Context. Bremsstrahlung scattering of fast electrons on ions can be enhanced by the microwave radiation present in the solar corona. It can account for the electron diffusive transport along magnetic loops and high precipitation rates. This process can also dominate the transport of thermal electrons confined in such loops, which calls for a dedicated study. Aims. We aim to study the influence of stimulated Bremsstrahlung scattering on the electron transport, with a focus on the return current induced by the fast electron population trapped in magnetic loops. From a more general standpoint, transport coefficients need to be reevaluated in the radiation-dominated plasma, which is characterized by the stimulated action of radiation on the Bremsstrahlung electron–ion collision frequency, down to thermal velocities. Methods. We developed a theoretical framework for electron transport driven by a large bandwidth, bright, low-frequency part of the photon spectrum and computed a set of radiation-enhanced transport coefficients. Ultraviolet (UV), X-ray and extreme ultraviolet (XEUV) and hard X-ray signals from flares were reinterpreted in order to put maximum constraints on the fast-electron model and its return-current component. These observations evidence anomalous resistivity, thermal-conduction inhibition, and high precipitation rates of fast electrons, which are quantified by the model. Results. Under the assumption of a near-isotropic solar microwave spectrum, stimulated Bremsstrahlung scattering provides the enhancement of electron collision frequency needed to account for the observed anomalous resistivity in flares. The anomalous resistivity systematically dominates over the classical resistivity in the flaring corona plasma by at least an order of magnitude. The runaway effect due to Coulomb collisions is suppressed. Thermal conduction is inhibited compared to the Spitzer conduction, in agreement with coronal seismology of slow-mode waves. Conclusions. Stimulated Bremsstrahlung scattering is found to be a key collisional process in flaring events of the solar corona. It can explain the above loop-top hard X-ray signal due to the fast electrons and the measured electrical conductivity due to the thermal electrons. As a perspective, the corresponding transport coefficients can be used in radiation MHD codes. To that aim, a simple model is proposed to self-consistently describe the transport coefficients and the infrared part of radiation spectrum. The radiation model could also be applied to stimulate large-angle electron scattering in the kinetic or hybrid models used to study the reconnecting regions of the solar corona.
Context. Globular clusters (GCs) are inherently non-spherical systems that in many cases show internal rotation. Typically, rotation is considered as the main driver of a GC’s morphology; however, the relationship between ellipticity and rotational support does not involve simple one-to-one mapping, and other multi-scale dynamical processes could contribute. Aims. We aim to provide a comprehensive interpretation of how morphology evolves in realistic models of rotating GCs and how it correlates with key physical ingredients and processes, including mass loss, stellar evolution, external tidal field, and two-body relaxation. Methods. Using the ROLLIN’ suite of direct N-body simulations, we compared two classical methods for measuring a GC’s shape – iso-density contours and the second-moment tensor method – and quantified their differences in measured ellipticity. We then measured the intrinsic ellipticity and triaxiality of our models using the tensor method and explored how these quantities change with time and the main physical mechanisms that drive them. Results. We find that the early evolution of GCs can be dominated by dynamical instabilities driven by internal rotation and velocity anisotropy, leading to the formation of bar-like structures that rapidly erode due to collisional effects around the time of the first core collapse. These bars are stronger and longer-lived (≲800 Myr) in strongly rotating clusters with longer relaxation times and subject to stellar evolution. In the long term, clusters evolve towards a less flattened and gradually triaxial configuration. This trend is more pronounced for clusters that (i) experience stronger mass loss, (ii) are more tidally filling and isotropic, and (iii) have lower rotational support. The inner (r50%) regions also show different morphologies. The inner parts of our models remain oblate and are dominated by internal rotation, while the outer region is triaxial and resembles the shape of the Roche lobe due to the external tidal field. Conclusions. Our models provide a physical explanation for the observational V/σ–ellipticity relation and demonstrate that morphology can serve as a reliable tracer of the dynamical state of the GC. Initially retrograde, dense, and inclined rotating models deviate systematically from the overall trend, providing physically motivated explanations for observational outliers. Upcoming large-scale photometric surveys will significantly benefit from comprehensive physical frameworks capable of interpreting GC evolution in terms of morphology and kinematics.
Context. With Gaia’s third Data Release (DR3), low-resolution BP/RP (XP) spectra became available for more than 219 million sources. In previous work, we developed a fast method for detecting absorption and emission lines while measuring their equivalent widths directly from the XP spectral coefficients. Aims. We conduct a systematic search for Hα emitters in the Gaia DR3 XP spectra. Methods. We measured the equivalent width of the Balmer Hα line for all sources with Gaia XP spectra and selected an initial sample of 556 100 sources depicting an Hα emission-like feature at 2σ level or above. Using a semi-supervised classifier complemented by manual spectral labelling, we removed sources that merely masqueraded as emission-line stars. Results. We find that 95% of the initial candidates are most likely cool M-type stars, where a molecular absorption band gives rise to a local maximum that can mimic Hα emission. Subsequently, 28 394 sources were flagged as bona fide Hα emitters using the Gaia DR3 XP spectra. The majority are Be-like candidates (26 287), M-type emission-line stars (717), Herbig Ae/Be candidates (525), quasi-stellar objects (204), Wolf–Rayet stars (177), cataclysmic variables (51), and carbon stars (18), among other sources. Our equivalent widths and stellar classifications agree well with results from higher-resolution studies and recent Gaia XP spectra-based catalogues. Ground-based spectroscopic follow-up for some of the newly found Hα emitters reinforces our confidence in the catalogue.
X-ray observations are a primary probe of the intracluster medium, and are widely used to infer galaxy cluster masses and scaling relations. We present and validate a pipeline to generate X-ray emission maps of galaxy groups and clusters from large cosmological simulations, and use it to study the origin of deviations from self-similarity across the group-to-cluster transition. We applied this pipeline to the Illustris-TNG300 and MillenniumTNG simulations, constructing X-ray emission maps and spectra based on APEC cooling functions. For simulations that do not explicitly track individual chemical abundances, we introduced a metallicity-based prescription that accurately reproduces the full spectral emission. We derived the LX–M500, Mgas–M500, and T–M500 scaling relations over 1012.5 ≤ M500 ≤ 1015.5 M⊙, compared them with observational data, and quantified the hydrostatic equilibrium and spectroscopic temperature biases through synthetic X-ray analyses. The simulated scaling relations are in good overall agreement with observations and are best described by broken power laws with a pivot at M500 = 1013.67 M⊙. At high masses, the slopes are close to self-similar expectations; at lower masses the relations steepen significantly, reflecting the growing importance of active galactic nuclei (AGN) feedback. X-ray hydrostatic masses are systematically underestimated by 15%, independently of cluster mass. When spectroscopic effects are included, the bias becomes mass-dependent, ranging from 15% at low masses to 21% at high masses. The recovered X-ray luminosity, measured in the 0.15–1 R500 aperture, is also mass-dependent: high-mass clusters are underestimated by 18%, while low-mass systems show discrepancies of up to 33%, driven by single-temperature spectral modelling of the gas outside the core. Despite these biases, the slope and normalisation of the M500–L500 scaling relation are only weakly affected, because the spectroscopic pipeline shifts both mass and luminosity simultaneously, absorbing a fraction of the bias along the intrinsic scaling relation rather than perpendicular to it.
Lopsidedness is a common feature in disk galaxies, but its origin and evolution remain poorly understood. Previous works have generally focused on stellar or gas lopsidedness separately. However, a combined analysis can help identify the mechanisms driving lopsidedness and constrain recent galactic evolution and environmental interactions. We study lopsidedness in the density and kinematics of the stellar and atomic hydrogen (HI) components of nine Milky Way-type galaxies from the Auriga Superstars cosmological zoom-in simulations. The high stellar mass resolution (mathrm m_ * =800, M_⊙ ) of the simulations improves the visibility of features in the stellar disk and halo, reduces the effect of noise on the measurement of lopsidedness, and provides a refined sampling of the gravitational potential. We quantified morphological and kinematical lopsidedness through the amplitude of the first Fourier mode (m=1) of the face-on projected mass distribution and radial velocity maps consistently for stars and gas and over 0.5-1, opt , where R_ opt is the stellar optical radius. At z=0, the morphological lopsidedness of the old stellar component ($>0.5$ Gyr old) strongly correlates with that of HI, tracing distortions in the global gravitational potential, while the young stellar population ($<0.5$ Gyr old) primarily traces asymmetric star formation along strong spiral arms. Morphological and kinematical lopsidedness are strongly correlated in stars but weakly correlated in HI, which typically exhibits stronger kinematical asymmetries. We also find that stellar disk lopsidedness is anticorrelated with bar strength, with strongly barred galaxies hosting more symmetric disks and higher central stellar mass densities. Based on our simulations of lopsidedness evolution over the past 7 Gyr, we observed that tidal interactions with massive satellites (mass ratio $>1$:50) produce coherent lopsided responses in stars and HI. In contrast, smooth gas accretion primarily drives lopsidedness in HI and young stars, while the total stellar component remains largely symmetric. Individual galaxies often experience multiple mechanisms, producing complex coupling and decoupling among distinct tracers.
