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Highly Ionized Plasma in the Large Magellanic Cloud: Evidence for ⋆ Outflows and a Possible Galactic Wind

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Highly Ionized Plasma in the Large Magellanic Cloud: Evidence for ⋆ Outflows and a Possible Galactic Wind Mon. Not. R. Astron. Soc. 377, 687–704 (2007) doi:10.1111/j.1365-2966.2007.11631.x Highly ionized plasma in the Large Magellanic Cloud: evidence for ⋆ outflows and a possible galactic wind N. Lehner† and J. C. Howk Department of Physics, University of Notre Dame, 225 Nieuwland Science Hall, Notre Dame, IN 46556, USA Accepted 2007 February 15. Received 2007 February 15; in original form 2006 November 30 ABSTRACT Based on an analysis of the interstellar highly ionized species C IV,SiIV,NV and O VI observed in the Far Ultraviolet Spectroscopic Explorer (FUSE) and Hubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS) E140M spectra of four hot stars in the Large Magellanic Cloud (LMC), we find evidence for a hot LMC halo fed by energetic outflows from the LMC disc and even possibly an LMC galactic wind. Signatures for such outflows 1 are the intermediate- and high-velocity components (vLSR 100 km s− ) relative to the LMC disc observed in the high- and low-ion absorption profiles. The stellar environments produce strong, narrow (T 2 104 K) components of C IV and Si IV associated with the LMC disc; × in particular they are likely signatures of H II regions and expanding shells. Broad components are observed in the profiles of C IV,SiIV and O VI with their widths implying hot, collisionally ionized gas at temperatures of a few times 105 K. There is a striking similarity in the O VI/C IV ratios for the broad LMC and high-velocity components, suggesting much of the material at 1 vLSR 100 km s− is associated with the LMC. The velocity of the high-velocity component is large enough to escape altogether the LMC, polluting the intergalactic space between the LMC and the Milky Way. The observed high-ion ratios of the broad LMC and high-velocity components are consistent with those produced in conductive interfaces; such models are also favoured by the apparent kinematically coupling between the high and the weakly ionized species. Key words: ISM: clouds – ISM: kinematics and dynamics – ISM: structure – galaxies: indi- vidual: Large Magellanic Cloud – galaxies: ISM – ultraviolet: ISM. fountain (Shapiro & Field 1976); in a low-mass system, this material 1 INTRODUCTION may escape the galaxy altogether in a galactic wind (Breitschwerdt, The structure and energetics of the interstellar medium (ISM) in Voelk & McKenzie 1991). Galactic fountains and winds affect the galaxies are strongly influenced by stellar winds and supernovae scale over which newly produced metals are distributed in the ISM (e.g. Abbot 1982). This is particularly true for spiral and irregular of a galaxy and/or in the intergalactic medium. For example, out- galaxies with on-going massive star formation, such as the Milky flows can affect the [α/Fe] ratios inside and outside dwarf galax- Way (MW), and the Large and Small Magellanic Clouds (LMC, ies (Recchi, Matteucci & D’Ercole 2001). Gas dynamics are there- SMC). The input of energy from winds and supernovae is expected fore critical in determining the evolution of galaxies (e.g. Matteucci to affect the ISM on kiloparsec scales, creating regions of super- 2003). Characterizing the phenomena of infall and outflow in nearby heated gas (e.g. Norman & Ikeuchi 1989). The 106-K gas that ex- galaxies is crucial for understanding the role they may play in galaxy pands into the halo of a disc galaxy may eventually cool and fall evolution and in the enrichment and evolution of the intergalactic back on to the disc in a massive system, participating in a galactic medium. The LMC is the nearest gas-rich disc galaxy to our own MW. As ⋆ such it has been the target of intense studies of its ISM, including Based on observations made with the NASA-CNES-CSA Far Ultraviolet its dust (e.g. Meixner et al. 2006), its molecular gas (Tumlinson Spectroscopic Explorer (FUSE). FUSE is operated for NASA by the Johns et al. 2002), its warm H I (Kim et al. 2003), its warm ionized gas Hopkins University under NASA contract NAS5-32985. Based on obser- vations made with the NASA/ESA Hubble Space Telescope, obtained at traced by Hα emission (e.g. Danforth et al. 2002), its warm-hot the Space Telescope Science Institute, which is operated by the Association gas traced by O VI (Howk et al. 2002; Danforth & Blair 2006) and of Universities for Research in Astronomy, Inc. under NASA contract no. its hot gas traced by X-rays (e.g. Snowden & Petre 1994). In this NAS5-26555. work, we report observations of interstellar absorption from the †E-mail: [email protected] Li-like ions N V,CIV,SiIV and O VI towards stars within the LMC. C C 2007 The Authors. Journal compilation 2007 RAS 688 N. Lehner and J. C. Howk These ions give us the means to study the highly ionized material LMC using FUSE and STIS. Two of these sight lines probe promi- produced by feedback processes in the MW and in the Magellanic nent superbubbles, while the other two probe H II regions, allowing Clouds. Si IV,CIV,NV and O VI peak in abundance at (0.6, 1.0, 1.8 us to compare the highly ionized species and their connection with and 2.8) 105 K, respectively, in collisional ionization equilibrium weaker ions towards different types of interstellar regions, which is (CIE; Sutherland× & Dopita 1993). They are, however, unlikely to useful for differentiating local from global effects. The organization be in equilibrium since these temperatures correspond to the peak of this paper is as follows. After describing the observations, data of the interstellar cooling curve, and gas at those temperatures is reduction and analysis of the data (measurements of the kinemat- likely to cool faster than it recombines. Instead, these ions may ics, column densities and column density ratios) in Section 2, we trace interfaces between very hot gas (T 106 K) and warm or cool present an overview of the kinematics in Section 3. In Section 4 (T 104 K) material and serve as a probe of the interaction between we discuss the origin of the highly ionized plasma in the LMC and the phases of the ISM having the most mass (warm/cool gas) and the implications of our results. A summary of the main results is the most direct connection to feedback processes (hot gas). presented in Section 5. Observational searches for a hot corona about the LMC using the highly ionized species started with IUE (International Ultra- 2 OBSERVATIONS AND ANALYSIS violet Explorer) spectra of C IV,SiIV and N V towards the hot star Sk 67 104 (de Boer & Savage 1980). However, the crude spectral ◦ 2.1 Data base resolution− of IUE and the fact that the hot star could be partly re- sponsible for producing C IV and Si IV brought Chu et al. (1994) to From the data archives of FUSE and HST/STIS E140M, we selected conclude these observations were not sufficient to prove the exis- four LMC targets. The selection criteria were to have the four critical tence of a hot LMC halo (N V was only marginally detected at best). highly ionized species (Si IV,CIV,NV and O VI) observable in the These highly ionized species could be either produced by photoion- far-ultraviolet (FUV) and ultraviolet (UV) and to be able to model ization from the star itself or in the superbubble via shocks. Wakker these ions without too much uncertainties (i.e. with a continuum et al. (1998) found C IV absorption in the spectra of cooler stars near the relevant ions that can be modelled without too much un- (O9.7 B1) using modest resolution Hubble Space Telescope (HST) certainty). This resulted in the sample of the four stars presented in − spectroscopy. This was the first evidence of C IV not related directly Table 1. Fortuitously, these stars are also located in different inter- to local effects, as these stars were deemed unable to produce sig- stellar environments (from Chu et al. 1994, and references therein). nificant high-ion absorption via photoionization. (i) Sk 65 22 is projected within the supergiant shell LMC 1, More recently, Far Ultraviolet Spectroscopic Explorer (FUSE) ◦ which is−about 1 kpc across (Meaburn 1980). It is located in a faint has contributed to the effort of examining the hot gas in the LMC H II region DEM 48 at the southeast rim. This supergiant shell con- by probing diffuse O VI in both Magellanic Clouds (Danforth et al. tains the OB association LH 15. 2002; Hoopes et al. 2002; Howk et al. 2002; Danforth & Blair (ii) Sk 67 211 is in the H II region DEM 241 that contains the 2006). Howk et al. (2002) used FUSE observations of sight lines to ◦ OB association− LH 82. The bright UV continuum and the early the LMC to study the O VI content and kinematics of the LMC. Their spectral type (O2 III(f ); Walborn et al. 2002) imply the star can survey of 12 sight lines through the near side of the LMC probed ∗ likely produce strong C IV and Si IV absorption associated with its collisionally ionized gas at T 3 105 K in the LMC and shows stellar environment. in particular that the LMC contains∼ ×a significant amount of highly (iii) Sk 69◦246 is a WR star situated in the X-ray bright super- ionized gas, with O VI column densities equal or greater than those bubble 30−Dor, and more precisely is in the south end of Shell 5, found along extended paths through the MW; and much of the O VI in which is the X-ray-bright ‘chimney’ to the north of the LH 100 OB the LMC must reside in an extended ‘halo’ or thick disc distribution, association (Y.-H.
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