LMC Paper II

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[edit] Abstract

We present a survey of interstellar O VI absorption in the Large Magellanic Cloud (LMC) towards 70 sightlines based on Far Ultraviolet Spectroscopic Explorer (FUSE) observations. The survey covers a large range of objects and has a wide coverage of the LMC providing measurements of O VI column densities in different environmental conditions of the LMC. Overall, a high abundance of O VI is noted with mean logN(O VI) = 14.23 atoms cm-2 . High abundance of O VI is present in active as well as inactive regions of the LMC. There is no correlation observed between O VI and emission from hot gas (X-ray surface brightness) or from the warm gas (Hα surface brightness). O VI absorption in the LMC is patchy and the properties are similar to that of the Milky Way (MW). In comparison to the Small Magellanic Cloud (SMC), O VI is lower in abundance even though SMC has a lower metallicity compared to the LMC and the MW. We have presented O VI data for ten superbubbles, of which five have been detected first time. It is found that superbubbles show an excess O VI absorption of about 40% compared to non-superbubble sightlines. We have also studied the properties of O VI absorption in the 30 Doradus region. Even though, O VI does not show any correlation with X-ray emission for the LMC as a whole, it shows a good correlation with X-ray surface brightness and also its abundance decreases with increasing distance from the star cluster R 136 for 30 Doradus region.

[edit] Introduction

The interstellar medium (ISM) of the Milky Way (MW) and other galaxies is a complex mix of gas and dust. The processes involved in maintaining the mass, energy, and ionization balance of the ISM are not properly understood. High resolution ultraviolet ({\it UV}) spectra provides information about these processes as many absorption lines of atoms, ions and molecules of ISM are present in the {\it UV} band of the electromagnetic spectrum, one of the most important of which is O+5 (O {\small VI}), a diagnostic of temperatures of about $3 \times 10^{5} $ K \citep{Cox05}. Such temperatures are at an interface of hot (T > 106 K) and warm (T ~ 104 K) ionized gas in the ISM. Thus, O {\small VI} absorption lines at 1031.9 \AA and 1037.6 \AA are crucial diagnostics of the energetic processes of interface environments in the ISM of galaxies. Gas at such temperatures may be cooling radiatively and may be essentially independent of density, metallicity and the heating mechanism \citep{Edgar86, Heckman02}. O {\small VI} formation by photoionization is unable to explain the observed abundances, given the energy of photons needed to get such high ionization (114 eV). O {\small VI} is mostly produced by shock heating and is collisionally ionized \citep{Indebetouw04} and mainly resides at the interface of warm and hot gas and therefore, is an important tracer of such environments of the ISM.

Previous studies of O {\small VI} are limited to observations by the {\it Copernicus} satellite \citep{Jenkins78a, Jenkins78b} and the {\it Hopkins Ultraviolet Telescope} \citep{Dixon96}. \citet{Shelton94} reanalyzed the {\it Copernicus} data and have concluded that the hot gas may exist in discrete regions rather than being continuously present in the ISM. The launch of {\it Far Ultraviolet Spectroscopic Explorer (FUSE)}; \citep{Moos00, Sahnow00} has enabled a wider and more descriptive study of O {\small VI} absorption and emission in the ISM and the intergalactic medium (IGM). With a spectral resolution of ~ 20,000, {\it FUSE} has been able to resolve fine details of O {\small VI} in many different environments. {\it FUSE} has observed O {\small VI} absorption lines in the local ISM of the MW \citep{Savage00, Wakker03, Oegerle05, Savage06, Welsh08}, disk of the MW \citep{Bowen08}, halo of the MW \citep{Savage03}, the Large Magellanic Cloud (LMC; \citet{Howk02a, Lehner07}), the Small Magellanic Cloud (SMC; \citet{Hoopes02}), starburst galaxies \citep{Grimes09}, IGM \citep{Danforth05, Danforth06} etc. Apart from absorption studies, {\it FUSE} has also recorded O {\small VI} spectra in emission from observations of diffuse ISM in the MW \citep{Shelton01, Shelton02, Dixon06, Dixon08} and superbubbles in the LMC \citep{Sankrit07}. These studies have not only augmented our knowledge about the formation and distribution of O {\small VI} in the MW and the Magellanic Clouds but have also helped in better understanding of the complexities of the ISM.

Owing to the contiguity to the MW ($\sim$ 50 kpc; \citet{Feast99}) and being nearly a face-on galaxy with a low inclination angle ($\sim$ 35$^{\circ}$; \citet{Marel01}), the LMC has been the subject of numerous studies to understand and interpret the properties of ISM. Recent ISM studies on the LMC include observations of diffuse UV emission \citep{Cole99a, Cole99b, Pradhan10}, the {\it SPITZER} infrared dust survey (SAGE; \citet{Meixner06, Bernard08}), the HI survey \citep{Kim03}, the H$_{\alpha}$ survey \citep{Gaustad01}, the O {\small VI} distribution \citep{Howk02a}, and the survey of hot gas in the X-ray bands \citep{Snowden94}. More recently, \citet{Lehner07} have studied the absorption of O {\small VI}, C {\small IV}, Si {\small IV} and N {\small V} ions towards four early type stars in the LMC and provide crucial details about the environments that may be probed through the study of these ions.

\citet{Howk02a} surveyed the distribution and kinematics of O {\small VI} towards 12 early type stars in the LMC. The authors show that strong O {\small VI} absorption is present in each line of sight with the mean value of log N(O {\small VI}) = 14.37 atom cm-2. They also find the distribution of O {\small VI} to be patchy and the properties to be similar to that of the MW. This survey of O {\small VI} absorption in the LMC was very selective and the targets were restricted to Wolf-Rayet stars and O-type stars with spectral types O7 and earlier.

Here we report an extensive survey of O {\small VI} absorption at 1032 \AA in the LMC. The observation log is given in Table \ref{Tab1}. The results presented in this survey become important as they not only provide a detailed mapping of the interface gas traced by O {\small VI} but also throw light on the small scale structure of the O {\small VI} distribution in various regions of the LMC. Fig. \ref{Fig1} shows the sightlines and different regions of the LMC including the superbubbles (marked by circles) that have been studied in this survey.

[edit] Observations and data analysis

[edit] FUSE data analysis and possible contamination

The {\it FUSE} mission and its operations are described by \citet{Moos00} and \citet{Sahnow00}. Observations are made through one of 3 apertures: the HIRS (\(1.25\) $\times$ \(20\)); the MDRS (\(4\) $\times$ \(20\)); and the LWRS (\(30\) $\times$ \(30\)), but all three obtained data simultaneously. Depending on the coating of the spectrograph, observations are possible through SiC and LiF channels that are further divided into eight different segements; the SiC 1A, SiC 2A, SiC 1B and SiC 2B segments covering the wavelength range 905 to 1105 \AA and LiF 1A, LiF 2A, LiF 1B and LiF 2B segments covering the wavelength range 1000 to 1187 \AA. The data from SiC 2B segment have been known to suffer from a fixed noise. The sensitivity of LiF 1A segment near 1032 \AA is almost double that of other segments and therefore, we have used only the LiF 1A observations. Most of the data is from the large aperture LWRS but some are also from the MDRS aperture.

The fully calibrated {\it FUSE} spectra were downloaded from the Multimission Archive at STScI (MAST) processed byt the latest {\it FUSE} data reduction pipeline (CALFUSE version 3.2; \citet{Dixon07}). {\it FUSE} has more than 600 pointings in and around the LMC. Several of the observations were rejected initially by just looking at the spectra (with non-existent or low signal-to-noise of O {\small VI} absorption). 70 unique {\it FUSE} targets were selected based mainly on the simplicity of the continuum fitting in the vicinity of O {\small VI} (at 1031.9 \AA). O {\small VI} absorption for 1 sightline has been reported by \citet{Friedman00}, 11 of these sightlines have been covered by \citet{Howk02a} survey, 3 sightlines by \citet{Danforth06a} and 1 sightline by \citet{Lehner07}. O {\small VI} data for the rest are being presented for the first time to the best of our knowledge. \citet{Danforth02} prepared an atlas of {\it FUSE} observations in the Magellanic Clouds where 57 LMC stars were included. \citet{Blair09} have extended this to produce a more extensive atlas that includes {\it FUSE} observations towards 287 stars from the Magellanic clouds and has been referred for spectral types and other information. {\it FUSE} offers a reasonably good resolution of 20 km~s$^{-1}$. We have downgraded all the spectra reported here to 35 km~s$^{-1}$ to have a higher signal-to-noise. This has been done for all the spectra irrespective of the quality to maintain uniformity in the data analysis procedure.

Fitting the stellar continuum in the neighbourhood of the O {\small VI} absorption profile has been discussed in detail by \citet{Friedman00} and \citet{Howk02a}. \citet{Howk02a} have limited their study to early type stars based on the fact that these stars have completely developed O {\small VI} P Cygni profiles that are easier to fit as the early type stars have a high mass loss rate and have minimal wind variations. \citet{Lehner01,Lehner03} have shown that for Galactic sight lines the stellar wind variability may have negligible effect on O {\small VI} absorption but may introduce substantial errors towards targets in the Magellanic clouds. \citet{Lehner01} did find variation in the equivalent width and column density towards a LMC star when estimated at two different times. This warrants for extra care while fitting the continuum for Magellanic cloud targets.

As discussed above, for the LMC, the estimation of stellar continuum in the vicinity of O {\small VI} absorption is not trivial. Our targets have been selected based on the fact that the the continuum near the O {\small VI} absorption at 1031.9 \AA\ is simple to fit. A few of the sightlines do show a complex behaviour near O {\small VI} absorption (for e.g., Sk-67D05, see \citet{Friedman00} for details). For such exceptional sightlines, the complexity in the continuum fitting is due to a local dip or a sudden rise near the O {\small VI} absorption and these targets needed a comparatively higher order polynomial for the fitting. Our background targets are mostly early O and B type stars with several Wolf-Rayets. Following the fitting procedure of \citet{Howk02a} and \citet{Sembach92}, the local stellar continuum was estimated for all the targets and were fitted by a Legendre polynomial fit of low order ($\leq 5$). Several continua were tested and the uncertainties involved for the complete data set were used in the measurement of the O {\small VI} column densities. It should be noted that we have not corrected for any stellar wind absorption features. The normalized spectra in the vicinity of O {\small VI} absorption are presented in Fig. \ref{Fig2}.

The O {\small VI} absorption may be contaminated by absorption from molecular hydrogen. However, the contamination by molecular hydrogen absorption in the LMC velocity range is minimal. This is due to the fact that the molecular fraction of H$_2$ in the LMC is only about 12\% of the molecular fraction in the Galactic disk \citep{Tumlinson02}. The closest absorption line of H$_2$ to O {\small VI} absorption in the LMC lies at 1032.35 \AA (at v$_{lsr} \sim$ +123 km~s$^{-1}$), which is due to (6--0) R(4) transition. An estimation of possible contamination by H$_2$ towards 12 sightlines in the LMC has been done by \citet{Howk2002a} where they find that H$_2$ absorption does not affect the LMC O {\small VI} column densities significantly. The H$_2$ absorption feature are usually not a problem for measurements of O {\small VI} absorption at velocities more than 20 km~s$^{-1}$ apart (\citet{Savage03} and references therein). Owing to this, we have neglected estimation of contamination by H$_2$ in our O {\small VI} column density measurements for the LMC.


[edit] Measurement of O {\small VI} column densities

The measurement of equivalent widths and column densities of O {\small VI} absorption for all the sightlines were done following \citet{Savage91, Sembach92} and \citet{Howk02a}. This apparent optical depth technique \citep{Savage91} is now commonly used in the analysis of interstellar absorption lines and is applicable to cases with non-saturated absorption profiles. Briefly the technique uses an apparent optical depth in terms of velocity, i.e., an instrumentally blurred version of the true optical depth, given as

\tau_a(v) = ln[I_{o}(v)/I_{obs}(v)],

where, $I_o$ is the estimated continuum intensity and $I_{obs}$ is the intensity of the absorption line as a function of velocity. If the resolution of the instrument is very high compared to the FWHM of the absorption line, the apparent optical depth is a very good representation of the true optical depth. The apparent column density ($N_a(v)$ [atoms cm$^{-2}$ (km~s$^{-1}$)$^{-1}$]) may be calculated by the following relation

N_a(v) = \frac{m_e c \tau_a(v)}{\pi e^2 f \lambda} = 3.768 \times 10^{14} \frac{\tau_a(v)}{f \lambda,

where \lambda is the wavelength (in \AA) and {\it f} is the oscillator strength of the atomic species (for O {\small VI}, {\it f} value of 0.1325 has been adopted from \citet{Yan98}). Similar to \citet{Howk02a}, we find that the 1032 \AA\ O {\small VI} profiles is broad and is fully resolved by {\it FUSE}. However, the weaker absorption of the O {\small VI} doublet at 1037.6 \AA is found to be inseparable from the CII* and H$_2$ absorption.

For the LMC, the details of the apparent optical depth measurements are listed in Table \ref{Tab2}. The overlap of the O {\small VI} absorption of the MW and the LMC does not allow a precise measurement of column densities for the LMC. For the line of sights (Sk-65D21, Sk-67D69, Sk-67D105, HV2543, Sk-66D100, HV5936, Sk-67D211, Sk-69 220, Sk-66D172, Sk-68D137, D301-1005, and D301-NW8) where the LMC O {\small VI} is distinct from the MW O {\small VI}, the limits of velocities over which integration was performed were easy to obtain. Taking cue from these lines of sights, we have estimated the velocity limits for the line of sights where the LMC O {\small VI} was not separated from the MW O {\small VI}. The errors in the equivalent widths and column densities are 1$\sigma$ uncertainties derived using the uncertainty in the {\it FUSE} data and the fitting procedures.

[edit] Distribution and properties of O VI in the LMC

[edit] Abundance and kinematics of O {\small VI}

The LMC spectra were selected based on the quality of the spectra around O {\small VI} spectral feature. The survey presented here have more-or-less complete coverage of the well known regions of the LMC such as 30 Doradus, N11, LMC 4, etc., and thus, is very useful in studying the variation of O {\small VI} at small scales as well. We find a significant amount of variation in the O {\small VI} column densities in a single region and in different regions of the LMC.

Our data covers a wide range of targets; O type stars, B type stars, Wolf-Rayet objects, supernova remnants etc. The complete list of the background targets is provided in Table \ref{Tab1}. These observations give fine details of the small scale structure of O {\small VI} column density in the LMC probing to a scale of $sim$ 10 pc in some regions. We find the O {\small VI} absorption in the LMC to be very patchy and this characteristic is similar to that of the MW. The O {\small VI} column density varies from the lowest value of 5.28 $\times 10^{13}$ atoms cm$^{-2}$ near a HII region (DEM 7; \citet{Davies76}) to the highest value of 3.74 $\times 10^{14}$ atoms cm$^{-2}$ just north of the LMC bar. \citet{Howk02a} report the distribution of O {\small VI} in the LMC to be patchy as well with the log N (O {\small VI}) value in the LMC varying from 13.9 to 14.6 atoms cm$^{-2}$ and a mean of 14.37 atoms cm$^{-2}$. Their data did not provide any small scale variation in the column density of O {\small VI} with the smallest scale probed is about 450--500 pc.

Given the coverage of our data, we have studied the variation in O {\small VI} absorption in different regions of the LMC. We find that the 30 Doradus and SN 1987 A regions dominate in terms of the abundance of O {\small VI}. The mean and median values of log N(O {\small VI}) for targets in and around 30 Doradus and SNR 1987A are 14.30 and 14.27 atoms cm$^{-2}$ respectively. 30 Doradus is the largest HII region of LMC with a dense concentration of early type massive stars and has thus, attracted extensive research in different wavelength bands \citep{Walborn92, Parker93, Malumuth94, Walborn97, Townsley06, Indebetouw09}. We discuss in detail about the O {\small VI} distribution and its properties in 30 Doradus region in section 5. N11 region, which is associated with several OB associations and has a superbubble at its center, shows a high value of O {\small VI} column density. The mean and median values of log N(O {\small VI}) in the N11 region are 14.21 and 14.16 atoms cm$^{-2}$ respectively. Another interesting region is the LMC4 Supergiant shell that includes the Shapley Constellation III, which is one of the largest region associated with star formation \citep{Dopita85, Dolphin98}. The log N(O {\small VI}) value in LMC4 supershell varies from a minimum of 13.86 atoms cm$^{-2}$ to a maximum of 14.45 atoms cm$^{-2}$ with a mean value of 14.20 atoms cm$^{-2}$ and a median value of 14.25 atoms cm$^{-2}$. Other regions of the LMC also show patchiness in the O {\small VI} distribution. Statistically, the mean of the O {\small VI} column density in the LMC is 1.88 $\times 10^{14}$ atoms cm$^{-2}$ which is lower than 2.34 $\times 10^{14}$ atoms cm$^{-2}$ given by Howk et al. (2002) for 12 sightlines. The difference is most likely due to the wide coverage of background targets in our data. The median value of the O {\small VI} column density is 1.66 $\times 10^{14}$ atoms cm$^{-2}$ which also is less than the value reported by \citet{Howk02a}. Overall we find that there is an ubiquitous presence of O {\small VI} throughout the LMC.

The kinematics of O {\small VI} in the LMC is difficult to study because of the ambiguity in separating the LMC O {\small VI} absorption from the MW absorption. For some of the {\it FUSE} targets, i.e., for Sk-65D21, Sk-67D69, Sk-67D105, HV2543, Sk-66D100, HV5936, Sk-67D211, Sk-69\_220, Sk-66D172, Sk-68D137, D301-1005, and D301-NW8, O {\small VI} absorption at LMC velocities are distinct from the MW (Fig. \ref{Fig2}). For these sightlines, kinematics for the LMC may be studied with relatively less error. The LMC O {\small VI} absorption profiles have all been fitted with a single Gaussian. The corresponding FWHM for these sightlines are 55, 85, 78, 90, 90, 113, 100, 85, 105, 94, 107, and 91 km~s$^{-1}$ respectively. The temperature range estimated from these widths is T $\sim$ 1 $\times$ 10$^6$ -- 5 $\times$ 10$^6$ K. O {\small VI} abundance is maximum at a temperature of 3 $\times$ 10$^5$ K, thus, higher FWHM of these profiles may be due to other broadening mechanisms such as more than one velocity component and/or collision and turbulence. This may also represent the kinematic flow structure of O {\small VI} in the LMC.

The line widths for the MW O {\small VI} profiles are narrower than the LMC profiles \citep{Savage03, Oegerle05, Savage06}. For the Galactic halo, \citet{Savage03} report a range for $\sigma$ (linewidth) from 16 to 65 km~s$^{-1}$ (corresponding FWHM range is 38--153 km~s$^{-1}$). For the local ISM, \citet{Oegerle05} report average $\sigma$ to be 16 km~s$^{-1}$ (FWHM = 38 km~s$^{-1}$) while \citet{Savage06} report $\sigma$ values ranging from 15 km~s$^{-1}$ to 36 km~s$^{-1}$ (FWHM ranging from 35 km~s$^{-1}$ to 85 km~s$^{-1}$). The linewidth of the SMC O {\small VI} absorption profile are comparable to that of the LMC. The FWHM range for the SMC O {\small VI} absorption is from 82 to 115 km~s$^{-1}$ with a mean of 94 km~s$^{-1}$ \citep{Hoopes02}. We obtained a mean FWHM value of 91 km~s$^{-1}$ for the LMC selected sightlines. \citet{Howk02a} have compared O {\small VI} absorption profiles of the LMC with Fe II absorption and find that the Fe II profiles are much narrower suggesting that the thermal broadening effect for O {\small VI} absorption is much more significant.

[edit] Comparisons with the MW and the SMC

The MW and the Small Magellanic Cloud (SMC) offer a different ISM environment compared to the LMC especially due to the difference in the metallicity. The absorption profiles of the MW O {\small VI} are different from the LMC O {\small VI} and sometimes these profiles are difficult to separate in an unambiguous manner, thus, a comparison of the kinematics is not possible. \citet{Howk02a} have compared the O {\small VI} absorption with Fe II absorption at 1125.45 \AA (fig. 9 in \citet{Howk02a}). While, the Fe II profiles for the MW and the LMC are clearly separated from each other, the O {\small VI} absorption suffers an overlap. This is due to the difference in width of the two absorption. Due to the overlap of the MW O {\small VI} absorption profile with the LMC O {\small VI} absorption, \citet{Howk02a} were unable to arrive at a definite conclusion about the existence of outflows from the LMC. Following the discussion about the distribution of O {\small VI} in the LMC in the previous section, we now present an overview of O {\small VI} in the MW and the SMC.

O {\small VI} in the MW has been extensively studied since the launch of {\it FUSE} \citep{Savage00, Howk02b, Wakker03, Savage03, Oegerle05, Savage06, Bowen08}. \citet{Savage00} were the first to study the O {\small VI} absorption in the disk and halo of MW as seen towards 11 extragalactic objects (active galactic nuclei) using {\it FUSE}, confirming the large scale presence of hot gas in the halo \citep{Spitzer56}. The authors find that log N$_{\perp}$(O {\small VI}) varies from 13.80 to 14.64 atom cm$^{-2}$ and the distribution of O {\small VI} is quite patchy. To compare with the projected O {\small VI} column density on the plane of the MW, we calculated the projection of O {\small VI} column on to the plane of the LMC. Taking the inclination angle of the LMC to be 33$^{\circ}$, we find the mean value of log N$_{\perp}$(O {\small VI}) $\equiv$ log N(O {\small VI})cos$\theta$ = 14.16 atom cm$^{-2}$. \citet{Savage00} quote a mean value of 14.29 atom cm$^{-2}$ for their sample. The median value of our sample is 14.14 atom cm$^{-2}$, while for the \citet{Savage00} sample, it is 14.21 atom cm$^{-2}$.

\citet{Savage03} report the {\it FUSE} observation of O {\small VI} absorption towards 100 extragalactic sightlines to study the properties and distribution of O {\small VI} in the galactic halo. The average log N (O {\small VI}) for the complete sample is 14.36 atom cm$^{-2}$ while log N (O {\small VI}) sin$|b|$ value for the complete sample is 14.21 atom cm$^{-2}$. The results reveal that there are substantial differences in the values of log N(O {\small VI}) and log N(O {\small VI}) sin$|b|$ in the northern Galactic hemisphere compared to the southern Galactic hemisphere. The patchiness in the distribution of O {\small VI} absorption is found to be similar over angular scales extending from $\leq$ 1$^{\circ}$ to 180$^{\circ}$. An extensive survey of O {\small VI} in the MW disk has been reported by \citet{Bowen08} in which the authors have studied O {\small VI} column density towards 148 early type stars. A correlation between O {\small VI} column density and effective distance to a star exists establishing the fact that the O {\small VI} is interstellar in nature. This correlation also shows that O {\small VI} absorption is observed universally and the interstellar phenomena that gives rise to O {\small VI} is present throughout the Galaxy.

\citet{Hoopes02} have surveyed O {\small VI} absorption towards 18 early type stars in the SMC and report a widespread presence of O {\small VI}. The mean value of log N(O {\small VI}) in the SMC is 14.53, which is higher than the LMC and the MW values. The column density in the SMC correlates with the distance from NGC 346, a star forming region that shows the highest abundance of O {\small VI} in the SMC.


[edit] Comparison with X-ray and H-alpha

The LMC has been the focus of H$_{\alpha}$ and X-ray surveys to search for ionized structures in the ISM. H$_{\alpha}$ surveys have revealed the presence of HII regions, supernova remnants, and large scale structures including superbubbles and super-shells \citep{Davies76}, whereas, X-ray studies have been done to study bright X-ray sources \citep{Trumper91}, the hot gas in the ISM \citep{Wang89, Wang91} and diffuse X-ray emission \citep{Bomans94}. Since, O {\small VI} traces the ISM gas with temperatures $\sim$ 10$^5$K, which is at the interface between hot gas (temperature $\geq$ 10$^6$K) traced by X-rays and warm gas (temperature $\sim$ 10$^4$K) traced by H$_{\alpha}$, correlation between O {\small VI} abundance and X-ray and H$_{\alpha}$ emissions is expected.

The O {\small VI} observations cover specific regions of the LMC with varying environmental conditions. To get an idea about the variation of O {\small VI} column densities with different environments in the LMC, we have overlaid O {\small VI} column densities as circles on H$_{\alpha}$ image (\citet{Gaustad01}; Fig. \ref{Fig3}). The area of the circle is linearly proportional to log N(O {\small VI}). Interestingly, it is noted that O {\small VI} abundance is high in regions with low H$_{\alpha}$ and X-ray emissions, i.e., regions that are relatively inactive. However, regions like superbubbles are O {\small VI} rich, for e.g., 30 Doradus C and N11. The gross picture suggests that O {\small VI} does not correlate with either H$_{\alpha}$ or X-ray emissions. To get a better insight, we have plotted log N(O {\small VI}) against log relative H$_{\alpha}$ and X-ray surface brightnesses. Fig. \ref{Fig4} and Fig. \ref{Fig5} show the correlation of log N(O {\small VI}) with H$_{\alpha}$ and X-ray surface brightnesses for the LMC barring five sightlines in the X-ray correlation plot. Four of the excluded sightlines (Sk-67D250, D301-1005, D301-NW8 and Sk65.63) are not covered by the X-ray observations and one (Sk-69D257) has extremely high X-ray emission (about 2 orders of magnitude higher than other sightlines). The H$_{\alpha}$ and X-ray surface brightness are measured by re-binning the corresponding images to match the {\it FUSE} LWRS and MDRS aperture size. The lack of correlation is evident in both plots, which is similar to previous result (Howk et al. 2002). Since, H$_{\alpha}$ traces warm ISM and star formation, there seems to be no direct relation between O {\small VI} formation and these processes. A better correlation with X-ray is expected as the hot gas traced by X-ray is presumed to cool through temperatures where O {\small VI} formation takes place. There is a high abundance of O {\small VI} in supernova remnants and superbubbles of the LMC, whereas, bright X-ray emission is observed mostly from the supernova remnants of the LMC. X-ray emission associated with supernova remnants in the LMC is a factor of 2 to 3 times greater than the X-ray emission associated with supergiant shells and the Bar in the LMC \citep{Points01}.

[edit] O VI in superbubbles of the LMC

Superbubbles may be formed due to the strong stellar wind from young stars and/or supernova explosions forming a local cavity in the surrounding ISM. These are excellent examples of the interaction of young hot stars and the ISM. Superbubbles contain hot gas ($\sim 10^{6} K$) that is heated by shocks created by stellar winds \citep{Castor75, Weaver77, Chu90}, which is an ideal condition for the formation of O {\small VI}. The LMC hosts more than 20 superbubbles and recently O {\small VI} has been detected in a superbubble N70 \citep{Danforth06a}, where they have reported around 60\% excess in abundance of O {\small VI} in comparison to non-superbubble sightlines in the proximity of N70. The authors conclude that superbubbles act as local O {\small VI} reservoirs and have a different absorption profile compared to the non-superbubble O {\small VI} absorption profiles. They have considered four different mechanisms for the formation of O {\small VI} in superbubbles and prioritize the thermal conduction between the interior hot, X-ray producing gas and the cool, photoionized shell of N70 over other processes. \citep{Sankrit07} have detected O {\small VI} in emission in several superbubbles in the LMC emphasizing superbubbles to be important contributors to the overall O {\small VI} budget.

We have 22 O {\small VI} observations covering 10 superbubbles. Of the 22 observations, 3 are in N70 (Sk-67D250, D301-1005 and D301-NW8) that have already been reported by \citet{Danforth06a} and 4 are in N144, N204, N206 and N154 respectively that have been reported by \citet{Howk02a}. We report 15 new observations of O VI absorption in the superbubbles 30 Doradus C, N158, N11, N51, and N57. Significant variation in O {\small VI} abundance exists towards these sightlines. The minimum value of log N(O {\small VI}) is 14.04 atoms cm$^{-2}$ for superbubble N206 and the maximum value of log N(O {\small VI}) is 14.57 atoms cm$^{-2}$ in superbubble N70. The properties of O {\small VI} in superbubbles reported here are tabulated in Table \ref{Tab3}. Comparing the abundance of O {\small VI} for the superbubble and non-superbubble sightlines, we find that the mean log N(O {\small VI}) for superbubble sightlines is $\langle N_{SB}\rangle$ = 14.35 atoms cm$^{-2}$ while for the non-superbubble sightlines this is $\langle N_{non-SB}\rangle$ = 14.19 atoms cm$^{-2}$. Thus, an excess O {\small VI} abundance of about 40\% in superbubbles of the LMC is found in comparison to non-superbubble regions. Combining the \citet{Danforth06a} and \citet{Howk02a} data for superbubble and non-superbubble sightlines (excluding Sk--67 05 sightline; see \citet{Howk02a} for details), the O {\small VI} excess in superbubbles is about 46\%, which is comparable to our results. Thus, results reported here support and confirm that superbubbles do show higher O {\small VI} abundance in comparison to the general halo absorption seen in other LMC sightlines. Some of the non-superbubble sightlines show an enhanced O {\small VI} column density owing to local effects. \citet{Lehner07} compared superbubble and non-superbubble sightlines for the LMC and found that some quiescent environments showed an enhanced O {\small VI} abundance, sometimes even larger than that of superbubbles.

Even in a single superbubble, there are significant variations in the O {\small VI} column densities. We have four observations each for the superbubbles 30 Doradus C and N11. In the case of 30 Doradus C, we find that the variation in N(O {\small VI}) is more than a factor of 2 (from the minimum value of N(O {\small VI}) to the maximum value). For N11, this variation is about a factor of 2.5. For N70, the variation in N(O {\small VI}) is not much (for the three sightlines included here) and this corroborates with the \citet{Danforth06a} data.

[edit] Properties of O VI in 30 Doradus

The 30 Doradus region of the LMC is ideally suited to study the interaction between a high rate of star formation and the surrounding ISM. This region is dominated by the star forming cluster NGC 2070 that contains a very interesting star cluster R136 at the center. The proximity of 30 Doradus has allowed extensive research on the stellar content \citep{Parker92, Parker93a, Walborn97} and initial mass function (\citet{Andersen09} and references therein). 30 Doradus has also been investigated in detail in the infrared bands to study the dust properties \citep{Sturm00, Vermeij02, Meixner06}. \citet{Indebetouw09} studied the 30 Doradus in the mid-infrared wavelength band to determine the physical conditions of the ionized gas. \citet{Indebetouw09} find that the local effects of hot stars in 30 Doradus appear to dominate over any large-scale trend with distance from the central cluster R136.

O {\small VI} abundance in 30 Doradus is higher compared to other regions of the LMC. The highest value of log N(O {\small VI}) in this region is 14.56 atoms cm$^{-2}$ near the center of the cluster R136. Since, our data explores the small scale structure in the O {\small VI}, we studied the variation in the log N(O {\small VI}) values with the distance from the the center of R136. Fig. \ref{Fig6} shows the change in the log N(O {\small VI}) values from the center of R136 up to an angular distance of 1 degree. We find that there is an overall decrease in the O {\small VI} abundance away from the center of R136. An interpretation of this plot could be that the processes involved in the formation of O {\small VI} may be associated with stellar radiation field but the effect of local processes cannot be neglected on a large angular scale. It should be noted that we do not find any correlation between O {\small VI} absorption and H$_{\alpha}$ emission for LMC and thus, no relation between star formation is established on a larger scale.

We have also studied the correlation between log N(O {\small VI}) and X-ray surface brightness. As mentioned earlier, log N(O {\small VI}) does not correlate with X-ray emission for the LMC as a whole but surprisingly we find a good correlation for the 30 Doradus sightlines (Fig. \ref{Fig7}). We have studied the correlation between log N(O {\small VI}) and X-ray emission from 30 Doradus considering all the sightlines within 1 degree around R136 except for one sightline (SK-69D257) that has exceptionally high value of X-ray surface brightness. One of the reasons for such high X-ray emission from SK-69D257 may be its proximity to a high mass X-ray binary LMC X-1 \citep{Points01}. The correlation in 30 Doradus suggests that O {\small VI} that is observed in this region is present in the ISM gas surrounding the X-ray emitting plasma. This may be due to the compactness, high density and non-uniform structure of 30 Doradus. The correlation confirms that the X-ray emitting gas cools through temperatures where O {\small VI} is being formed. This also supports the general consensus about the formation of O {\small VI} by collisional ionization in the interface regions between cooler photoionized ISM gas and hot exterior ISM gas \citep{Slavin02, Indebetouw04}.

[edit] Summary and Conclusions

We have presented O {\small VI} column density measurements for the LMC using {\it FUSE} data for 70 sightlines. This is the largest survey of O {\small VI} absorption that has the most wide coverage of the LMC. The results reported here reveal significant variation in O {\small VI} column densities over a very small angular scale thus confirming the patchiness of O {\small VI} distribution in the LMC. The significant inferences drawn from this work are following:

(i) This survey probes 70 sightlines with varying environmental conditions. We find strong O {\small VI} absorption in the LMC that is not restricted to active regions. High O {\small VI} abundance is present even in relatively inactive regions of the LMC.

(ii) There is significant variations in the velocity profiles of O {\small VI} absorption. The O {\small VI} absorption profile is broader than the MW absorption for many sightlines but it should be noted that unambiguous separation of the MW and the LMC components may not be possible for most of the sightlines. This proves to be a significant hurdle in interpreting exact kinematics for O {\small VI} in the LMC (for e.g. outflows from the LMC, etc.).

(iii) The maximum column density measured for the LMC is log N(O {\small VI}) = 14.57 atoms cm$^{-2}$ and minimum value is log N(O {\small VI}) = 13.72 atoms cm$^{-2}$. The mean value of O {\small VI} column density is $<$log N(O {\small VI})$>$ = 14.23 atoms cm$^{-2}$, which is slightly lower than the earlier reported value. The median value of O {\small VI} column density in the LMC comes to be 14.22 atoms cm$^{-2}$. The results corroborates with the previous finding that the distribution of O {\small VI} in the LMC is patchy.

(iv) Despite the fact that the LMC has lower metallicity compared to the MW, the abundance of O {\small VI} and properties of O {\small VI} absorption are similar in both the galaxies. The mean of log N$_{\perp}$(O {\small VI}) value for the MW is 14.29 atoms cm$^{-2}$ while the projected column density for the LMC, i.e., log N$_{\perp}$(O {\small VI}) is 14.15 atoms cm$^{-2}$. A more extensive study for the MW suggests this value to be 14.21 atoms cm$^{-2}$. SMC with even lower metallicity has higher O {\small VI} abundance with mean log N(O {\small VI}) = 14.53 atoms cm$^{-2}$.

(v) O {\small VI} absorption in the LMC does not correlate with H$_{\alpha}$ (warm gas) or X-ray (hot gas) emission but we find a good correlation between O {\small VI} absorption and X-ray emission in the 30 Doradus region. It is also seen that the O {\small VI} absorption is decreasing with increasing angular distance from the star cluster R136 suggesting some loose correlation with star formation.

(vi) The work reported here covers 10 superbubbles of the LMC and for 5 superbubbles (30 Doradus C, N158, N51, N11 and N57) O {\small VI} absorption is reported for the first time. Superbubbles are O {\small VI} rich and have 40\% excess compared to non-superbubble sightlines.

The O {\small VI} absorption reported here covers variation in O {\small VI} column densities over a very small angular scale. This information is extremely useful in modelling the formation of highly ionized material of the ISM of the LMC. Any such modelling is beyond the scope of this paper. The data reported here are useful in understanding better the properties of the ISM of the LMC.

[edit] Figures

[edit] OVI Absorption Profile plots of all the 70 sight lines

The figure gives the normalized profile for 70 sightlines in the LMC.

[edit] log N(O VI) versus log relative H-alpha surface brightness

The figure shows that there is no correlation between H-alpha emission and O VI absorption in the LMC.

[edit] log N(O VI) versus log relative X-ray surface brightness

The figure shows that there is no correlation between X-ray emission and O VI absorption in the LMC.

[edit] log N(O VI) versus distance from the centre of 30 Doradus

The figure shows the variation of log N(O VI) with increasing distance from the centre of 30 Doradus (R 136). Sightlines within 1 degree angular scale of R 136 have been included

[edit] 30Dor log N(O VI) - Xray correlation

The figure shows correlation between log N(O VI) and X-ray luminosity for 30 Doradus. No such correlation has been seen for complete LMC and for other individual regions in the LMC. 1 sightline (SK-69D257) has been excluded as it shows an exceptionally high X-ray emission due to its proximity to X-ray binary LMC-X1 (Points et al. 2001).

[edit] H-alpha image of the LMC showing distribution of N(OVI)

H-alpha image of the LMC (Gaustad et al. 2001) with circles showing O VI absorption around the 70 targets. The area of the circle is linearly proportional to the column density of OVI at LMC velocities.

[edit] R-band image of the LMC showing Superbubbles and OVI pointings

R-band image of the LMC (Bothun & Thompson 1988) showing the targets towards which O VI absorption have been studied. Superbubbles studied here have been shown by circles.

[edit] Reference

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[edit] Comments on Paper (JM)

  1. If I'm reading the paper correctly, you are only using the 1032 A line. If so, please state it explicitly.

This is now mentioned in the last paragraph of Introduction

  1. The selection criteria are not clear. You say "quality of the spectra". Can you quantify in terms of S/N or exposure time.

The main selection criteria is the simplicity of fitting the continuum. For most of the sightlines, a Legendre polynomial of order 5 or less is used to fit the continuum. There are exceptional targets that need higher order polynomial because they have a local dip or rise near the OVI absorption. This is now mentioned in section 2.1, para 4.

  1. I question the complete coverage of the LMC. You are only looking at specific lines of sight. See Fig. 1.

This has been corrected.

  1. Do you expect a correlation of OVI with the background source? I wouldn't.

Howk et al have a plot with no correlation with the background source. This statement is now removed.

  1. What does it mean to say that the OVI distribution is similar to the Milky Way? I don't know what that is.

This is made more clear now. Both the LMC and the MW show a similar abundance and patchiness in OVI absorption.

  1. Your second paragraph in Section 3 is much too long and has too many different parts to it. I'm having a hard time keeping separate which parts are your results and which are other peoples'.

Now I have two paragraphs instead of one. Things should be more clear now.

  1. The section heading is abundance of OVI but then you talk about the kinematics of OVI.

Section heading is changed.

  1. If the OVI line is blended with the Milky Way line for kinematic purposes, how do you separate the two for abundance purposes?

There are some sightlines that show a clear separation between MW and LMC OVI absorption. The velocity limits have been estimated from these sightlines for all of the targets. This is now clearly mentioned in section 2.2, para 2.

  1. When I look at Fig. 3, I see very little variation in the circle diameters.

We are working on this figure to make it more clear. We are also working on Fig. 1. Captions for each figure comes at the bottom of the page leaving a gap of few inches between the figure and the caption. The MNRAS style is not as good as ApJ.

  1. You find that the OVI drops with distance from the centre of R136. Could this be due to a difference in the stellar radiation field rather than star formation?

I have changed this to stellar radiation field.

  1. I would take out the words "first survey" everywhere. Why not just say "the largest survey of OVI"

Relevant changes made.

[edit] Second set of comments

  1. Gas at such temperatures may be cooling radiatively and may be essentially independent of density, metallicity and the heating mechanism (Edgar & Chevalier 1986; Heckman et al. 2002).You don't

say what is independent

I mean the cooling process. I have changed the statement to "The gas at such temperatures is cooling radiatively and the colling is essentially independent of density, metallicity and the heating mechanism \citep{Edgar86, Heckman02}."

  1. You qualify with "may" too much. Shelton & Cox concluded that the hot gas exists in discrete regions ...

I have reduced the number of "may"s.

  1. Use \AA\ for angstroms so that you leave the extra space after.

Corrected.

  1. In those targets where you had a target in the MDRS, could you not observe the diffuse radiation in the LWRS?

I have not used any diffuse spectra here.

  1. Danforth et al. (2002) prepared an atlas of FUSE observations in the Magellanic Clouds where 57 LMC stars were included. Blair et al. (2009) have extended this to produce a more exten- sive atlas that includes FUSE observations towards 287 stars from the Magellanic clouds and has been referred for spec- tral types and other information. Why not just say that spectral types and other stellar information was taken from Danforth et al. 2002 and Blair et al. 2009.

The statement is changed as suggested.

  1. If you did, in fact, use the diffuse data you should mention that explicitly. It is interesting.
  2. Did you use both OVI lines or only the one?

Only one line at 1032 Ang. because the other one is not separated from the CII line.

  1. It is lines of sight not line of sights.

Corrected.

  1. The word data is plural.

This is rectified.

  1. Are you confusing kinematics with temperature? When I think of kinematics, I think of velocity structure whereas I think you are using it for the line width. I don't believe kinematics is hte right word here.

I have changed the section heading here to "Abundance and linewidth of O {\small VI}" and have made changes in the paragraph as well where I mention "kinematics".

  1. I don't understand what you mean by "local processes" on page 6.

I have changed this to "local effects". I just mean that a local stellar radiation field may result in high abundance of OVI.

  1. When you say "good" or "bad" correlation, it is better to quote the coefficient.

I have to get the coefficient for the X-ray data from Sujatha. I will add this.

  1. It is still not clear to me how you separate the OVI of the LMC from that of the Milky Way. If the abundances are similar, how can you be sure it comes from the LMC and not the Milky Way.

OVI absorption is for the Milky Way and LMC may not be separated because the LMC is very near to the MW. Sometimes the two profiles may be separated out and for these lines of sight, one can resolve the LMC OVI absorption and get the velocity limits (for the LMC). For other lines of sight, there is just one single profile and for the MW and the LMC. Based on the velocity limits where the two profiles are separated out, one may guess about the LMC velocity limits. I integrate within these limits to get the LMC OVI. It is true that this OVI column density is not accurate but there is no other way. I have given these velocity limits in Table 2. For the MW, one needs to integrate from either 0 or -ve velocity to about +150 - +160 km/s. Beyond this, it is the LMC absorption that extends up to about +350 km/s. TO get the LMC OVI column density, I integrated from about +160 to about +350 km/s.

If you can answer the last point satisfactorily, I have no need to see the paper again before submission.

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