The Magellanic System: What have we learnt from FUSE?
N. Lehner
备注：
A contributed paper to the FUSE Annapolis Conference "Future Directions in Ultraviolet Spectroscopy.", 5 pages. To appear as an AIP Conference Proceeding
I review some of the findings on the Magellanic System produced by the Far Ultraviolet Spectroscopic Explorer (FUSE) during and after its eight years of service. The Magellanic System with its high-velocity complexes provides a nearby laboratory that can be used to characterize phenomena that involve interaction between galaxies, infall and outflow of gas and metals in galaxies. These processes are crucial for understanding the evolution of galaxies and the intergalactic medium. Among the FUSE successes I highlight are the coronal gas about the LMC and SMC, and beyond in the Stream, the outflows from these galaxies, the discovery of molecules in the diffuse gas of the Stream and the Bridge, an extremely sub-solar and sub-SMC metallicity of the Bridge, and a high-velocity complex between the Milky Way and the Clouds.
We present an atlas of 57 Large Magellanic Cloud (LMC) and 37 Small Magellanic Cloud (SMC) observations obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite. The atlas highlights twelve interstellar absorption line transitions at a resolution of ~15 km/s. These transitions cover a broad range of temperatures, ionization states, and abundances. The species included are OVI, which probes hot (T~3x10^5 K) ionized gas; CIII and FeIII, which probe warm (T~10^4 K) ionized gas; SiII, PII, CII, FeII, and OI, warm neutral gas; and six different molecular hydrogen transitions, which…
We observed five central stars of planetary nebulae (CSPN) in the Large Magellanic Cloud (LMC) and three in the Small Magellanic Cloud (SMC) with the Far Ultraviolet Spectroscopic Explorer (FUSE), in the range 905-1187 Ang. We performed a model-based analysis of these spectra in conjunction with Hubble Space Telescope (HST) spectra in the UV and optical range to determine stellar and nebular parameters. The signature of hot (T >~ 2000 K) circumstellar molecular hydrogen is found in the FUSE spectra of most objects. We also find evidence of X-rays in the wind of LMC-SMP 76.
We present an extinction analysis of 9 paths through the LMC and SMC based on FUSE observations. To date, just two LMC sight lines have probed dust grain composition and size distributions in the Clouds using spectra including wavelengths as short as 950 A. We supplement these with results from 4 regions distinguished by their IR through UV extinction curves and grouped as LMCAvg, LMC2, SMC bar and SMC wing. Despite the distinct characters of Milky Way and Magellanic Cloud extinction, our results are generally analogous to those found for Galactic curves in that the FUSE portions of each extin…
We compare the stellar wind features in far-UV spectra of Sk -67 111, an O7 Ib(f) star in the LMC, with Sk 80, an O7 Iaf+ star in the SMC. The most striking differences are that Sk 80 has a substantially lower terminal velocity, much weaker O VI absorption, and stronger S IV emission. We have used line-blanketed, hydrodynamic, non-LTE atmospheric models to explore the origin of these differences. The far-UV spectra require systematically lower stellar temperatures than previous determinations for O7 supergiants derived from plane-parallel, hydrostatic models of photospheric line profiles. At t…
We present the results of a FUSE survey of O VI 1031.93 Angstrom and 1037.62 Angstrom absorption toward 18 OB stars in the Small Magellanic Cloud (SMC). We detect O VI along every sight line. The average value of the O VI column density in the SMC is log <N(O VI)>=14.53. This value is 1.7 times higher than the average value for the Milky Way halo (perpendicular to the Galactic plane) of log N_perp(O VI)=14.29 found by FUSE, even though the SMC has much lower metallicity than the Galaxy. The column density in the SMC is higher along sight lines that lie close to star-forming regions, in particu…
We present Far Ultraviolet Explorer (FUSE) spectra for three Magellanic Cloud O stars (Sk 80, Sk -67 05 and Sk -67 111) with repeated observations. The data demonstrate the capabilities of FUSE to perform time-resolved spectroscopy on extragalactic stars. The wavelength coverage of FUSE provides access to resonance lines due to less abundant species, such as sulfur, which are unsaturated in O supergiants. This allows us to examine wind variability at all velocities in resonance lines for stars with higher mass loss rates than can be studied at longer (lambda > 1150 A) wavelengths. The FUSE wav…
We discuss FUSE observations of two early-type stars, DI1388 and DGIK975, in the low density and low metallicity gas of Magellanic Bridge (MB). Toward DI1388, the FUSE observations show molecular hydrogen, O VI, and numerous other atomic or ionic transitions in absorption, implying the presence of multiple gas phases in a complex arrangement. The relative abundance pattern in the MB is attributed to varying degrees of depletion onto dust similar to that of halo clouds. The N/O ratio is near solar, much higher than N/O in damped Ly-alpha systems, implying subsequent stellar processing to explai…
We present new results from our survey of diffuse O VI-emitting gas in the interstellar medium with the Far Ultraviolet Spectroscopic Explorer (FUSE). Background observations obtained since 2005 have yielded eleven new O VI detections of 3-sigma significance, and archival searches have revealed two more. An additional 15 sight lines yield interesting upper limits. Combined with previous results, these observations reveal the large-scale structure of the O VI-bearing gas in the quadrant of the sky centered on the Magellanic Clouds. The most prominent feature is a layer of low-velocity O VI emis…
We have undertaken a FUSE survey of Magellanic Cloud supernova remnants, looking primarily for O VI and C III emission lines. Work in earlier cycles with FUSE indicates that optical and/or X-ray characteristics of supernova remnants are not always good predictors of the objects that will be bright and detectable in the UV. The goal of our survey is to test this concept by obtaining spectra of a random sample of Magellanic Cloud remnants with a broad range of radio, optical, and X-ray properties. Previously observed objects and remnants with known high extinction (or known high column densities…
We describe a moderate-resolution FUSE survey of H2 along 70 sight lines to the Small and Large Magellanic Clouds, using hot stars as background sources. FUSE spectra of 67% of observed Magellanic Cloud sources (52% of LMC and 92% of SMC) exhibit absorption lines from the H2 Lyman and Werner bands between 912 and 1120 A. Our survey is sensitive to N(H2) >= 10^14 cm^-2; the highest column densities are log N(H2) = 19.9 in the LMC and 20.6 in the SMC. We find reduced H2 abundances in the Magellanic Clouds relative to the Milky Way, with average molecular fractions <f_H2> = 0.010 (+0.005, -0.002)…
We use a Sobolev with Exact Integration model to analyze the winds lines of 25 LMC O stars. The data include FUSE profiles of C III, N III, S IV, P V, S VI, and O VI and IUE or HST data for Si IV, C IV, and N V. Several of the FUSE lines are unsaturated, so meaningful optical depths (equivalently, mass loss rate times ionization fractions), as a function of wind velocity can be determined. Ratios of these quantities give the relative ionization as a function of velocity and demonstrate that, except for O VI in all stars and S VI in the later stars, the wind ionization shifts toward lower stage…
We have undertaken quantitative analysis of four LMC and SMC O4--9.7 extreme supergiants using far-ultraviolet FUSE, ultraviolet IUE/HSTand optical VLT UVES spectroscopy. Extended, non-LTE model atmospheres that allow for the consistent treatment of line blanketing are used to analyse wind & photospheric spectral features simultaneously. Using Halpha to constrain dM/dt, HeI-II photospheric lines reveal T_eff's which are systematically (5-7.5kK) & substantially (15-20%) lower than previously derived from unblanketed, plane-parallel, non-LTE photospheric studies. We have confidence in these revi…
We present Far Ultraviolet Spectroscopic Explorer observations of the atomic and molecular absorption in high velocity cloud HVC 287.5+22.5+240, which lies in front of the ultraviolet-bright nucleus of the Seyfert 1 galaxy NGC 3783. We detect H2, N I, N II, Si II, and Fe II absorption and set limits on the amount of absorption due to P III, Ar I, and Fe III. We extend the earlier metallicity and dust-depletion measurements made by Lu and collaborators by examining the relative gas-phase abundances of Si, P, S, and Fe. Corrections to the derived gas-phase abundances due to ionized gas in the HV…
We summarize recent studies of the predominantly neutral gas in the Magellanic Clouds (MC), based on combinations of high-resolution optical spectra and UV spectra obtained with IUE, HST, and FUSE. Similarities in relative gas-phase abundances for Galactic and MC sightlines with relatively low N(H) had suggested that the depletion patterns might be similar in the three galaxies -- at least where the depletions are relatively mild. New STIS spectra of one higher N(H) SMC sightline, however, have revealed significant differences, relative to the Galactic patterns, for SMC clouds characterized by…
We report a detection of far ultraviolet absorption from the supernova remnant SNR 0057 - 7226 in the Small Magellanic Cloud (SMC). The absorption is seen in the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum of the LBV/WR star HD 5980. Absorption from O VI 1032 and C III 977 is seen at a velocity of +300 km/s with respect to the Galactic absorption lines, +170 km/s with respect to the SMC absorption. The O VI 1038 line is contaminated by H_2 absorption, but is present. These lines are not seen in the FUSE spectrum of Sk80, only ~1' (~17 pc) away from HD 5980. No blue-shifted O VI 1032…
We observed seven central stars of planetary nebulae (CSPN) in the Large Magellanic Cloud (LMC) with the Far Ultraviolet Spectroscopic Explorer (FUSE), and performed a model-based analysis of these spectra in conjunction with Hubble Space Telescope (HST) spectra in the UV and optical range to determine the stellar and nebular parameters. Most of the objects show wind features, and they have effective temperatures ranging from 38 to 60 kK with mass-loss rates of ~= 5x10^-8 Msun/yr. Five of the objects have typical LMC abundances. One object (SMP LMC 61) is a [WC4] star, and we fit its spectra w…
We present a new analysis of the diffuse gas in the Magellanic Bridge (RA>3h) based on HST/STIS E140M and FUSE spectra of 2 early-type stars lying within the Bridge and a QSO behind it. We derive the column densities of HI (from Lyα), NI, OI, ArI, SiII, SII, and FeII of the gas in the Bridge. Using the atomic species, we determine the first gas-phase metallicity of the Magellanic Bridge, [Z/H]=-1.02+/-0.07 toward one sightline, and -1.7<[Z/H]<-0.9 toward the other one, a factor 2 or more smaller than the present-day SMC metallicity. Using the metallicity and N(HI), we show that the Bridge gas …
FUSE spectra of the four brightest H II regions in M 33 show absorption by interstellar gas in the Galaxy and in M 33. On three lines of sight molecular hydrogen in M 33 is detected. This is the first measurement of diffuse H_2 in absorption in a Local Group galaxy other than the Magellanic Clouds. A quantitative analysis is difficult because of the low signal to noise ratio and the systematic effects produced by having multiple objects in the FUSE aperture. We use the M 33 FUSE data to demonstrate in a more general manner the complexity of interpreting interstellar absorption line spectra tow…
The Far Ultraviolet Spectroscopic Explorer (FUSE) is a NASA astronomy mission that will explore the 905-1187 A wavelength region at high spectral resolution. Funded by NASA's Explorer Program, this Origins mission is scheduled for a 1999 launch and at least three years of operations. The development of FUSE is being led by the Johns Hopkins University, with major contributions to the program from the University of Colorado, the University of California-Berkeley, the space agencies of Canada and France, and corporate partners. FUSE will have approximately 10,000 times the sensitivity of its pio…
Based on an analysis of the interstellar highly ionised species C IV, Si IV, N V, and O VI observed in the FUSE and 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 disk and even possibly an LMC galactic wind. Signatures for such outflows are the intermediate and high-velocity components (v_LSR>100 km/s) relative to the LMC disk observed in the high- and low-ion absorption profiles. The stellar environments produce strong, narrow (T<20,000 K) components of C IV and Si IV associated with the …
We describe a moderate-resolution FUSE mini-survey of H2 in the Milky Way and Magellanic Clouds, using four hot stars and four AGN as background sources. FUSE spectra of nearly every stellar and extragalactic source exhibit numerous absorption lines from the H2 Lyman and Werner bands between 912 and 1120 A. One extragalactic sightline, PKS 2155-304, with low N(HI) shows no detectable H2 and could be the "Lockman Hole" of molecular gas, of importance for QSO absorption-line studies. We measure H2 column densities in low rotational states (J = 0 and 1) to derive rotational and/or kinetic tempera…
Measurements of molecular hydrogen (H_2) column densities are presented for the first six rotational levels (J=0 to 5) for 73 extragalactic targets observed with FUSE. All of these have a final signal-to-noise ratio larger than \snlimit, and are located at galactic latitude |b|>20 deg. The individual observations were calibrated with the FUSE calibration pipeline CalFUSE version 2.1 or higher, and then carefully aligned in velocity. The final velocity shifts for all the FUSE segments are listed. H_2 column densities or limits are determined for the 6 lowest rotational (J) levels for each HI co…
We present the first far ultraviolet (FUV) spectra of the four known Balmer-dominated supernova remnants (SNRs) in the Large Magellanic Cloud, acquired with the Far Ultraviolet Spectroscopic Explorer. The remnants DEM L 71 (0505-67.9), 0509-67.5, 0519-69.0 and 0548-70.4 are all in the non-radiative stages of evolution and exhibit expansion speeds ranging from ~ 500 km/s to ~ 5000 km/s. We have detected broad emission lines of Ly beta, Ly gamma, C III and O VI in DEM L 71 (V(FWHM) ~ 1000 km/s) and have detected broad Ly beta and O VI emission in 0519-69.0, (V(FWHM) ~ 3000 km/s). In addition, br…
We report on measurements of interstellar O VI, H2, P II, Si II, Ar I, and Fe II absorption along the line of sight to Sk -67 05, a B0 Ia star in a diffuse H II region in the western edge of the Large Magellanic Cloud (LMC). We find log N(O VI) = 14.40 +/- 0.04 in the Milky Way (MW) component and, using the C IV column density from previous IUE observations, N(C IV) / N(O VI) = 1.00 +/- 0.16, a value similar to other halo measurements made with FUSE. In the LMC component, log N(O VI) = 13.89 +/- 0.05, and N(C IV) / N(O VI) < 0.4 (3 sigma), since only an upper limit on N(C IV) is available. Alo…
The Far Ultraviolet Spectroscopic Explorer (FUSE) has observed over three hundred fifty sightlines to extragalactic targets. About one hundred fifty of these are of sufficient quality to measure OVI absorption in and near the Milky Way. High-velocity OVI absorption is detected in about 80% of the sightlines, with the detection rate going up for sightlines with higher signal-to-noise ratios. OVI is almost always seen in directions with previously known HI HVCs, including HVCs complex C, complex A, complex WD, the Magellanic Stream, and the Outer Arm. Studies of several sightlines through comple…
We discuss the interstellar absorption lines found in FUSE spectra of the Wolf-Rayet binary Sk 108, located in the northeastern part of the main ``bar'' of the Small Magellanic Cloud. The spectra cover the wavelength range 988-1187 Angstroms, at a resolution of about 12,000 and S/N of 20--40. We use detailed component information from higher resolution near-UV and optical spectra to model the far-UV lines of similarly distributed species. Both the Galactic and SMC gas toward Sk 108 seem to be predominantly neutral, though a significant fraction of the SMC gas is ionized. The column densities o…
We have used the Far Ultraviolet Spectroscopic Explorer (FUSE) to conduct a snap-shot survey of O VI variability in the winds of 66 OB-type stars in the Galaxy and the Magellanic Clouds. These time series consist of two or three observations separated by intervals ranging from a few days to several months. Although these time series provide the bare minimum of information required to detect variations, this survey demonstrates that the O VI doublet in the winds of OB-type stars is variable on various scales both in time and velocity. For spectral types from O3 to B1, 64% vary in time. At spect…
We have used moderate-resolution (FWHM ~ 25 km/sec) spectra of AGNs and QSOs observed by the Far Ultraviolet Spectroscopic Explorer (FUSE) to make the first definitive measurements of absorption by hot gas in high velocity clouds (HVCs) at large distances from the Galactic plane. Seven of the 11 sight lines studied exhibit high velocity (|V(LSR)| > 100 km/sec) O VI 1031.93 A absorption with log N(O VI) = 13.79-14.62. High velocity O VI absorption is detected in the distant gas of H I HVC Complex C, the Magellanic Stream, several HVCs believed to be in the Local Group, and the outer Galaxy. The…
In this contribution the current state of knowledge about the high-velocity clouds (HVCs) is summarized. Recent progress has shown that the HVCs are a diverse phenomenon. The intermediate-velocity clouds (IVCs) are likely to be part of a Galactic Fountain. The Magellanic Stream is a tidal remnant. HVC complex C (possibly complexes A and GCN) are low-metallicity clouds near the Galaxy; they could be remnants of the formation of the Galaxy or old tidal streams extracted from nearby dwarf galaxies. Having a substantial number of HI HVCs dispersed throughout the Local Group seems incompatible with…
We present FUSE observations of OVI absorption in a sample of 100 extragalactic targets and 2 distant halo stars. We describe the details of the calibration, alignment in velocity, continuum fitting, and manner in which contaminants were removed (Galactic H2, absorption intrinsic to the background target and intergalactic Ly-beta lines). We searched for OVI absorption in the velocity range -1200 to 1200 km/s. With a few exceptions, we only find OVI between -400 and 400 km/s; the exceptions may be intergalactic OVI. We discuss the separation of the observed OVI absorption into components associ…
We report the results of an extensive FUSE study of high velocity OVI absorption along 102 complete sight lines through the Galactic halo. The high velocity OVI traces a variety of phenomena, including tidal interactions with the Magellanic Clouds, accretion of gas, outflow from the Galactic disk, warm/hot gas interactions in a highly extended Galactic corona, and intergalactic gas in the Local Group. We identify 85 high velocity OVI features at velocities of -500 < v(LSR) < +500 km/s along 59 of the 102 sight lines. Approximately 60% of the sky (and perhaps as much as 85%) is covered by high …
Optical emission lines have now been detected from about 20 high velocity clouds. These emission lines -- primarily H-alpha, secondarily [N II] and [S II] -- are very faint and diffuse, spread over the surfaces of the clouds. We compile emission line measurements and present a model in which the H-alpha is recombination caused by photoionizing radiation escaping the Milky Way. In such a model, we infer HVC distances of 5--30 kpc. The photoionization model fails to explain the relatively strong H-alpha emission from the Magellanic Stream, and the O VI absorption seen by FUSE in HVCs and the MS,…
We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of interstellar molecular hydrogen (H_2) in two Galactic high-velocity clouds (HVCs). Molecular hydrogen absorption is detected in the Magellanic Stream (abundance ~0.3 solar) toward the Seyfert galaxy Fairall 9 in the lowest three rotational states (J=0-2) at v(LSR)=+190 km/s, yielding a total H_2 column density of log N(H_2)=16.40(+0.26)(-0.53). In contrast, no H_2 absorption is seen in the high-velocity cloud Complex C (abundance ~0.1 solar) toward the quasar PG 1259+593 (log N(H_2)<13.96 at v(LSR)=-130 km/s) although bot…
We have obtained high resolution FUSE and HST/STIS echelle observations of the quasar PG 1116+215. The semi-continuous coverage of the ultraviolet spectrum over the wavelength range 916-2800 provides detections of Galactic and high velocity cloud (HVC) absorption over a wide range of ionization species over the local standard of rest velocity range -100 - +200 km/s. The high dispersion of these spectra (6.5-20 km/s) reveals that low ionization species consist of five discrete components, three at low-to-intermediate velocities, and two at high velocities (v = +100, +184 km/s). Over the same ve…
