Does cosmic-ray (CR) pressure matter for the circumgalactic medium (CGM)? While prior studies suggest that CRs influence the thermal and dynamical state of the CGM, their overall importance remains an open question due to limited observational constraints and the strong dependence of simulation predictions on the implementation of CR physics. In the context of the multiphase CGM, the role of CRs remains underexplored, especially in governing the formation and survival of cold gas clouds and structures that are expected to fuel future star formation in the host galaxy. We investigate how CRs affect ram-pressure-stripped cold gas clouds originating from satellite galaxies in a Milky Way-like halo. Using high-resolution simulations with varying CR energy densities, we find that CRs can significantly modify the size and survival of stripped clouds. Specifically, CR pressure "puffs up" cold clouds, increasing their surface area and enabling more efficient mixing-layer cooling, which allows them to grow in mass. This enhanced growth results in higher cold gas inflow rates into the central galaxy, leading to an increased star formation rate compared to the no-CR case at later times. Moreover, CRs can boost the total cold gas mass in the CGM by up to a factor of 4. These effects are most pronounced in simulations where the CR energy density is in equipartition with the thermal gas. We show that CRs can play a critical role in regulating the cold phase of the CGM contributed by satellites, and therefore their ability to feed galaxies.

Modern X-ray and gamma-ray instruments are revealing a growing class of Galactic nonthermal sources whose emission centroids are measurably offset from the nearest plausible sites of cosmic-ray (CR) acceleration. Such "displaced" sources are seen in keV X-rays and TeV-PeV gamma-rays but not in GeV gamma-rays, have hard spectra, and are not associated with gas clumps, suggesting a leptonic origin. We develop a general framework for understanding displacement, whereby relativistic CR electrons (CRe) injected into the interstellar medium with a strongly anisotropic pitch-angle distribution propagate a finite distance from their acceleration site before scattering processes isotropize their directions sufficiently for the emission to become visible. We use CR transport simulations to investigate under what circumstances displacement is likely, finding that it requires an initial pitch angle distribution less than or similar to 45 degrees wide, a line of sight broadly edge-on to the magnetic field, and that the source be measured in a waveband where emission is dominated by CRe for which the radiative-loss and pitch-angle-scattering timescales are comparable. For typical Galactic conditions, the latter condition is satisfied only for CRe energies greater than or similar to 10 TeV, explaining why displaced sources appear at X-ray and TeV but not GeV energies. We further show that, when displacement is detected, it allows for a direct inference of the CRe pitch-angle scattering rate.

Feedback from active galactic nuclei (AGNs) is a key physical mechanism proposed to regulate galaxy formation and suppress star formation, primarily in massive galaxies. Cosmic rays (CRs) associated with AGN jets can efficiently suppress cooling flows and quench star formation, but the locus of CR production and their coupling to gas are crucial to self-regulation. We conduct high-resolution, non-cosmological magnetohydrodynamic (MHD) simulations of a massive 10(14) M-(R) halo using the FIRE-2 (Feedback In Realistic Environments) stellar feedback model. We explore AGN jet feedback with CRs by varying the CR energy fraction in jets, the CR coupling sites (in the vicinity of the black hole versus at the shock fronts of large-scale jet cocoons), and jet precession parameters. Our findings indicate that injecting CRs near the black hole efficiently inhibits accretion by lowering the local gas density before the jet propagates to large radii. This produces episodic accretion and leaves the jet with insufficient energy flux to reach large radii and impact cooling flows. By contrast, injecting CRs at the shock front of the jet cocoon sustains a higher jet energy flux for longer and disperses CRs to larger radii. This configuration more effectively suppresses the cooling flow. The period and angle of jet precession influence shock-front positions. We identify an optimal range of precession periods of order tens of Myr that places shocks in the inner circumgalactic medium (CGM), where cooling flows are most severe. We report that this configuration most effectively suppresses cooling flows and quenches star formation.

The North Polar Spur (NPS) is a prominent diffuse X-ray feature whose origin has remained uncertain for decades. Using a uniform analysis of archival Suzaku and XMM-Newton data with new Chandra observations, we constrain its thermal and chemical properties. The NPS emission is fully absorbed by the neutral interstellar medium, demonstrating that the plasma lies beyond the Galactic disk and is not a local supernova remnant or nearby superbubble. The spectra require a two-temperature model with a warm-hot component (kT approximate to 0.2 keV) and a hotter component (kT = 0.4-0.7 keV), with emission measures of (41.8 +/- 4.9) & times; 10-3 and (12.9 +/- 2.2) & times; 10-3cm-6 pc, respectively. A key result is the detection of supersolar abundance ratios in the warm-hot phase, with N/O =3.6 +/- 0.3 and Ne/O =1.9 +/- 0.1 solar. A Suzaku observation of the outer South Polar Spur (SPS) shows similar absorption, temperatures, and enhanced abundances (N/O =2.9 +/- 0.4, Ne/O =1.6 +/- 0.2), though with lower emission measures. The similar supersolar abundance ratios suggest a common enrichment history. These properties are consistent with those measured along other sightlines through the X-ray-bright shells of the Galactic bubbles. Together, these results support that the NPS and SPS trace opposite limbs of the Galactic bubbles. The chemical properties suggest a strong contribution from stellar feedback in shaping the Galactic bubbles.

Recent observations have revealed a supervirial temperature gas phase at log( T /K) ∼ 7 in the Milky Way, challenging existing galaxy formation models. This hot gas phase was discovered toward extragalactic absorption sightlines and blank-sky emission fields, both at high Galactic latitudes. The location of this hot component is unknown; is it in the extended circumgalactic medium (CGM) or in the interstellar medium (ISM) instead? We analyzed X-ray spectra from Chandra’s High-Energy Transmission Grating observations of 27 Galactic X-ray binaries (XRBs) to investigate whether the hot gas component is present in the ISM. We searched for absorption lines of S xvi K α , Si xiv K α , and Ne x K α , which are the telltale signatures of the hot gas and which have been detected toward extragalactic sightlines. Of the 27 targets, these lines were detected in the spectra of only 7, with two sources displaying broad line features likely intrinsic to the XRB systems. Additionally, most of the detected lines are time variable, reinforcing their likely association with the XRBs. Our results suggest that the supervirial temperature gas is not a widespread component of the ISM but may instead be located in extraplanar regions or the extended CGM, which aligns with some recent simulation results.

We present the first simultaneous detection of four distinct highly ionized z=0 absorbing phases using Chandra and XMM-Newton grating spectra toward the blazar PKS 2155-304. We detect the MgXII Kα absorption line for the first time in the circumgalactic medium (CGM) of the Milky Way. Along with MgXII Kα, we detect SiXIV Kα absorption, which are the tell-tale signatures of the hot 'super-virial' gas in the CGM. Both from the model-independent calculations and hybrid-ionization modeling, we infer four phases at distinct temperatures, hot 'super-virial' (5.4^+1.9_-0.8× 10^7 K), warm-hot 'virial' (1.8^+0.3_-0.2× 10^6 K), warm 'sub-virial' (2.2± 0.5 × 10^5 K), and cool phase (<1.7 × 10^5 K). The warm-hot and hot phases are α-enhanced, and [C/O] and [Ne/O] are super-solar in the warm-hot phase, while [Mg/O] and [Si/O] are super-solar in the hot phase. The low-ionization lines are blue-shifted (v_ los≈ -100 km s^-1), whereas the high-ionization lines are red-shifted. It suggests a scenario of infalling sub-virial, quasi-static virial, and outflowing super-virial phases along this sightline. Earlier studies on individual sightlines were confined to the Northern Hemisphere. Our sightline is located in the Southern hemisphere, demonstrating that hot super-virial gas is also present at Southern Galactic latitudes as well. This confirms a more widespread distribution of the super-virial gas across both hemispheres.

Shadow observations provide a powerful tool to separate foreground components of the soft diffuse X-ray background (SDXB) from the background components. Such observations have now established that the “local” foreground is made of the solar wind charge exchange and the local bubble, and the background emission is from the extended circumgalactic medium (CGM) of the Milky Way and from the unresolved extragalactic sources. New data and careful analyses of the SDXB led to two new discoveries in recent years: (1) excess emission near 0.5 keV that is identified as the NVII emission line, and (2) excess emission near 0.8-1.0 keV that is identified with an additional, super-virial temperature hot thermal component of the CGM. The goal of this paper is to use Suzaku shadow observations along six sightlines to determine whether either of these components is from the “local” sources. We eliminate the ambiguity regarding the origin of NVII emission, ruling out the local origin. We confirm that the Milky Way CGM contains nitrogen-rich plasma, with a super-solar average (N/O) of 2.6+-0.5, and suggest that nitrogen-enhanced plasma is widespread throughout the CGM. We find super-solar Ne abundance in two sighlines, also from the CGM. Similarly, we rule out the local origin of the hot thermal component and confirm that it is present beyond the shadowing clouds. Furthermore, we provide a revised model of the soft diffuse X-ray background, which is crucial for extragalactic astronomy.

ABSTRACT Hot, $\log (T/\mathrm{K})$$\sim$ 7.5, gas was recently discovered in the Milky Way in extragalactic sightlines. In order to determine its location, here we present sightlines to Galactic X-ray binaries (XRBs) passing through the Interstellar Medium (ISM). In this pilot study, we investigate absorption features of S$_{\mathrm{XVI}}$, Si$_{\mathrm{XIV}}$, and Ne$_{\mathrm{X}}$ in the spectra of three XRBs, namely 4U 1735–44, 4U 1820–30, and Cyg X-2, using Chandra High Energy Transmission Grating archival observations. We do not detect any of these lines. We determine the 2$\sigma$ upper limit for the equivalent widths of the undetected absorption lines and the column densities of the corresponding ions. We note that the 2$\sigma$ upper limits for S$_{\mathrm{XVI}}$ K$\alpha$ and Si$_{\mathrm{XIV}}$ K$\alpha$ are an order of magnitude smaller than those previously detected in the extragalactic sightlines. Our finding suggests that if any gas at $\log (T/\mathrm{K})$$\gt\ 7$ is present in the Galactic ISM, it is unlikely to be ubiquitous. This is an important result because it implies that S$_{\mathrm{XVI}}$, Si$_{\mathrm{XIV}}$, and Ne$_{\mathrm{X}}$ absorption detected in extragalactic sightlines is not from the ISM, but is likely from a hot gas phase in the extraplanar region beyond the ISM or in the extended Circumgalactic Medium.

In an effort to understand the presence of super-virial gas detected in the Milky Way, we present our findings from isolated galaxy simulations of Milky Way-like systems using GIZMO with the FIRE-2 (Feedback In Realistic Environments) stellar feedback model. It unveils the presence of a significant super-virial temperature ( T>6×10^6K) gas component within 20 kpc from the galactic center. This super-virial gas has a mass of 1-2×10^7 M_⊙ and is found close to the disk, where typical gas densities are 0.004-0.01 cm^-3. We find that some of the virial gas (T∼10^6K) forms a rotating hot inflow, where gravitational energy is converted to heat mainly via compressive heating. This process causes gas infalling close to the rotation axis to reach super-virial temperatures just before cooling and joining the disk. Stellar feedback heating accounts for less than 1

Recent observations and simulations reveal that the circumgalactic medium (CGM) surrounding galaxies is multiphase, with the gas temperatures spanning a wide range at most radii, similar to 10(4) K to the virial temperature (similar to 10(6) K for Milky Way). Traditional CGM models using simple density profiles are inadequate at reproducing observations that indicate a broad temperature range. Alternatively, a model based on probability distribution functions (PDFs) with parameters motivated by simulations can better match multiwavelength observations. In this work, we use log-normal distributions, commonly seen in the simulations of the multiphase interstellar and circumgalactic media, to model the multiphase CGM. We generalize the isothermal background model by Faerman et al. to include more general CGM profiles. We extend the existing probabilistic models from 1D-PDFs in temperature to 2D-PDFs in density-temperature phase space and constrain its parameters using a Milky Way-like Illustris TNG50-1 halo. We generate various synthetic observables such as column densities of different ions, UV/X-ray spectra, and dispersion and emission measures. X-ray and radio (Fast Radio Burst) observations mainly constrain the hot gas properties. However, interpreting cold/warm phase diagnostics is not straightforward since these phases are patchy, with inherent variability in intercepting these clouds along arbitrary lines of sight. We provide a tabulated comparison of model predictions with observations and plan to expand this into a comprehensive compilation of models and data. Our modelling provides a simple analytical framework that is useful for describing important aspects of the multiphase CGM.

The origin of the cold phase in the CGM is a highly debated question. We investigate the contribution of satellite galaxies to the cold gas budget in the circumgalactic medium (CGM)of a Milky Way-like host galaxy. We perform controlled experiments with three different satellite mass distributions and identify several mechanisms by which satellites can add cold gas to the CGM, including ram pressure stripping and induced cooling in the mixing layer of the stripped cold gas. These two mechanisms contribute a comparable amount of cold gas to the host CGM. We find that the less massive satellites ($\leq 10^9 M_\odot$) not only lose all of their cold gas in a short period ($\sim$ 0.5-1 Gyr), but their stripped cold clouds also mix with the hot CGM gas and get heated up quickly. However, stellar feedback from these less massive satellites can hugely alter the fate of their stripped gas. Feedback speeds up the destruction of the stripped cold clouds from these satellites by making them more diffuse with more surface area. On the other hand, the more massive satellites (LMC or SMC-like $\sim 10^{10} M_\odot$) can add cold gas to the total gas budget of the host CGM for several Gyrs.

ABSTRACT We discuss the production of γ-rays from cosmic rays (CR) in the circumgalactic medium (CGM) of Andromeda (M31) in light of the recent detection of γ-rays from an annular region of ∼5.5–120 kpc away from the M31 disc. We consider the CRs accelerated as a result of the star formation in the M31 disc, which are lifted to the CGM by advection due to outflow and CR diffusion. The advection time-scale due to bulk flow of gas triggered by star formation activity in the M31 disc is comparable (∼Gyr) to the diffusion time-scale with diffusion coefficient ≥1029 cm2 s−1 for the propagation of CR protons with energy ∼412 GeV that are responsible for the highest energy photons observed. We show that a leptonic origin of the γ-rays from cosmic ray (CR) electrons has difficulties, as the inverse Compton time-scale (∼Myr) is much lower than advection time-scale (∼Gyr) to reach 120 kpc. Invoking CR electrons accelerated by accretion shocks in the CGM at ∼100–120 kpc does not help since it would lead to diffuse X-ray features that are not observed. We, therefore, study the production of γ-rays via hadronic interaction between CR protons and CGM gas with the help of numerical two-fluid (thermal + CR) hydrodynamical simulation. We find that a combination of these mechanisms that are related to the star formation processes in M31 in the last ∼Gyr, along with diffusion and hadronic interaction, can explain the observed flux from the CGM of M31.

We constrain the cosmic-ray (CR) population in the circumgalactic medium (CGM) of the Milky Way by comparing the observations of absorption lines of O viii ions with predictions from analytical models of the CGM: the precipitation (PP) and isothermal (IT) models. For a CGM in hydrostatic equilibrium, the introduction of CR suppresses thermal pressure and affects the O viii ion abundance. We explore the allowances given to the ratio of CR pressure to thermal pressure (P (CR)/P (th) = eta), with varying boundary conditions, CGM mass content, photoionization by extragalactic ultraviolet background, and temperature fluctuations. We find that the allowed maximum values of eta are eta less than or similar to 10 in the PP model and eta less than or similar to 6 in the IT model. We also explore the spatial variation of eta: rising (eta = Ax) or declining (eta = A/x) with radius, where A is the normalization of the profiles. In particular, the models with a declining ratio of CR to thermal pressure fare better than those with a rising ratio with suitable temperature fluctuation (higher sigma (lnT) for PP and lower for IT). The declining profiles allow A less than or similar to 8 and A less than or similar to 10 in the case of the IT and PP models, respectively, thereby accommodating a large value of eta (similar or equal to 200) in the central region but not in the outer regions. These limits, combined with the limits derived from the gamma-ray and radio background, can be useful for building models of the Milky Way CGM including the CR population. However, the larger amount of CRs can be packed in the cold phase, which may be one way to circumvent these constraints.

We consider a model of the circumgalactic medium (CGM) in which feedback maintains a constant ratio of cooling time to free-fall time throughout the halo, so that the entire CGM is marginally unstable to multiphase condensation. This 'precipitation model' is motivated by observations of multiphase gas in the cores of galaxy clusters and the haloes of massive ellipticals. From the model, we derive the density and temperature profiles for the CGM around galaxies with masses similar to the Milky Way. After taking into consideration the geometrical position of our Solar system in the Milky Way, we show that the CGM model is consistent with observed O VI, O VII, O VIII column densities and the ratio of O VII and O VIII column densities only if temperature fluctuations with a lognormal dispersion sigma(lnT) similar to 0.6-1.0 are included. We show that O VI column densities observed around star-forming galaxies require systematically greater values of sigma(lnT) than around passive galaxies, implying a connection between star formation in the disc and the state of the CGM. Photoionization by an extragalactic ultraviolet background radiation does not significantly change these CGM features for galaxies like the Milky Way but has much greater and significant effects on the CGM of lower mass galaxies.

We study the interaction of cosmic rays (CRs) with the diffuse circumgalactic gas of the Milky Way (MW) galaxy that results in hadronic gamma-ray emission and radio synchrotron emission. We aim to constrain the CR population in our circumgalactic medium (CGM) with the help of the observed isotropic gamma-ray background (IGRB), its anisotropy, and radio continuum. We modify different models of CGM gas in hydrostatic equilibrium discussed in the literature by including a cosmic-ray population, parameterized by eta P-CR/P-th. For the simplest isothermal model, while the IGRB intensity allows eta less than or similar to 3, the anisotropy resulting from the solar system's off-center position in MW rules out all values of eta. For the precipitation model, in which the cooling of the CGM gas is regulated with an optimum ratio of cooling time to freefall time, while the observed IGRB intensity allows eta less than or similar to 230, the observed anisotropy allows only very large values of eta, of order greater than or similar to 100. The radio continuum limits eta less than or similar to 400 for the precipitation model and does not constrain the isothermal model; however, these constraints are mitigated by synchrotron loss time being comparable to CR diffusion timescales. These bounds are relevant for current numerical simulations that indicate a significant CR population in CGM of galaxies of MW mass.