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1. X-Ray Properties of NGC 253's Starburst-driven Outflow

   https://doi.org/10.3847/1538-4357/aca65e  

   Lopez, Sebastian; Lopez, Laura A.; Nguyen, Dustin D.; Thompson, Todd A.; Mathur, Smita; Bolatto, Alberto D.; Vulic, Neven; Sardone, Amy (January 2023, The Astrophysical Journal)

   Abstract We analyze image and spectral data from ≈365 ks of observations from the Chandra X-ray Observatory of the nearby, edge-on starburst galaxy NGC 253 to constrain properties of the hot phase of the outflow. We focus our analysis on the −1.1 to +0.63 kpc region of the outflow and define several regions for spectral extraction where we determine best-fit temperatures and metal abundances. We find that the temperatures and electron densities peak in the central ∼250 pc region of the outflow and decrease with distance. These temperature and density profiles are in disagreement with an adiabatic spherically expanding starburst wind model and suggest the presence of additional physics such as mass loading and nonspherical outflow geometry. Our derived temperatures and densities yield cooling times in the nuclear region of a few million years, which may imply that the hot gas can undergo bulk radiative cooling as it escapes along the minor axis. Our metal abundances of O, Ne, Mg, Si, S, and Fe all peak in the central region and decrease with distance along the outflow, with the exception of Ne, which maintains a flat distribution. The metal abundances indicate significant dilution outside of the starburst region. We also find estimates of the mass outflow rates, which are 2.8M_⊙yr⁻¹in the northern outflow and 3.2M_⊙yr⁻¹in the southern outflow. Additionally, we detect emission from charge exchange and find it makes a significant contribution (20%–42%) to the total broadband (0.5–7 keV) X-ray emission in the central and southern regions of the outflow. 

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2. Resolving the Unresolved Galactic Winds in Multiphase Models. I. Methodology and Application

   https://doi.org/10.3847/1538-4357/ae63d1  

   Xu, Xinfeng; Fielding, Drummond; Heckman, Timothy; Bryan, Greg L; Henry, Alaina; Arellano-Córdova, Karla Z; Carr, Cody; Chisholm, John; Faucher-Giguère, Claude-André; Hayes, Matthew; et al (May 2026, The Astrophysical Journal)

   Abstract Galactic winds shape galaxy evolution; however, the outflowing gas is complex: it consists of multiple ionization phases, and its properties vary spatially. Therefore, methods that combine high-fidelity observations with state-of-the-art galactic-wind models are limited. Here we investigate methods for fitting the column density profiles derived from high-quality outflow observations with the multiphase, multiscale wind model from D. B. Fielding & G. L. Bryan. We identify three key outflow parameters: the initial hot-phase mass-loading factor (η_M,hot,0), the initial cool-phase mass-loading factor (η_M,cool,0), and the initial cool-cloud mass (M_cl,0). We obtain good fits (reducedχ² < 1.5) for most galaxies, with tight constraints onη_M,cool,0and moderate constraints on the other two parameters. We find the inferredη_M,cool,0andη_M,hot,0are mostly of order unity, with significant scatter. The constraints onη_M,hot,0suggest that the interaction between the cool and hot phases allow us to indirectly constrain the properties of the hot wind from cool-outflow observations. The model also predicts various radial trends. First, for all galaxies, the cool-phase outflow velocity increases rapidly between 1 and 2r₅₀, then reaches a plateau, wherer₅₀is the half-light radius of the galaxy. Second, most galaxies exhibit increasingη_M,cooland decreasingη_M,hotwith radius, with a few showing the reverse trends. These results should be interpreted as effective, model-conditional constraints but are consistent with other recent multiphase simulations and observations. This highlights that the velocity–radius mapping encoded in UV absorption profiles enables recovery of outflow spatial structures from spatially integrated spectra. Our method paves the way for future broad parameter studies and guides updates of outflow simulations in future work. 

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3. Kinematics, Structure, and Mass Outflow Rates of Extreme Starburst Galactic Outflows

   https://doi.org/10.3847/1538-4357/acc660  

   Perrotta, Serena; Coil, Alison L.; Rupke, David S.; Tremonti, Christy A.; Davis, Julie D.; Diamond-Stanic, Aleksandar M.; Geach, James E.; Hickox, Ryan C.; Moustakas, John; Rudnick, Gregory H.; et al (May 2023, The Astrophysical Journal)

   Abstract We present results on the properties of extreme gas outflows in massive (M_*∼ 10¹¹M_⊙), compact, starburst (star formation rate, SFR∼ 200M_⊙yr⁻¹) galaxies atz= 0.4–0.7 with very high star formation surface densities (Σ_SFR∼ 2000M_⊙yr⁻¹kpc⁻²). Using optical Keck/HIRES spectroscopy of 14 HizEA starburst galaxies, we identify outflows with maximum velocities of 820–2860 km s⁻¹. High-resolution spectroscopy allows us to measure precise column densities and covering fractions as a function of outflow velocity and characterize the kinematics and structure of the cool gas outflow phase (T∼ 10⁴K). We find substantial variation in the absorption profiles, which likely reflects the complex morphology of inhomogeneously distributed, clumpy gas and the intricacy of the turbulent mixing layers between the cold and hot outflow phases. There is not a straightforward correlation between the bursts in the galaxies’ star formation histories and their wind absorption line profiles, as might naively be expected for starburst-driven winds. The lack of strong Mgiiabsorption at the systemic velocity is likely an orientation effect, where the observations are down the axis of a blowout. We infer high mass outflow rates of ∼50–2200M_⊙yr⁻¹, assuming a fiducial outflow size of 5 kpc, and mass loading factors ofη∼ 5 for most of the sample. While these values have high uncertainties, they suggest that starburst galaxies are capable of ejecting very large amounts of cool gas that will substantially impact their future evolution. 

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4. CLASSY. VI. The Density, Structure, and Size of Absorption-line Outflows in Starburst Galaxies

   https://doi.org/10.3847/1538-4357/acbf46  

   Xu, Xinfeng; Heckman, Timothy; Henry, Alaina; Berg, Danielle A.; Chisholm, John; James, Bethan L.; Martin, Crystal L.; Stark, Daniel P.; Hayes, Matthew; Arellano-Córdova, Karla Z.; et al (May 2023, The Astrophysical Journal)

   Abstract Galaxy formation and evolution are regulated by the feedback from galactic winds. Absorption lines provide the most widely available probe of winds. However, since most data only provide information integrated along the line of sight, they do not directly constrain the radial structure of the outflows. In this paper, we present a method to directly measure the gas electron density in outflows (n_e), which in turn yields estimates of outflow cloud properties (e.g., density, volume filling factor, and sizes/masses). We also estimate the distance (r_n) from the starburst at which the observed densities are found. We focus on 22 local star-forming galaxies primarily from the COS Legacy Archive Spectroscopic SurveY (CLASSY). In half of them, we detect absorption lines from fine-structure excited transitions of Siii(i.e., Siii*). We determinen_efrom relative column densities of Siiiand Siii*, given Siii* originates from collisional excitation by free electrons. We find that the derivedn_ecorrelates well with the galaxy’s star formation rate per unit area. From photoionization models or assuming the outflow is in pressure equilibrium with the wind fluid, we getr_n∼ 1–2r_*or ∼5r_*, respectively, wherer_*is the starburst radius. Based on comparisons to theoretical models of multiphase outflows, nearly all of the outflows have cloud sizes large enough for the clouds to survive their interaction with the hot wind fluid. Most of these measurements are the first ever for galactic winds detected in absorption lines and, thus, will provide important constraints for future models of galactic winds. 

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5. Evidence for Intrinsic X-Ray Weakness among Red Quasars at Cosmic Noon

   https://doi.org/10.3847/1538-4357/ad710c  

   Ma_马, Yilun 逸伦; Goulding, Andy; Greene, Jenny E; Zakamska, Nadia L; Wylezalek, Dominika; Jiang_姜, Yan-Fei 燕飞 (October 2024, The Astrophysical Journal)

   Abstract Quasar feedback is a key ingredient in shaping galaxy evolution. A rare population of extremely red quasars (ERQs) atz= 2−3 are often associated with high-velocity [Oiii]λ5008 outflows and may represent sites of strong feedback. In this paper, we present an X-ray study of 50 ERQs to investigate the link between the X-ray and outflow properties of these intriguing objects. Using hardness ratio analysis, we confirm that the ERQs are heavily obscured systems with gas column density reachingN_H= 10²³⁻²⁴cm⁻². We identify 20 X-ray-nondetected ERQs at high mid-infrared (MIR) luminosities ofνL_ν,6μm≳ 3 × 10⁴⁶erg s⁻¹. By stacking the X-ray observations, we find that the nondetected ERQs are on average underluminous in X-rays by a factor of ∼10 for their MIR luminosities. We consider such X-ray weakness to be due to both heavy gas absorption and intrinsic factors. Moreover, we find that the X-ray-weak sources also display higher-velocity outflows. One option to explain this trend is that weaker X-rays facilitate more vigorous line-driven winds, which then accelerate the [Oiii]-emitting gas to kiloparsec scales. Alternatively, super-Eddington accretion could also lead to intrinsic X-ray weakness and more powerful continuum-driven outflow. 

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