ISM Simulations: An Overview of Models

Until recently the dynamical evolution of the interstellar medium (ISM) was simulated using collisional ionization equilibrium (CIE) conditions. However, the ISM is a dynamical system, in which the plasma is naturally driven out of equilibrium due to atomic and dynamic processes operating on different timescales. A step forward in the field comprises a multi-fluid approach taking into account the joint thermal and dynamical evolutions of the ISM gas.Comment: Overview paper (3 pages) presented by M. Avillez at the Special Session "Modern views of the interstellar medium", XXVIIIth IAU General Assembly, August 27-30, 2012, Beijing. Chin

The Generation and Dissipation of Interstellar Turbulence - Results from Large Scale High Resolution Simulations

We study, by means of adaptive mesh refinement hydro- and magnetohydrodynamical simulations that cover a wide range of scales (from kpc to sub-parsec), the dimension of the most dissipative structures and the injection scale of the turbulent interstellar gas, which we find to be about 75 pc, in agreement with observations. This is however smaller than the average size of superbubbles, but consistent with significant density and pressure changes in the ISM, which leads to the break-up of bubbles locally and hence to injection of turbulence. The scalings of the structure functions are consistent with log-Poisson statistics of supersonic turbulence where energy is dissipated mainly through shocks. Our simulations are different from previous ones by other authors as (i) we do not assume an isothermal gas, but have temperature variations of several orders of magnitude and (ii) we have no artificial forcing of the fluid with some ad hoc Fourier spectrum, but drive turbulence by stellar explosions at the Galactic rate, self-regulated by density and temperature thresholds imposed on the ISM gas.Comment: Five pages and three figures. Accepted for publication in Astrophysical Journal (Letters

Time-dependent galactic winds I. Structure and evolution of galactic outflows accompanied by cosmic ray acceleration

Cosmic rays are transported out of the galaxy by diffusion and advection due to streaming along magnetic field lines and resonant scattering off self-excited MHD waves. Thus momentum is transferred to the plasma via the frozen-in waves as a mediator assisting the thermal pressure in driving a galactic wind. The bulk of the Galactic CRs are accelerated by shock waves generated in SNRs, a significant fraction of which occur in OB associations on a timescale of several $10^7$ years. We examine the effect of changing boundary conditions at the base of the galactic wind due to sequential SN explosions on the outflow. Thus pressure waves will steepen into shock waves leading to in situ post-acceleration of GCRs. We performed simulations of galactic winds in flux tube geometry appropriate for disk galaxies, describing the CR diffusive-advective transport in a hydrodynamical fashion along with the energy exchange with self-generated MHD waves. Our time-dependent CR hydrodynamic simulations confirm the existence of time asymptotic outflow solutions (for constant boundary conditions). It is also found that high-energy particles escaping from the Galaxy and having a power-law distribution in energy ($\propto E^{-2.7}$) similar to the Milky Way with an upper energy cut-off at $\sim 10^{15}$ eV are subjected to efficient and rapid post-SNR acceleration in the lower galactic halo up to energies of $10^{17} - 10^{18}$ eV by multiple shock waves propagating through the halo. The particles can gain energy within less than $3\,$kpc from the galactic plane corresponding to flow times less than $5\cdot 10^6\,$years. The mechanism described here offers a natural solution to explain the power-law distribution of CRs between the "knee" and the "ankle". The mechanism described here offers a natural and elegant solution to explain the power-law distribution of CRs between the "knee" and the "ankle".Comment: 15 pages, 7 figure

Persistent infection of pets within a household with three Bartonella species.

We monitored by blood culture and immunofluorescence assay (IFA) bartonella infection in one dog and eight cats in a household to determine the prevalence and persistence of the infection as well as its transmissibility to humans. Ectoparasite control was rigorously exercised. During a 3-year period, Bartonella clarridgeiae was recovered from one cat on two occasions, and B. henselae was isolated from another cat on four occasions. During a 16-month period, B. vinsonii subsp. berkhoffii was isolated from the dog on 8 of 10 culture attempts. Despite extensive household contact, the pet owner was seronegative to all three species by IFA for Bartonella-specific immunoglobulin G

The Milky Way's Kiloparsec Scale Wind: A Hybrid Cosmic-Ray and Thermally Driven Outflow

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 wind's 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.Comment: 14 pages, 11 figures; Accepted to Ap

Hemotropic mycoplasmas in little brown bats (Myotis lucifugus).

BackgroundHemotropic mycoplasmas are epicellular erythrocytic bacteria that can cause infectious anemia in some mammalian species. Worldwide, hemotropic mycoplasmas are emerging or re-emerging zoonotic pathogens potentially causing serious and significant health problems in wildlife. The objective of this study was to determine the molecular prevalence of hemotropic Mycoplasma species in little brown bats (Myotis lucifugus) with and without Pseudogymnoascus (Geomyces) destrucans, the causative agent of white nose syndrome (WNS) that causes significant mortality events in bats.MethodsIn order to establish the prevalence of hemotropic Mycoplasma species in a population of 68 little brown bats (Myotis lucifugus) with (n = 53) and without (n = 15) white-nose syndrome (WNS), PCR was performed targeting the 16S rRNA gene.ResultsThe overall prevalence of hemotropic Mycoplasmas in bats was 47%, with similar (p = 0.5725) prevalence between bats with WNS (49%) and without WNS (40%). 16S rDNA sequence analysis (~1,200 bp) supports the presence of a novel hemotropic Mycoplasma species with 91.75% sequence homology with Mycoplasma haemomuris. No differences were found in gene sequences generated from WNS and non-WNS animals.ConclusionsGene sequences generated from WNS and non-WNS animals suggest that little brown bats could serve as a natural reservoir for this potentially novel Mycoplasma species. Currently, there is minimal information about the prevalence, host-specificity, or the route of transmission of hemotropic Mycoplasma spp. among bats. Finally, the potential role of hemotropic Mycoplasma spp. as co-factors in the development of disease manifestations in bats, including WNS in Myotis lucifugus, remains to be elucidated

Time-dependent galactic winds

Cosmic rays (CRs) are transported out of the galaxy by diffusion and advection due to streaming along magnetic field lines and resonant scattering off self-excited Magneto-Hydro-Dynamic (MHD) waves. Thus momentum is transferred to the plasma via the frozen-in waves as a mediator assisting the thermal pressure in driving a galactic wind. Galactic CRs (GCRs) are accelerated by shock waves generated in supernova remnants (SNRs), and they propagate from the disc into the halo. Therefore CR acceleration in the halo strongly depends on the inner disc boundary conditions. We performed hydrodynamical simulations of galactic winds in flux tube geometry appropriate for disc galaxies, describing the CR diffusive-advective transport in a hydrodynamical fashion (by taking appropriate moments of the Fokker-Planck equation) along with the energy exchange with self-generated MHD waves. Our time-dependent CR hydrodynamic simulations confirm that the evolution of galactic winds with feedback depends on the structure of the galactic halo. In case of a wind-structured halo, the wind breaks down after the last super nova (SN) has exploded. The mechanism described here offers a natural and elegant solution to explain the power-law distribution of CRs between the `knee' and the `ankle'. The transition will be naturally smooth, because the Galactic CRs accelerated at SN shocks will be `post-accelerated' by shocks generated at the inner boundary and travelling through the halo.Comment: Galaxies: evolution -- ISM: jets and outflows -- Galaxies: starburst -- supernova remnants -- cosmic ray

An XMM-Newton Observation of the Local Bubble Using a Shadowing Filament in the Southern Galactic Hemisphere

We present an analysis of the X-ray spectrum of the Local Bubble, obtained by simultaneously analyzing spectra from two XMM-Newton pointings on and off an absorbing filament in the Southern galactic hemisphere (b ~ -45 deg). We use the difference in the Galactic column density in these two directions to deduce the contributions of the unabsorbed foreground emission due to the Local Bubble, and the absorbed emission from the Galactic halo and the extragalactic background. We find the Local Bubble emission is consistent with emission from a plasma in collisional ionization equilibrium with a temperature $\log T_{LB} = 6.06^{+0.02}_{-0.04}$ and an emission measure of 0.018 cm^{-6} pc. Our measured temperature is in good agreement with values obtained from ROSAT All-Sky Survey data, but is lower than that measured by other recent XMM-Newton observations of the Local Bubble, which find $\log T_{LB} \approx 6.2$ (although for some of these observations it is possible that the foreground emission is contaminated by non-Local Bubble emission from Loop I). The higher temperature observed towards other directions is inconsistent with our data, when combined with a FUSE measurement of the Galactic halo O VI intensity. This therefore suggests that the Local Bubble is thermally anisotropic. Our data are unable to rule out a non-equilibrium model in which the plasma is underionized. However, an overionized recombining plasma model, while observationally acceptable for certain densities and temperatures, generally gives an implausibly young age for the Local Bubble (\la 6 \times 10^5 yr).Comment: Accepted for publication in the Astrophysical Journal. 16 pages, 9 figure

Metal Enrichment of the Intergalactic Medium in Cosmological Simulations

Observations have established that the diffuse intergalactic medium (IGM) at z ~ 3 is enriched to ~0.1-1% solar metallicity and that the hot gas in large clusters of galaxies (ICM) is enriched to 1/3-1/2 solar metallicity at z=0. Metals in the IGM may have been removed from galaxies (in which they presumably form) during dynamical encounters between galaxies, by ram-pressure stripping, by supernova-driven winds, or as radiation-pressure driven dust efflux. This study develops a method of investigating the chemical enrichment of the IGM and of galaxies, using already completed cosmological simulations. To these simulations, we add dust and (gaseous) metals, distributing the dust and metals in the gas according to three simple parameterized prescriptions, one for each enrichment mechanism. These prescriptions are formulated to capture the basic ejection physics, and calibrated when possible with empirical data. Our results indicate that dynamical removal of metals from >~ 3*10^8 solar mass galaxies cannot account for the observed metallicity of low-column density Ly-alpha absorbers, and that dynamical removal from >~ 3*10^10 solar mass galaxies cannot account for the ICM metallicities. Dynamical removal also fails to produce a strong enough mass-metallicity relation in galaxies. In contrast, either wind or radiation-pressure ejection of metals from relatively large galaxies can plausibly account for all three sets of observations (though it is unclear whether metals can be distributed uniformly enough in the low-density regions without overly disturbing the IGM, and whether clusters can be enriched quite as much as observed). We investigate in detail how our results change with variations in our assumed parameters, and how results for the different ejection processes compare. (Abridged)Comment: Minor revision, 1 figure added addressing diffusion of metals after their ejection. Accepted by ApJ. 31 EmulateApj Pages with 13 embedded postscript figure