.org/10.3847/1538-4357/acd2d5  

   Caputo, Miranda ; Soam, Archana ; Andersson, B-G ; Dennis, Remy ; Chambers, Ed ; Güsten, Rolf ; Knee, Lewis B. G. ; Stutzki, Jürgen ( June 2023 , The Astrophysical Journal)

   We used high-resolution [Cii] 158μm mapping of two nebulae IC 59 and IC 63 from SOFIA/upGREAT in conjunction with ancillary data of the gas, dust, and polarization to probe the kinematics, structure, and magnetic properties of their photodissociation regions (PDRs). The nebulae are part of the Sh 2-185 Hiiregion that is illuminated by the B0 IVe starγCas. The velocity structure of each PDR changes with distance fromγCas, which is consistent with driving by the radiation. Based on previous far-ultraviolet (FUV) flux measurements of, and the known distance to,γCas, along with the predictions of 3D distances to the clouds, we estimated the FUV radiation field strength (G₀) at the clouds. Assuming negligible extinction between the star and clouds, we find their 3D distances fromγCas. For IC 63, our results are consistent with earlier estimates of distance from Andersson et al., locating the cloud at ∼2 pc fromγCas at an angle of 58° to the plane of the sky behind the star. For IC 59, we derive a distance of 4.5 pc at an angle of 70° in front of the star. We do not detect any significant correlation between the orientation of the magnetic field and the velocity gradients of [Cii] gas, which indicates a moderate magnetic field strength. The kinetic energy in IC 63 is estimated to be an order of 10 higher than the magnetic energies. This suggests that kinetic pressure in this nebula is dominant.

    

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