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Register a new userThe Milky Ways Kiloparsec Scale Wind: A Hybrid Cosmic-Ray and Thermally Driven Outflow
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We apply a wind model, driven by combined cosmic-ray and thermal-gas pressure, to the Milky Way, and show that the observed Galactic diffuse soft X-ray emission can be better explained by a wind than by previous static gas models. We find that cosmic-ray pressure is essential to driving the observed wind. Having thus defined a best-fit model for a Galactic wind, we explore variations in the base parameters and show how the winds properties vary with changes in gas pressure, cosmic-ray pressure and density. We demonstrate the importance of cosmic rays in launching winds, and the effect cosmic rays have on wind dynamics. In addition, this model adds support to the hypothesis of Breitschwerdt and collaborators that such a wind may help explain the relatively small gradient observed in gamma-ray emission as a function of galactocentric radius.
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During its 2005 outburst, GRO J1655-40 was observed twice with the Chandra High Energy Transmission Grating Spectrometer; the second observation revealed a spectrum rich with ionized absorption lines from elements ranging from O to Ni (Miller et al. 2006a, 2008; Kallman et al. 2009), indicative of an outflow too dense and too ionized to be driven by radiation or thermal pressure. To date, this spectrum is the only definitive evidence of an ionized wind driven off the accretion disk by magnetic processes in a black hole X-ray binary. Here we present our detailed spectral analysis of the first Chandra observation, nearly three weeks earlier, in which the only signature of the wind is the Fe XXVI absorption line. Comparing the broadband X-ray spectra via photoionization models, we argue that the differences in the Chandra spectra cannot possibly be explained by the changes in the ionizing spectrum, which implies that the properties of the wind cannot be constant throughout the outburst. We explore physical scenarios for the changes in the wind, which we suggest may begin as a hybrid MHD/thermal wind, but evolves over the course of weeks into two distinct outflows with different properties. We discuss the implications of our results for the links between the state of the accretion flow and the presence of transient disk winds.
The centre of the Milky Way is the site of several high-energy processes that have strongly impacted the inner regions of our Galaxy. Activity from the super-massive black hole, Sgr A*, and/or stellar feedback from the inner molecular ring expel matter and energy from the disc in the form of a galactic wind. Multiphase gas has been observed within this outflow, from hot highly-ionized, to warm ionized and cool atomic gas. To date, however, there has been no evidence of the cold and dense molecular phase. Here we report the first detection of molecular gas outflowing from the centre of our Galaxy. This cold material is associated with atomic hydrogen clouds travelling in the nuclear wind. The morphology and the kinematics of the molecular gas, resolved on ~1 pc scale, indicate that these clouds are mixing with the warmer medium and are possibly being disrupted. The data also suggest that the mass of molecular gas driven out is not negligible and could impact the rate of star formation in the central regions. The presence of this cold, dense, high-velocity gas is puzzling, as neither Sgr A* at its current level of activity, nor star formation in the inner Galaxy seem viable sources for this material.
A long-lasting debate in space plasma physics concerns the nature of subproton-scale fluctuations in solar wind (SW) turbulence. Over the past decade, a series of theoretical and observational studies were presented in favor of either kinetic Alfven wave (KAW) or whistler turbulence. Here, we investigate numerically the nature of the subproton-scale turbulent cascade for typical SW parameters by means of unprecedented high-resolution simulations of forced hybrid-kinetic turbulence in two real-space and three velocity-space dimensions. Our analysis suggests that small-scale turbulence in this model is dominated by KAWs at $betagtrsim1$ and by magnetosonic/whistler fluctuations at lower $beta$. The spectral properties of the turbulence appear to be in good agreement with theoretical predictions. A tentative interpretation of this result in terms of relative changes in the damping rates of the different waves is also presented. Overall, the results raise interesting new questions about the properties and variability of subproton-scale turbulence in the SW, including its possible dependence on the plasma $beta$, and call for detailed and extensive parametric explorations of driven kinetic turbulence in three dimensions.
We investigate the efficiency and time-dependence of thermally and cosmic ray driven galactic winds for the metal enrichment of the intra-cluster medium (ICM) using a new analytical approximation for the mass outflow. The spatial distribution of the metals are studied using radial metallicity profiles and 2D metallicity maps of the model clusters as they would be observed by X-ray telescopes like XMM-Newton. Analytical approximations for the mass loss by galactic winds driven by thermal and cosmic ray pressure are derived from the Bernoulli equation and implemented in combined N-body/hydrodynamic cosmological simulations with a semi-analytical galaxy formation model. Observable quantities like the mean metallicity, metallicity profiles, and 2D metal maps of the model clusters are derived from the simulations. We find that galactic winds alone cannot account for the observed metallicity of the ICM. At redshift $z=0$ the model clusters have metallicities originating from galactic winds which are almost a factor of 10 lower than the observed values. For massive, relaxed clusters we find, as in previous studies, a central drop in the metallicity due to a suppression of the galactic winds by the pressure of the ambient ICM. Combining ram-pressure stripping and galactic winds we find radial metallicity profiles of the model clusters which agree qualitatively with observed profiles. Only in the inner parts of massive clusters the observed profiles are steeper than in the simulations. Also the combination of galactic winds and ram-pressure stripping yields too low values for the ICM metallicities. The slope of the redshift evolution of the mean metallicity in the simulations agrees reasonably well with recent observations.
Giant lobes of plasma extend 55 degrees above and below the Galactic Center, glowing in emission from gamma rays (the Fermi Bubbles) to microwaves (the WMAP haze) and polarized radio waves. We use ultraviolet absorption-line spectra from the Hubble Space Telescope to constrain the velocity of the outflowing gas within these regions, targeting the quasar PDS 456 (Galactic coordinates l,b=10.4, +11.2 degrees). This sightline passes through a clear biconical structure seen in hard X-ray and gamma-ray emission near the base of the northern Fermi Bubble. We report two high-velocity metal absorption components, at v_LSR=-235 and +250 km/s, which cannot be explained by co-rotating gas in the Galactic disk or halo. Their velocities are suggestive of an origin on the front and back side of an expanding biconical outflow emanating from the Galactic Center. We develop simple kinematic biconical outflow models that can explain these observed profiles with an outflow velocity of ~900 km/s and a full opening angle of ~110 degrees (matching the X-ray bicone). This indicates Galactic Center activity over the last ~2.5-4.0 Myr, in line with age estimates of the Fermi Bubbles. The observations illustrate the use of UV absorption-line spectroscopy to probe the properties of swept-up gas venting into the Fermi Bubbles.
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