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Ultraviolet and Optical Observations of OB Associations and Field Stars in the Southwest Region of the Large Magellanic Cloud  [PDF]
Joel Wm. Parker,Dennis Zaritsky,Theodore P. Stecher,Jason Harris,Phil Massey
Physics , 2000, DOI: 10.1086/318765
Abstract: Using photometry from the Ultraviolet Imaging Telescope (UIT) and photometry and spectroscopy from three ground-based optical datasets we have analyzed the stellar content of OB associations and field areas in and around the regions N 79, N 81, N 83, and N 94 in the LMC. We compare data for the OB association Lucke-Hodge 2 (LH 2) to determine how strongly the initial mass function (IMF) may depend on different photometric reductions and calibrations. We also correct for the background contribution of field stars, showing the importance of correcting for field star contamination in determinations of the IMF of star formation regions. It is possible that even in the case of an universal IMF, the variability of the density of background stars could be the dominant factor creating the differences between calculated IMFs for OB associations. We have also combined the UIT data with the Magellanic Cloud Photometric Survey to study the distribution of the candidate O-type stars in the field. We find a significant fraction, roughly half, of the candidate O-type stars are found in field regions, far from any obvious OB associations. These stars are greater than 2 arcmin (30 pc) from the boundaries of existing OB associations in the region, which is a distance greater than most O-type stars with typical dispersion velocities will travel in their lifetimes. The origin of these massive field stars (either as runaways, members of low-density star-forming regions, or examples of isolated massive star formation) will have to be determined by further observations and analysis.
Ultraviolet Imaging Telescope (UIT) Observations of the SMC  [PDF]
Robert H. Cornett,Theodore P. Stecher,the UIT Science Team
Physics , 1996,
Abstract: A mosaic of four UIT far-UV (FUV; 1620A) images, which covers most of the SMC bar, is presented, with derived stellar and HII region photometry. The UV morphology of the SMC's Bar shows that recent star formation there has left striking features including: a) four concentrations of UV-bright stars spread from northeast to southwest at nearly equal (~30 arcmin=0.5 kpc) spacings; b) one concentration comprising a well-defined 8-arcmin diameter ring surrounded by a larger H-alpha ring, suggestive of sequential star formation. FUV PSF photometry is obtained for 11,306 stars, and FUV photometry is obtained for 42 H-alpha-selected HII regions, both for the stars and for the total emission contained in the apertures defined by Kennicutt & Hodge. The flux- weighted average ratio of total to stellar FUV flux is 2.15; the stellar FUV luminosity function indicates that most of the excess total flux is due to scattered FUV radiation, rather than faint stars. Both stellar and total emission are well correlated with H-alpha fluxes, and yield FUV/H-alpha ratios that are consistent with models of single-burst clusters with SMC metallicity, ages from 1-5 Myr, and moderate (E(B-V)=0.0-0.1 mag) internal SMC extinction.
Far Ultraviolet Diffuse Emission from the Large Magellanic Cloud  [PDF]
Ananta C. Pradhan,Amit Pathak,Jayant Murthy
Physics , 2010, DOI: 10.1088/2041-8205/718/2/L141
Abstract: We present the first observations of diffuse radiation in the far ultraviolet (1000 -- 1150 \AA) from the Large Magellanic Cloud based on observations made with the {\it Far Ultraviolet Spectroscopic Explorer}. The fraction of the total radiation in the field emitted as diffuse radiation is typically 5 -- 20\% with a high of 45\% near N70 where there are few exciting stars, indicating that much of the emission is not due to nearby stars. Much less light is scattered in the far ultraviolet than at longer wavelengths with the stellar radiation going into heating the interstellar dust.
Observations of Far-Ultraviolet Diffuse Emission from the Small Magellanic Cloud  [PDF]
Ananta C. Pradhan,Jayant Murthy,Amit Pathak
Physics , 2011, DOI: 10.1088/0004-637X/743/1/80
Abstract: We report the first observations of far-ultraviolet (FUV: 1000 -- 1150 \AA) diffuse radiation from the Small Magellanic Cloud (SMC) using observations from the {\em Far Ultraviolet Spectroscopic Explorer (FUSE)}. The strength of FUV diffuse surface brightness in the SMC ranges from the detection limit of 2000 photons cm$^{-2}$ s$^{-1}$ sr$^{-1}$ \AA$^{-1}$ to a maximum of $3 \times 10^{5}$ photons cm$^{-2}$ s$^{-1}$ sr$^{-1}$ \AA$^{-1}$ at 1004 \AA. The contribution of diffuse emission to the total radiation field was found to be 34% at 1004 \AA to 44% at 1117 \AA with a maximum observed uncertainty of 30%. There is a striking difference between the FUV diffuse fraction from the SMC and the Large Magellanic Cloud (LMC) with the SMC fraction being higher probable because the higher dust albedo. The FUV diffuse emission correlates with H$\alpha$ emission in the H {\small II} regions of the SMC.
The Influence of Far-Ultraviolet Radiation on the Properties of Molecular Clouds in the 30 Dor Region of the Large Magellanic Cloud  [PDF]
Jorge L. Pineda,Juergen Ott,Ulrich Klein,Tony Wong,Erik Muller,Annie Hughes
Physics , 2009, DOI: 10.1088/0004-637X/703/1/736
Abstract: We present a complete 12CO J = 1-0 map of the prominent molecular ridge in the Large Magellanic Cloud (LMC) obtained with the 22-m ATNF Mopra Telescope. The region stretches southward by ~2 degrees, (or 1.7kpc) from 30 Doradus, the most vigorous star-forming region in the Local Group. The location of this molecular ridge is unique insofar as it allows us to study the properties of molecular gas as a function of the ambient radiation field in a low-metallicity environment. We find that the physical properties of CO-emitting clumps within the molecular ridge do not vary with the strength of the far-ultraviolet (FUV) radiation field. Since the peak CO brightness of the clumps shows no correlation with the radiation field strength, the observed constant value for CO-to-H_2 conversion factor along the ridge seems to require an increase in the kinetic temperature of the molecular gas that is offset by a decrease in the angular filling factor of the CO emission. We find that the difference between the CO-to-H_2 conversion factor in the molecular ridge and the outer Milky Way is smaller than has been reported by previous studies of the CO emission: applying the same cloud identification and analysis methods to our CO observations of the LMC molecular ridge and CO data from the outer Galaxy survey by Dame et al. (2001), we find that the average CO-to-H_2 conversion factor in the molecular ridge is X_CO = (3.9+/-2.5)10^20 cm^-2 (K km s^-1)^-1, approximately twice the value that we determine for the outer Galaxy clouds. The mass spectrum and the scaling relations between the properties of the CO clumps in the molecular ridge are similar, but not identical, to those that have been established for Galactic molecular clouds.
Ultraviolet Imaging Polarimetry of the Large Magellanic Cloud. II. Models  [PDF]
Andrew A. Cole,Kenneth Wood,Kenneth H. Nordsieck
Physics , 1999, DOI: 10.1086/301105
Abstract: Motivated by new sounding-rocket wide-field polarimetric images of the Large Magellanic Cloud, we have used a three-dimensional Monte Carlo radiation transfer code to investigate the escape of near-ultraviolet photons from young stellar associations embedded within a disk of dusty material (i.e. a galaxy). As photons propagate through the disk, they may be scattered or absorbed by dust. Scattered photons are polarized and tracked until they escape to be observed; absorbed photons heat the dust, which radiates isotropically in the far-infrared, where the galaxy is optically thin. The code produces four output images: near- UV and far-IR flux, and near-UV images in the linear Stokes parameters Q and U. From these images we construct simulated UV polarization maps of the LMC. We use these maps to place constraints on the star + dust geometry of the LMC and the optical properties of its dust grains. By tuning the model input parameters to produce maps that match the observed polarization maps, we derive information about the inclination of the LMC disk to the plane of the sky, and about the scattering phase function g. We compute a grid of models with i = 28 deg., 36 deg., and 45 deg., and g = 0.64, 0.70, 0.77, 0.83, and 0.90. The model which best reproduces the observed polarization maps has i = 36 +2/-5 degrees and g ~0.7. Because of the low signal-to-noise in the data, we cannot place firm constraints on the value of g. The highly inclined models do not match the observed centro-symmetric polarization patterns around bright OB associations, or the distribution of polarization values. Our models approximately reproduce the observed ultraviolet photopolarimetry of the western side of the LMC; however, the output images depend on many input parameters and are nonunique.
UIT Astro-2 Observations of NGC 4449  [PDF]
Robert S. Hill,Michael N. Fanelli,Denise A. Smith,Theodore P. Stecher,the UIT Team
Physics , 1996, DOI: 10.1063/1.52826
Abstract: The bright Magellanic irregular galaxy NGC 4449 was observed by the Ultraviolet Imaging Telescope (UIT) during the Astro-2 Spacelab mission in March, 1995. Far ultraviolet (FUV) images at a spatial resolution of ~3 arcsec show bright star-forming knots that are consistent with the general optical morphology of the galaxy and are often coincident with bright H II regions. Comparison of FUV with H-alpha shows that in a few regions, sequential star formation may have occurred over the last few Myr. The bright star forming complexes in NGC 4449 are superposed on a smooth, diffuse FUV background that may be associated with the H-alpha "froth."
Ultraviolet Imaging Polarimetry of the Large Magellanic Cloud. I. Observations  [PDF]
Andrew A. Cole,Kenneth H. Nordsieck,Steven J. Gibson,Walter M. Harris
Physics , 1999, DOI: 10.1086/301104
Abstract: We have used the rocket-borne Wide-Field Imaging Survey Polarimeter (WISP) to image a 1.5 deg. by 4.8 deg. area of the western side of the LMC at a wavelength of 215 nm and a resolution of 1' x 1.5'. These are the first wide-field ultraviolet polarimetric images in astronomy. We find the UV background light of the LMC to be linearly polarized at levels ranging from 0.04 to ~0.4. In general, the polarization in a pixel increases as the flux decreases; the weighted mean value of polarization across the WISP field is 0.126 +/- 0.023. The LMC's diffuse UV background, in uncrowded areas, rises from a minimum of 23.6 +/- 0.5 mag/arcsec**2 to 23.1 +/- 0.2 in regions near the bright OB associations. We use our polarization maps to investigate the geometry of the interstellar medium in the LMC, and to search for evidence of a significant contribution of scattered light from OB associations to the diffuse galactic light of the LMC. Through a statistical analysis of our polarization map, we identify 9 regions of intense UV emission which may be giving rise to scattering halos in our image. We find that starlight from the N11 complex and the LH 15 association are the strongest contributors to the scattered light component of the LMC's diffuse galactic light. This region of the northwestern LMC can be thought of as a kiloparsec-scale reflection nebula, in which OB stars illuminate distant dust grains, which scatter the light into our sightline. In contrast, the polarization map does not support the scattering of light from the large B2 complex in the southern WISP field; this effect may be astrophysical, or it may be the result of bias in our analysis.
Far Ultraviolet Spectroscopic Explorer Observations of a Supernova Remnant in the Line of Sight to HD 5980 in the Small Magellanic Cloud  [PDF]
Charles G. Hoopes,Kenneth R. Sembach,J. Christopher Howk,William P. Blair
Physics , 2001, DOI: 10.1086/323514
Abstract: 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 absorption from the SNR is seen in the FUSE spectrum. The O VI 1032 line in the SNR is well described by a Gaussian with FWHM=75 km/s. We find log N(O VI)=14.33-14.43, which is roughly 50% of the rest of the O VI column in the SMC (excluding the SNR) and greater than the O VI column in the Milky Way halo along this sight line. The N(C IV)/N(O VI) ratio for the SNR absorption is in the range of 0.12-0.17, similar to the value seen in the Milky Way disk, and lower than the halo value, supporting models in which SNRs produce the highly ionized gas close to the plane of the Galaxy, while other mechanisms occur in the halo. The N(C IV)/N(O VI) ratio is also lower than the SMC ratio along this sight line, suggesting that other mechanisms contribute to the creation of the global hot ionized medium in the SMC. The O VI, C IV, and Si IV apparent column density profiles suggest the presence of a multi-phase shell followed by a region of higher temperature gas.
Search for Gamma-ray Emission from Dark Matter Annihilation in the Large Magellanic Cloud with the Fermi Large Area Telescope  [PDF]
Matthew R. Buckley,Eric Charles,Jennifer M. Gaskins,Alyson M. Brooks,Alex Drlica-Wagner,Pierrick Martin,Geng Zhao
Physics , 2015, DOI: 10.1103/PhysRevD.91.102001
Abstract: At a distance of 50 kpc and with a dark matter mass of $\sim10^{10}$ M$_{\odot}$, the Large Magellanic Cloud (LMC) is a natural target for indirect dark matter searches. We use five years of data from the Fermi Large Area Telescope (LAT) and updated models of the gamma-ray emission from standard astrophysical components to search for a dark matter annihilation signal from the LMC. We perform a rotation curve analysis to determine the dark matter distribution, setting a robust minimum on the amount of dark matter in the LMC, which we use to set conservative bounds on the annihilation cross section. The LMC emission is generally very well described by the standard astrophysical sources, with at most a $1-2\sigma$ excess identified near the kinematic center of the LMC once systematic uncertainties are taken into account. We place competitive bounds on the dark matter annihilation cross section as a function of dark matter particle mass and annihilation channel.
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