Numerical Study of Turbulent Mixing Layers with Non-Equilibrium Ionization Calculations
01 Jan 2010-Vol. 215
TL;DR: In this article, the authors investigate the physical properties of turbulent mixing layers and the production of high ions (C IV, N V, and O VI) using hydrodynamic simulations with radiative cooling and non-equilibrium ionization calculations.
Abstract: Highly ionized species, such as C IV, N V, and O VI, are commonly observed in diffuse gas in various places in the universe, such as in our Galaxy's disk and halo, high velocity clouds (HVCs), external galaxies, and the intergalactic medium. These ions are often used to trace hot gas whose temperature is a few times 105 K. One possible mechanism for producing high ions is turbulent mixing of cool gas (such as that in a high or intermediate velocity cloud) with hotter (a few times 106 K) gas in locations where these gases slide past each other. By using hydrodynamic simulations with radiative cooling and non-equilibrium ionization (NEI) calculations, we investigate the physical properties of turbulent mixing layers and the production of high ions (C IV, N V, and O VI). We find that most of the mixing occurs on the hot side of the hot/cool interface, where denser cool gas is entrained and mixed into the hotter, more diffuse gas. Our simulations reveal that the mixed region separates into a tepid zone containing radiatively cooled, C IV-rich gas and a hotter zone which is rich in C IV, N V, and O VI. The hotter zone contains a mixture of low and intermediate ions contributed by the cool gas and intermediate and high-stage ions contributed by the hot gas. Mixing occurs faster than ionization or recombination, making the mixed gas a better source of C IV, N V, and O VI in our NEI simulations than in our collisional ionization equilibrium (CIE) simulations. In addition, the gas radiatively cools faster than the ions recombine, which also allows large numbers of C IV, N V, and O VI ions to linger in the NEI simulations. For these reasons, our NEI calculations predict more C IV, N V, and O VI than our CIE calculations predict. We also simulate various initial configurations and find that more C IV is produced when the shear speed is smaller or the hot gas has a higher temperature. We find no significant differences between simulations having different perturbation amplitudes in the initial boundary between the hot and cool gas. We discuss the results of our simulations, compare them with observations of the Galactic halo and highly ionized HVCs, and compare them with other models, including other turbulent mixing calculations. The ratios of C IV to N V and N V to O VI are in reasonable agreement with the averages calculated from observations of the halo. There is a great deal of variation from sightline to sightline and with time in our simulations. Such spatial and temporal variation may explain some of the variation seen among observations.
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TL;DR: The circumgalactic medium (CGM) as mentioned in this paper is a multiphase medium characterized by rich dynamics and complex ionization states, and it is a source for a galaxy's star-forming fuel, the venue for galactic feedback and recycling, and perhaps the key regulator of the galactic gas supply.
Abstract: The gas surrounding galaxies outside their disks or interstellar medium and inside their virial radii is known as the circumgalactic medium (CGM). In recent years this component of galaxies has assumed an important role in our understanding of galaxy evolution owing to rapid advances in observational access to this diffuse, nearly invisible material. Observations and simulations of this component of galaxies suggest that it is a multiphase medium characterized by rich dynamics and complex ionization states. The CGM is a source for a galaxy's star-forming fuel, the venue for galactic feedback and recycling, and perhaps the key regulator of the galactic gas supply. We review our evolving knowledge of the CGM with emphasis on its mass, dynamical state, and coevolution with galaxies. Observations from all redshifts and from across the electromagnetic spectrum indicate that CGM gas has a key role in galaxy evolution. We summarize the state of this field and pose unanswered questions for future research.
546 citations
TL;DR: The local interstellar medium (ISM) is a flow of low-density, warm, and partially ionized interstellar material that has been sampled directly by in situ measurements of interstellar neutral gas and dust in the heliosphere.
Abstract: The Solar System is embedded in a flow of low-density, warm, and partially ionized interstellar material that has been sampled directly by in situ measurements of interstellar neutral gas and dust in the heliosphere. Absorption line data reveal that this interstellar gas is part of a larger cluster of local interstellar clouds, which is spatially and kinematically divided into additional small-scale structures indicating ongoing interactions. An origin for the clouds that is related to star formation in the Scorpius-Centaurus OB association is suggested by the dynamic characteristics of the flow. Variable depletions observed within the local interstellar medium (ISM) suggest an inhomogeneous Galactic environment, with shocks that destroy grains in some regions. Although photoionization models of the circumheliospheric ISM do an excellent job of reproducing the observed properties of the surrounding ISM, the unknown characteristics of the very low-density hot plasma filling the Local Bubble introduces uncertainty about the source of ionization and nature of cloud boundaries. Recent observations of small cold clouds provide new insight into the processes affecting the local ISM. A fuller understanding of the local ISM can provide insights into the past and future Galactic environment of the Sun, and deeper knowledge of the astrospheres of nearby stars.
235 citations
TL;DR: In this article, the authors analyze the physical conditions in the highly ionized circumgalactic medium (CGM) traced by OVI absorption lines observed in COS-Halos spectra.
Abstract: The total contribution of diffuse halo gas to the galaxy baryon budget strongly depends on its dominant ionization state. In this paper, we address the physical conditions in the highly ionized circumgalactic medium (CGM) traced by OVI absorption lines observed in COS-Halos spectra. We analyze the observed ionic column densities, absorption-line widths and relative velocities, along with the ratios of NV/OVI for 39 fitted Voigt profile components of OVI. We compare these quantities with the predictions given by a wide range of ionization models. Photoionization models that include only extragalactic UV background radiation are ruled out; conservatively, the upper limits to NV/OVI and measurements of N$_{\rm OVI}$ imply unphysically large path lengths > 100 kpc. Furthermore, very broad OVI absorption (b > 40 km s$^{-1}$) is a defining characteristic of the CGM of star-forming L* galaxies. We highlight two possible origins for the bulk of the observed OVI: (1) highly structured gas clouds photoionized primarily by local high energy sources or (2) gas radiatively cooling on large scales behind a supersonic wind. Approximately 20% of circumgalactic OVI does not align with any low-ionization state gas within $\pm$50 km s$^{-1}$ and is found only in halos with M$_{\rm halo}$ < 10$^{12}$ M$_{\odot}$. We suggest that this type of unmatched OVI absorption traces the hot corona itself at a characteristic temperature of 10$^{5.5}$ K. We discuss the implications of these very distinct physical origins for the dynamical state, gas cooling rates, and total baryonic content of L* gaseous halos.
171 citations
TL;DR: The Magellanic Stream as mentioned in this paper is an interwoven tail of filaments trailing the clouds in their orbit around the Milky Way, and it is a benchmark for hydrodynamical simulations of accreting gas and cloud/corona interactions.
Abstract: The Magellanic Clouds are surrounded by an extended network of gaseous structures. Chief among these is the Magellanic Stream, an interwoven tail of filaments trailing the Clouds in their orbit around the Milky Way. When considered in tandem with its Leading Arm, the Stream stretches over 200° on the sky. The Stream is thought to represent the result of tidal interactions between the Clouds and ram-pressure forces exerted by the Galactic corona, and its kinematic properties reflect the dynamical history of the pair of dwarf galaxies closest to the Milky Way. The Stream is a benchmark for hydrodynamical simulations of accreting gas and cloud/corona interactions. If the Stream survives these interactions and arrives safely in the Galactic disk, its cargo of over a billion solar masses of gas has the potential to maintain or elevate the Galactic star-formation rate. In this article, we review the current state of knowledge of the Stream, including its chemical composition, physical conditions, origin, and fa...
154 citations
01 Jan 2013
TL;DR: In this article, the authors used XMM-Newton Reflection Grating Spectrometer archival data and measured O VII K{alpha} absorption-line strengths toward 26 active galactic nuclei, LMC X-3, and two Galactic sources (4U 1820-30 and X1735-444).
Abstract: The Milky Way's million degree gaseous halo contains a considerable amount of mass that, depending on its structural properties, can be a significant mass component. In order to analyze the structure of the Galactic halo, we use XMM-Newton Reflection Grating Spectrometer archival data and measure O VII K{alpha} absorption-line strengths toward 26 active galactic nuclei, LMC X-3, and two Galactic sources (4U 1820-30 and X1735-444). We assume a {beta}-model as the underlying gas density profile and find best-fit parameters of n{sub circle} = 0.46{sup +0.74}{sub -0.35} cm{sup -3}, r{sub c} = 0.35{sup +0.29}{sub -0.27} kpc, and {beta} = 0.71{sup +0.13}{sub -0.14}. These parameters result in halo masses ranging between M(18 kpc) = 7.5{sub -4.6}{sup +}2{sup 2.0} x 10{sup 8} M{sub Sun} and M (200 kpc) = 3.8{sub -0.5}{sup +6.0} x 10{sup 10} M{sub Sun} assuming a gas metallicity of Z = 0.3 Z{sub Sun }, which are consistent with current theoretical and observational work. The maximum baryon fraction from our halo model of f{sub b} = 0.07{sup +0.03}{sub -0.01} is significantly smaller than the universal value of f{sub b} = 0.171, implying the mass contained in the Galactic halo accounts for 10%-50% of the missing baryons in the Milky Way.more » We also discuss our model in the context of several Milky Way observables, including ram pressure stripping in dwarf spheroidal galaxies, the observed X-ray emission measure in the 0.5-2 keV band, the Milky Way's star formation rate, spatial and thermal properties of cooler gas ({approx}10{sup 5} K), and the observed Fermi bubbles toward the Galactic center. Although the metallicity of the halo gas is a large uncertainty in our analysis, we place a lower limit on the halo gas between the Sun and the Large Magellanic Cloud (LMC). We find that Z {approx}> 0.2 Z{sub Sun} based on the pulsar dispersion measure toward the LMC.« less
127 citations
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