L. Matthew Haffner

Vertical Structure of a Supernova-driven Turbulent, Magnetized Interstellar Medium

Stellar feedback drives the circulation of matter from the disk to the halo of galaxies. We perform three-dimensional magnetohydrodynamic simulations of a vertical column of the interstellar medium with initial conditions typical of the solar circle in which supernovae drive turbulence and determine the vertical stratification of the medium. The simulations were run using a stable, positivity-preserving scheme for ideal MHD implemented in the FLASH code. We find that the majority (90%) of the mass is contained in thermally stable temperature regimes of cold molecular and atomic gas at T 3 kpc. The magnetic field in our models has no significant impact on the scale heights of gas in each temperature regime; the magnetic tension force is approximately equal to and opposite the magnetic pressure, so the addition of the field does not significantly affect the vertical support of the gas. The addition of a magnetic field does reduce the fraction of gas in the cold (<200 K) regime with a corresponding increase in the fraction of warm (~104 K) gas. However, our models lack rotational shear and thus have no large-scale dynamo, which reduces the role of the field in the models compared to reality. The supernovae drive oscillations in the vertical distribution of halo gas, with the period of the oscillations ranging from 30 Myr in the T < 200 K gas to ~100 Myr in the 106 K gas, in line with predictions by Walters & Cox.

THE TURBULENT WARM IONIZED MEDIUM: EMISSION MEASURE DISTRIBUTION AND MHD SIMULATIONS

We present an analysis of the distribution of Hemission measures for the warm ionized medium (WIM) of the GalaxyusingdatafromtheWisconsinHMapper(WHAM)NorthernSkySurvey.OursampleisrestrictedtoGalactic latitudes jbj > 10 � .Weremovedsightlinesintersecting19high-latitudeclassicalHiiregions,leavingonlysightlines that sample the diffuse WIM. The distribution of EM sin jbjfor the diffuse WIM sample is poorly characterized by a single normal distribution, but is extraordinarily wellfit by a lognormal distribution, with hlog EM sin jbj(pc cm � 6 ) � 1 i¼ 0:146 � 0:001 and standard deviationlog EM sin jbj ¼ 0:190 � 0:001. The value of log EM sin jbj hi drops from 0:260 � 0:002 at Galactic latitude 10 < jbj < 30 to 0:038 � 0:002 at Galactic latitude 60 < jbj < 90. The distribution maywidenslightly atlowGalacticlatitude.WecomparetheobservedEMdistributionfunctiontothepredictionsof three- dimensional magnetohydrodynamic simulations of isothermal turbulence within a nonstratified interstellar medium. We find that the distribution of EM sin jbj is well described by models of mildly supersonic turbulence with a sonic Mach number of � 1.4Y2.4. The distribution is weakly sensitive to the magnetic field strength. The model also successfully predictsthedistributionof dispersionmeasuresof pulsarsandHlineprofiles.InthebestfittingmodeltheturbulentWIM occupies a vertical path length of 400Y500 pc within the 1.0Y1.8 kpc scale height of the layer. The WIM gas has a lognormal distribution of densities with a most probable electron density npk � 0:03 cm � 3 . We also discuss the impli- cations of these results for interpreting the filling factor, the power requirement, and the magnetic field of the WIM. Subject headingg ISM: structure — MHD — turbulence

PHOTOIONIZATION OF HIGH-ALTITUDE GAS IN A SUPERNOVA-DRIVEN TURBULENT INTERSTELLAR MEDIUM

We investigate models for the photoionization of the widespread diffuse ionized gas (DIG) in galaxies. In particular, we address the long standing question of the penetration of Lyman continuum photons from sources close to the galactic midplane to large heights in the galactic halo. We find that recent hydrodynamical simulations of a supernova-driven interstellar medium (ISM) have low-density paths and voids that allow for ionizing photons from midplane OB stars to reach and ionize gas many kiloparsecs above the midplane. We find that ionizing fluxes throughout our simulation grids are larger than predicted by one-dimensional slab models, thus allowing for photoionization by O stars of low altitude neutral clouds in the Galaxy that are also detected in Hα. In previous studies of such clouds, the photoionization scenario had been rejected and the Hα had been attributed to enhanced cosmic ray ionization or scattered light from midplane H II regions. We do find that the emission measure distributions in our simulations are wider than those derived from Hα observations in the Milky Way. In addition, the horizontally averaged height dependence of the gas density in the hydrodynamical models is lower than inferred in the Galaxy. These discrepancies are likely due to the absence of magnetic fields in the hydrodynamic simulations and we discuss how magnetohydrodynamic effects may reconcile models and observations. Nevertheless, we anticipate that the inclusion of magnetic fields in the dynamical simulations will not alter our primary finding that midplane OB stars are capable of producing high-altitude DIG in a realistic three-dimensional ISM.

IONIZED GAS IN THE SMITH CLOUD

We present Wisconsin Hα Mapper observations of ionized gas in the Smith Cloud, a high-velocity cloud which Lockman et al. have recently suggested is interacting with the Galactic disk. Our Hα map shows the brightest Hα emission, 0.43 ± 0.04 R, coincident with the brightest Hi, while slightly fainter Hα emission (0.25 ± 0.02 R) is observed in a region with Hi intensities < 0.1 times as bright as the brightest Hi. We derive an ionized mass of 3 × 10 6 M� , comparable to the Hi mass, with the H + mass spread over a considerably larger area than that of Hi. An estimated Galactic extinction correction could adjust these values upward by 40%. Hα and [Sii] line widths toward the region of brightest emission constrain the electron temperature of the gas to be between 8000 K and 23,000 K. A detection of [Nii] λ6583 in the same direction with a line ratio [Nii]/Hα = 0.32 ± 0.05 constrains the metallicity of the cloud: for typical photoionization temperatures of 8000–12,000 K, the nitrogen abundance is 0.15–0.44 times solar. These results lend further support to the claim that the Smith Cloud is new material accreting onto the Galaxy.

Observational Evidence of Supershell Blowout in GS 018–04+44: The Scutum Supershell

Emission in the H I 21 cm line has been mapped for a region of the Galaxy that includes two known supershells, GS 018(04)44 and GS 034(06)65. We focus on the GS 018(04)44, hereafter referred to as the Scutum Supershell, which is an elongated shell about 5i in diameter extending to (7i below the Galactic plane. The Scutum shell lies at a kinematic distance of D3300 pc, implying a shell diameter of D290 pc with a vertical extension of D400 pc away from the Galactic plane. The Scutum shell con- tains 6.2 ) 105 swept into the walls. We observe that the top of the shell is missing, and a substan- M _ tial column of H I rises from the shell walls to b \( 11i, culminating in a large cloud of neutral hydrogen, 3.74 ) 104 located D630 pc from the plane. ROSAT data show X-ray emission that M _ , closely anticorrelates with the 21 cm emission. This emission probably originates from hot gas within the Scutum Supershell. After approximately correcting for the foreground absorption, we —nd that the 1.5 keV X-rays peak at the base of the shell, the 0.75 keV emission peaks in the interior and at the top of the shell, and the 0.25 keV emission extends to high latitudes above the shell. The X-ray luminosity is roughly D5 ) 1036 ergs s~1. The Wisconsin H-Alpha Mapper (WHAM) survey shows the presence of Ha emission that exhibits a morphology similar to that of the H I. Spectra indicate the presence of ionized hydrogen at velocities similar to the H I, placing ionized material at the same kinematic distance as the neutral material. IRAS images in the 60 and 100 km wavebands reveal the presence of dust corre- lated with the neutral hydrogen. Infrared surface brightness indicates an excess in the 100 km emission, which could indicate a molecular hydrogen component with a column density of 2.4 ) 1021 cm~2 in the densest regions of the high-latitude cloud of neutral hydrogen. IUE ultraviolet high dispersion spectra of HD 177989 and HD 175754 reveal the presence of very (l \ 17i.89, b \( 11i.88) (l \ 16i.40, b \( 9i.92) strong absorption by highly ionized gas at a velocity that associates the absorption with the ejecta of the Scutum Supershell. In the case of HD 177989, the high ion column density ratios suggest an origin in a turbulent mixing layer where hot and cool gases mix in the presence of shear —ows. The Ha and X-ray emission suggest that a multitude of energetic phenomena exist in this region, providing the necessary ionizing radiation. Indeed, there are multiple supernova remnants, H II regions, and hot stars, which could all contribute sizeable amounts of energy and ionizing radiation. The combination of these data sets indicates observational evidence of a ii blowout ˇˇ phenomena whereby hot material produced within the Scutum Supershell has blown through the top of the shell and been pushed to high latitude. Subject headings: Galaxy: structureISM: bubblesISM: individual (Scutum Supershell) ¨ ISM: structureradio lines: ISMsupernova remnants

SDSS-IV MaNGA : the impact of diffuse ionized gas on emission-line ratios, interpretation of diagnostic diagrams and gas metallicity measurements

Diffuse Ionized Gas (DIG) is prevalent in star-forming galaxies. Using a sample of 365 nearly face-on star-forming galaxies observed by MaNGA, we demonstrate how DIG in star-forming galaxies impacts the measurements of emission line ratios, hence the interpretation of diagnostic diagrams and gas-phase metallicity measurements. At fixed metallicity, DIG-dominated low H\alpha\ surface brightness regions display enhanced [SII]/H\alpha, [NII]/H\alpha, [OII]/H\beta, and [OI]/H\alpha. The gradients in these line ratios are determined by metallicity gradients and H\alpha\ surface brightness. In line ratio diagnostic diagrams, contamination by DIG moves HII regions towards composite or LI(N)ER-like regions. A harder ionizing spectrum is needed to explain DIG line ratios. Leaky HII region models can only shift line ratios slightly relative to HII region models, and thus fail to explain the composite/LI(N)ER line ratios displayed by DIG. Our result favors ionization by evolved stars as a major ionization source for DIG with LI(N)ER-like emission. DIG can significantly bias the measurement of gas metallicity and metallicity gradients derived using strong-line methods. Metallicities derived using N2O2 are optimal because they exhibit the smallest bias and error. Using O3N2, R23, N2=[NII]/H\alpha, and N2S2H\alpha\ (Dopita et al. 2016) to derive metallicities introduces bias in the derived metallicity gradients as large as the gradient itself. The strong-line method of Blanc et al. (2015; IZI hereafter) cannot be applied to DIG to get an accurate metallicity because it currently contains only HII region models which fail to describe the DIG.

Confirmation of a Faraday Rotation Measure Anomaly in Cygnus

We confirm the reality of a reversal of the sign of the Faraday rotation measure in the Galactic plane in Cygnus (Lazio et al. 1990), possibly associated with the Cygnus OB1 association. The rotation measure changes by several hundred rad m–2 over an angular scale of 2°-5°. We show that a simple model of an expanding plasma shell with an enhanced density and magnetic field can account for the magnitude and angular scale of this feature. This model is consistent with observations of Hα emission as well as other observations in this part of sky. We suggest that this structure is physically associated with a superbubble produced by the Cygnus OB1 association.

Models of diffuse Hα in the interstellar medium: the relative contributions from in situ ionization and dust scattering

JB acknowledges the support of an STFC studentship. LMH acknowledges support from the US National Science Foundation through award AST-1108911.

ERRATUM: “VERTICAL STRUCTURE OF A SUPERNOVA-DRIVEN TURBULENT, MAGNETIZED INTERSTELLAR MEDIUM” (2012, ApJ, 750, 104)

Alex S. Hill1,7, M. Ryan Joung2,3, Mordecai-Mark Mac Low3, Robert A. Benjamin4, L. Matthew Haffner1, Christian Klingenberg5, and Knut Waagan6 1 Department of Astronomy, University of Wisconsin-Madison, Madison, WI, USA; alex.hill@csiro.au 2 Department of Astronomy, Columbia University, New York, NY, USA 3 Department of Astrophysics, American Museum of Natural History, New York, NY, USA 4 Department of Physics, University of Wisconsin-Whitewater, Whitewater, WI, USA 5 Department of Mathematics, Würzburg University, Emil Fischer Strasse 30, Würzburg, Germany 6 Department of Applied Mathematics, University of Washington, Seattle, WA, USA Received 2012 November 2; published 2012 December 6

The Extragalactic Background and Its Fluctuations in the Far-Infrared Wavelengths

A Cosmic Far-InfraRed Background (CFIRB) has long been predicted that would traces the intial phases of galaxy formation. It has been first detected by Puget et al.(1996) using COBE data and has been later confirmed by several recent studies (Fixsen et al. 1998, Hauser et al. 1998, Lagache et al. 1999). We will present a new determination of the CFIRB that uses for the first time, in addition to COBE data, two independent gas tracers: the HI survey of Leiden/Dwingeloo (hartmann, 1998) and the WHAM H