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Radio continuum and polarization observations reveal best the magnetic field structure and strength in nearby spiral galaxies. They show a similar magnetic field pattern, which is of spiral shape along the disk plane and X-shaped in the halo, sometimes accompanied by strong vertical fields above and below the central region of the disk. The strength of the total halo field is comparable to that of the disk. The small- and large-scale dynamo action is discussed to explain the observations with special emphasis on the rôle of star formation on the α − Ω dynamo and the magnetic field strength and structure in the disk and halo. Recently, with RM-synthesis of the CHANG-ES observations, we obtained the first observational evidence for the existence of regular magnetic fields in the halo. The analysis of the radio scale heights indicate escape-dominated radio halos with convective cosmic ray propagation for many galaxies. These galactic winds may be essential for an effective dynamo action and may transport large-scale magnetic field from the disk into the halo.

Understanding the physical mechanisms that control galaxy formation is a fundamental challenge in contemporary astrophysics. Recent advances in the field of astrophysical feedback strongly suggest that cosmic rays (CRs) may be crucially important for our understanding of cosmological galaxy formation and evolution. The appealing features of CRs are their relatively long cooling times and relatively strong dynamical coupling to the gas. In galaxies, CRs can be close to equipartition with the thermal, magnetic, and turbulent energy density in the interstellar medium, and can be dynamically very important in driving large-scale galactic winds. Similarly, CRs may provide a significant contribution to the pressure in the circumgalactic medium. In galaxy clusters, CRs may play a key role in addressing the classic cooling flow problem by facilitating efficient heating of the intracluster medium and preventing excessive star formation. Overall, the underlying physics of CR interactions with plasmas exhibit broad parallels across the entire range of scales characteristic of the interstellar, circumgalactic, and intracluster media. Here we present a review of the state-of-the-art of this field and provide a pedagogical introduction to cosmic ray plasma physics, including the physics of wave–particle interactions, acceleration processes, CR spatial and spectral transport, and important cooling processes. The field is ripe for discovery and will remain the subject of intense theoretical, computational, and observational research over the next decade with profound implications for the interpretation of the observations of stellar and supermassive black hole feedback spanning the entire width of the electromagnetic spectrum and multi-messenger data.

Approximately 10% of active galactic nuclei exhibit relativistic jets, which are powered by the accretion of matter onto supermassive black holes. Although the measured width profiles of such jets on large scales agree with theories of magnetic collimation, the predicted structure on accretion disk scales at the jet launch point has not been detected. We report radio interferometry observations, at a wavelength of 1.3 millimeters, of the elliptical galaxy M87 that spatially resolve the base of the jet in this source. The derived size of 5.5 ± 0.4 Schwarzschild radii is significantly smaller than the innermost edge of a retrograde accretion disk, suggesting that the M87 jet is powered by an accretion disk in a prograde orbit around a spinning black hole.

Galaxies are surrounded by large reservoirs of gas, mostly hydrogen, that are fed by inflows from the intergalactic medium and by outflows from galactic winds. Absorption-line measurements along the lines of sight to bright and rare background quasars indicate that this circumgalactic medium extends far beyond the starlight seen in galaxies, but very little is known about its spatial distribution. The Lyman-α transition of atomic hydrogen at a wavelength of 121.6 nanometres is an important tracer of warm (about 10 4 kelvin) gas in and around galaxies, especially at cosmological redshifts greater than about 1.6 at which the spectral line becomes observable from the ground. Tracing cosmic hydrogen through its Lyman-α emission has been a long-standing goal of observational astrophysics 1 – 3 , but the extremely low surface brightness of the spatially extended emission is a formidable obstacle. A new window into circumgalactic environments was recently opened by the discovery of ubiquitous extended Lyman-α emission from hydrogen around high-redshift galaxies 4 , 5 . Such measurements were previously limited to especially favourable systems 6 – 8 or to the use of massive statistical averaging 9 , 10 because of the faintness of this emission. Here we report observations of low-surface-brightness Lyman-α emission surrounding faint galaxies at redshifts between 3 and 6. We find that the projected sky coverage approaches 100 per cent. The corresponding rate of incidence (the mean number of Lyman-α emitters penetrated by any arbitrary line of sight) is well above unity and similar to the incidence rate of high-column-density absorbers frequently detected in the spectra of distant quasars 11 – 14 . This similarity suggests that most circumgalactic atomic hydrogen at these redshifts has now been detected in emission. Lyman-α emission from atomic hydrogen shows the location of warm gas and is ubiquitous around galaxies between redshifts of 3 and 6, thereby covering nearly all the sky.

Ninety per cent of baryons are located outside galaxies, either in the circumgalactic or intergalactic medium 1 , 2 . Theory points to galactic winds as the primary source of the enriched and massive circumgalactic medium 3 – 6 . Winds from compact starbursts have been observed to flow to distances somewhat greater than ten kiloparsecs 7 – 10 , but the circumgalactic medium typically extends beyond a hundred kiloparsecs 3 , 4 . Here we report optical integral field observations of the massive but compact galaxy SDSS J211824.06+001729.4. The oxygen [O  ii ] lines at wavelengths of 3726 and 3729 angstroms reveal an ionized outflow spanning 80 by 100 square kiloparsecs, depositing metal-enriched gas at 10,000 kelvin through an hourglass-shaped nebula that resembles an evacuated and limb-brightened bipolar bubble. We also observe neutral gas phases at temperatures of less than 10,000 kelvin reaching distances of 20 kiloparsecs and velocities of around 1,500 kilometres per second. This multi-phase outflow is probably driven by bursts of star formation, consistent with theory 11 , 12 . Theory predicts that winds expel baryons from galaxies into intergalactic space; now optical observations of the massive, but compact, galaxy SDSS J211824.06+001729.4 show that it is ejecting an enormous ionized outflow of gas.

Collimated outflows (jets) appear to be a ubiquitous phenomenon associated with the accretion of material onto a compact object. Despite this ubiquity, many fundamental physics aspects of jets are still poorly understood and constrained. These include the mechanism of launching and accelerating jets, the connection between these processes and the nature of the accretion flow, and the role of magnetic fields; the physics responsible for the collimation of jets over tens of thousands to even millions of gravitational radii of the central accreting object; the matter content of jets; the location of the region(s) accelerating particles to TeV (possibly even PeV and EeV) energies (as evidenced by γ-ray emission observed from many jet sources) and the physical processes responsible for this particle acceleration; the radiative processes giving rise to the observed multi-wavelength emission; and the topology of magnetic fields and their role in the jet collimation and particle acceleration processes. This chapter reviews the main knowns and unknowns in our current understanding of relativistic jets, in the context of the main model ingredients for Galactic and extragalactic jet sources. It discusses aspects specific to active Galactic nuclei (especially blazars) and microquasars, and then presents a comparative discussion of similarities and differences between them

Black holes generate collimated, relativistic jets, which have been observed in gamma-ray bursts (GRBs), microquasars, and at the center of some galaxies [active galactic nuclei (AGN)]. How jet physics scales from stellar black holes in GRBs to the supermassive ones in AGN is still unknown. Here, we show that jets produced by AGN and GRBs exhibit the same correlation between the kinetic power carried by accelerated particles and the gamma-ray luminosity, with AGN and GRBs lying at the low- and high-luminosity ends, respectively, of the correlation. This result implies that the efficiency of energy dissipation in jets produced in black hole systems is similar over 10 orders of magnitude in jet power, establishing a physical analogy between AGN and GRBs.

Large haloes of diffuse molecular gas discovered around high-redshift starburst galaxies show that galactic feedback, coupled to turbulence and gravity, extends the starburst phase instead of quenching it. Far-away gas feeds star formation Starburst galaxies in the early Universe form stars so rapidly that the gas reservoirs that feed star formation should quickly be depleted. However, star formation goes on for longer than can be explained by the amount of gas inside the galaxies. CH + is a useful tracer of the physical conditions of these galaxies because it can form in cold gas only in the presence of strong ultraviolet radiation or mechanical shocks. Edith Falgarone et al . report spectra in which they see CH + in both emission and absorption in a sample of starburst galaxies at redshifts of around 2.5. In emission, the CH + lines are very wide and result from shocks. In absorption, the lines reveal the presence of very turbulent gas extending far outside the star-forming cores of the galaxies. These findings suggest that the feedback process that regulates star formation involves reservoirs of gas far outside the galaxies. Starburst galaxies at the peak of cosmic star formation 1 are among the most extreme star-forming engines in the Universe, producing stars over about 100 million years (ref. 2 ). The star-formation rates of these galaxies, which exceed 100 solar masses per year, require large reservoirs of cold molecular gas 3 to be delivered to their cores, despite strong feedback from stars or active galactic nuclei 4 , 5 . Consequently, starburst galaxies are ideal for studying the interplay between this feedback and the growth of a galaxy 6 . The methylidyne cation, CH + , is a most useful molecule for such studies because it cannot form in cold gas without suprathermal energy input, so its presence indicates dissipation of mechanical energy 7 , 8 , 9 or strong ultraviolet irradiation 10 , 11 . Here we report the detection of CH + ( J  = 1–0) emission and absorption lines in the spectra of six lensed starburst galaxies 12 , 13 , 14 , 15 at redshifts near 2.5. This line has such a high critical density for excitation that it is emitted only in very dense gas, and is absorbed in low-density gas 10 . We find that the CH + emission lines, which are broader than 1,000 kilometres per second, originate in dense shock waves powered by hot galactic winds. The CH + absorption lines reveal highly turbulent reservoirs of cool (about 100 kelvin), low-density gas, extending far (more than 10 kiloparsecs) outside the starburst galaxies (which have radii of less than 1 kiloparsec). We show that the galactic winds sustain turbulence in the 10-kiloparsec-scale environments of the galaxies, processing these environments into multiphase, gravitationally bound reservoirs. However, the mass outflow rates are found to be insufficient to balance the star-formation rates. Another mass input is therefore required for these reservoirs, which could be provided by ongoing mergers 16 or cold-stream accretion 17 , 18 . Our results suggest that galactic feedback, coupled jointly to turbulence and gravity, extends the starburst phase of a galaxy instead of quenching it.

The missing mass problem has been with us since the 1970s, as Newtonian gravity using baryonic mass cannot account for various observations. We investigate the viability of retardation theory, an alternative to the Dark Matter paradigm (DM) which does not seek to modify the General Principal of Relativity but to improve solutions within it by exploring its weak field approximation to solve the said problem in a galactic context. This work presents eleven rotation curves calculated using Retardation Theory. The calculated rotation curves are compared with observed rotation curves and with those calculated using MOND. Values for the change in mass flux to mass ratio are extracted from the fitting process as a free fitting parameter. Those quantities are interpreted here and in previous works using given galactic processes. Retardation Theory was able to successfully reproduce rotation curves and a preliminary correlation with star birthrate index is seen, suggesting a possible link between galactic winds and observed rotation curves. Retardation Theory shows promising results within current observations. More research is needed to elucidate the suggested mechanism and the processes which contribute to it. Galactic mass outflows carried by galactic winds may affect rotation curves.

Galactic superbubbles are triggered by stellar feedback in the discs of star-forming galaxies. They are important in launching galactic winds, which play a key role in regulating the mass and energy exchange in galaxies. Observations can only reveal projected information and the 3D structure of such winds is quite complex. Therefore, numerical simulations are required to further our understanding of such structures. Here, we describe hydrodynamical simulations targeting two spatial scales. Large-scale superbubble models reveal supernova-driven outflows, and their subsequent merging, which leads to galactic wind formation. Additionally, the turbulence parameter σ t not only affects disc formation, but also influences mass and energy characteristics, controlling gas distribution and the injection rate in the simulated star formation zone. Small-scale wind-multicloud models indicate that isolated clouds are susceptible to instabilities, leading to fragmentation and dense gas destruction. In contrast, in closer cloud configurations, the condensation mechanism becomes important owing to hydrodynamic shielding, which helps to maintain the cold material throughout the evolution of the system. These simulations provide a comprehensive picture of galactic winds, showing how large-scale superbubble dynamics create the environment where small-scale wind-multicloud interactions shape the interstellar and circumgalactic media, ultimately regulating galaxy evolution.