Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userVirial shocks are suppressed in cosmic ray-dominated galaxy halos
298
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We study the impact of cosmic rays (CRs) on the structure of virial shocks, using a large suite of high-resolution cosmological FIRE-2 simulations accounting for CR injection by supernovae. In massive ($M_{rm halo} gtrsim 10^{11},M_{odot}$), low-redshift ($zlesssim 1-2$) halos, which are expected to form hot halos with slowly-cooling gas in quasi-hydrostatic equilibrium (with a stable virial shock), our simulations without CRs do exhibit clear virial shocks. The cooler phase condensing out from inflows becomes pressure-confined to over-dense clumps, embedded in low-density, volume-filling hot gas whose cooling time is much longer than inflow time. The gas thus transitions sharply from cool free-falling inflow, to hot and thermal-pressure supported at approximately the virial radius ($approx R_{rm vir}$), and the shock is quasi-spherical. With CRs, we previously argued that halos in this particular mass and redshift range build up CR-pressure-dominated gaseous halos. Here, we show that when CR pressure dominates over thermal pressure, there is no significant virial shock. Instead, inflowing gas is gradually decelerated by the CR pressure gradient and the gas is relatively subsonic out to and even beyond $R_mathrm{vir}$. Rapid cooling also maintains sub-virial temperatures in the inflowing gas within $sim R_mathrm{vir}$.
rate research
Read More
We investigate the impact of cosmic rays (CRs) on the circumgalactic medium (CGM) in FIRE-2 simulations, for ultra-faint dwarf through Milky Way (MW)-mass halos hosting star-forming (SF) galaxies. Our CR treatment includes injection by supernovae, anisotropic streaming and diffusion along magnetic field lines, collisional and streaming losses, with constant parallel diffusivity $kappasim3times10^{29},mathrm{cm^2 s^{-1}}$ chosen to match $gamma$-ray observations. With this, CRs become more important at larger halo masses and lower redshifts, and dominate the pressure in the CGM in MW-mass halos at $zlesssim 1-2$. The gas in these CR-dominated halos differs significantly from runs without CRs: the gas is primarily cool (a few $sim10^{4},$K), and the cool phase is volume-filling and has a thermal pressure below that needed for virial or local thermal pressure balance. Ionization of the low and mid ions in this diffuse cool gas is dominated by photo-ionization, with O VI columns $gtrsim 10^{14.5},mathrm{cm^{-2}}$ at distances $gtrsim 150,mathrm{kpc}$. CR and thermal gas pressure are locally anti-correlated, maintaining total pressure balance, and the CGM gas density profile is determined by the balance of CR pressure gradients and gravity. Neglecting CRs, the same halos are primarily warm/hot ($Tgtrsim 10^{5},$K) with thermal pressure balancing gravity, collisional ionization dominates, O VI columns are lower and Ne VIII higher, and the cool phase is confined to dense filaments in local thermal pressure equilibrium with the hot phase.
Cosmic rays (CRs) with ~GeV energies can contribute significantly to the energy and pressure budget in the interstellar, circumgalactic, and intergalactic medium (ISM, CGM, IGM). Recent cosmological simulations have begun to explore these effects, but almost all studies have been restricted to simplified models with constant CR diffusivity and/or streaming speeds. Physical models of CR propagation/scattering via extrinsic turbulence and self-excited waves predict transport coefficients which are complicated functions of local plasma properties. In a companion paper, we consider a wide range of observational constraints to identify proposed physically-motivated cosmic-ray propagation scalings which satisfy both detailed Milky Way (MW) and extra-galactic $gamma$-ray constraints. Here, we compare the effects of these models relative to simpler diffusion+streaming models on galaxy and CGM properties at dwarf through MW mass scales. The physical models predict large local variations in CR diffusivity, with median diffusivity increasing with galacto-centric radii and decreasing with galaxy mass and redshift. These effects lead to a more rapid dropoff of CR energy density in the CGM (compared to simpler models), in turn producing weaker effects of CRs on galaxy star formation rates (SFRs), CGM absorption profiles and galactic outflows. The predictions of the more physical CR models tend to lie in between models which ignore CRs entirely and models which treat CRs with constant diffusivity.
We present joint Suzaku and Chandra observations of MKW4. With a global temperature of 1.6 keV, MKW4 is one of the smallest galaxy groups that have been mapped in X-rays out to the virial radius. We measure its gas properties from its center to the virial radius in the north, east, and northeast directions. Its entropy profile follows a power-law of $propto r^{1.1}$ between R$_{500}$ and R$_{200}$ in all directions, as expected from the purely gravitational structure formation model. The well-behaved entropy profiles at the outskirts of MKW4 disfavor the presence of gas clumping or thermal non-equilibrium between ions and electrons in this system. We measure an enclosed baryon fraction of 11% at R$_{200}$, remarkably smaller than the cosmic baryon fraction of 15%. We note that the enclosed gas fractions at R$_{200}$ are systematically smaller for groups than for clusters from existing studies in the literature. The low baryon fraction of galaxy groups, such as MKW4, suggests that their shallower gravitational potential well may make them more vulnerable to baryon losses due to AGN feedback or galactic winds. We find that the azimuthal scatter of various gas properties at the outskirts of MKW4 is significantly lower than in other systems, suggesting that MKW4 is a spherically symmetric and highly relaxed system.
For the first time, we present the simultaneous detection and characterization of three distinct phases at $>10^5$ K in $z=0$ absorption, using deep $it{Chandra}$ observations toward Mrk 421. The extraordinarily high signal-to-noise ratio ($geqslant60$) of the spectra has allowed us to detect a $it{hot}$ phase of the Milky Way circumgalactic medium (CGM) at 3.2$^{+1.5}_{-0.5}times$ 10$^7$ K, coexisting with a $textit{warm-hot}$ phase at 1.5$pm$0.1$times$10$^6$ K and a $textit{warm}$ phase at 3.0$pm$0.4$times$10$^5$ K. The $textit{warm-hot}$ phase is at the virial temperature of the Galaxy, and the $textit{warm}$ phase may have cooled from the $textit{warm-hot}$ phase, but the super-virial $textit{hot}$ phase remains a mystery. We find that [C/O] in the $textit{warm}$ and $textit{warm-hot}$ phases, [Mg/O] in the $textit{warm-hot}$ phase and [Ne/O] in the $textit{hot}$ phase are super-solar, and the $textit{hot}$ and the $textit{warm-hot}$ phases are $alpha-$enhanced. Non-thermal line broadening is evident in the $textit{warm-hot}$ and the $textit{hot}$ phases and it dominates the total line broadening. Our results indicate that the $>10^5$ K CGM is a complex ecosystem. It provides insights on the thermal and chemical history of the Milky Way CGM, and theories of galaxy evolution.
We present a suite of high-resolution cosmological simulations, using the FIRE-2 feedback physics together with explicit treatment of magnetic fields, anisotropic conduction and viscosity, and cosmic rays (CRs) injected by supernovae (including anisotropic diffusion, streaming, adiabatic, hadronic and Coulomb losses). We survey systems from ultra-faint dwarf ($M_{ast}sim 10^{4},M_{odot}$, $M_{rm halo}sim 10^{9},M_{odot}$) through Milky Way masses, systematically vary CR parameters (e.g. the diffusion coefficient $kappa$ and streaming velocity), and study an ensemble of galaxy properties (masses, star formation histories, mass profiles, phase structure, morphologies). We confirm previous conclusions that magnetic fields, conduction, and viscosity on resolved ($gtrsim 1,$pc) scales have small effects on bulk galaxy properties. CRs have relatively weak effects on all galaxy properties studied in dwarfs ($M_{ast} ll 10^{10},M_{odot}$, $M_{rm halo} lesssim 10^{11},M_{odot}$), or at high redshifts ($zgtrsim 1-2$), for any physically-reasonable parameters. However at higher masses ($M_{rm halo} gtrsim 10^{11},M_{odot}$) and $zlesssim 1-2$, CRs can suppress star formation by factors $sim 2-4$, given relatively high effective diffusion coefficients $kappa gtrsim 3times10^{29},{rm cm^{2},s^{-1}}$. At lower $kappa$, CRs take too long to escape dense star-forming gas and lose energy to hadronic collisions, producing negligible effects on galaxies and violating empirical constraints from $gamma$-ray emission. But around $kappasim 3times10^{29},{rm cm^{2},s^{-1}}$, CRs escape the galaxy and build up a CR-pressure-dominated halo which supports dense, cool ($Tll 10^{6}$ K) gas that would otherwise rain onto the galaxy. CR heating (from collisional and streaming losses) is never dominant.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
