LMC Paper
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[edit] Abstract
We present the first observations of diffuse far ultraviolet radiation from the Large Magellanic Cloud (LMC) based on observations made with the Far Ultraviolet Spetroscopic Explorer (FUSE). This radiation is primarily due to light from hot stars in the LMC scattered off of local dust. Dust in LMC is known to be different and these observations will provide insights into the properties of the dust in galaxy other than Milky Way.We have good correlations between the FUV emission & both total hydrogen column density & the 100 micron Infrared emission observed with Infrared Astronomical Satelite (IRAS).
[edit] INTRODUCTION
The Magellanic clouds provide a very different view of the interstellar medium than is possible from either within the Milky Way or from other, more distant galaxies. Unlike the Milky Way, can get an overall view of the interstellar medium without being confused by the small scale structure. At the same time we can distinguish between the different sources, particularly between the stars and the dust which is not possible in more distant galaxies.This was used to great advantage by Cole et al (1999) in developing a three dimensional scattering code for the analysis of the observed radiation. They found a fairly complex relationship between the diffuse radiation in LMC and the star and the dust distribution. It is not sufficient to merely have bright stars or to have dust: both must be present to show scattered light. The diffuse light escaping from galaxies is a diagnostic of the amount of dust and the interaction of the radiation field with the dust. It will affect the appearance of external galaxies and will affect the amount of radiation escaping to ionize the intergalactic medium (IGM). We have previously (Murthy & Sahnow 2005) shown that Far Ultraviolet Spectroscopic Explorer (FUSE) can be used for observations of diffuse sources brighter than about 2000 photons cm-2 s-1 sr-1 A-1. As a result, we decided to look for serendipitous observations of the diffuse radiation in place where the signal may be expected to be bright. One such location was the Large Magellanic Cloud (LMC) where we identified a number of observations. The Large Magellanic Cloud provides an ideal location to probe the scattering of starlight by interstellar dust grains. It is far enough away that local effects are not one of the best studied galaxies in the Universe. It is a small irregular galaxy which is a companion to our own Milky Way. Because it is so close, the different components can be studied in great detail. We have obtained the first diffuse observations in the Far Ultraviolet (FUV) of the radiation field in the LMC.We have obtained a total of 48 observations in the far ultraviolet.
[edit] OBSERVATIONS & DATA ANALYSIS
Far Ultraviolet Spectroscopic Explorer (FUSE) spacecraft and mission has been described by Moos et al and by Sahnow et al (2000). It observes the light in far ultraviolet spectral region, 905-1187 A with a high spectral resolution of 20000. The instrument consists of four co-aligned optical channels, two of which coated with Silicon carbide (Sic) & two with Lithium fluoride (LiF) over Aluminium. FUSE spacecraft was launched in 1999 June 24 on a Delta II rocket & has been observing astronomical objects since then.The instrument has four apertures: the LWRS(30*30), the MDRS (4*20) aperture , the HIRS (1.25*20) aperture & the PINH (0.5 diameter) aperture & the observation can be made through any of the three former apertures. Eventhough the FUSE spacecraft has exposures in both DAY & NIGHT part of the orbit, we have used only the NIGHT part of the exposures eliminating airglow contamination other than the Lyman lines of atmospheric hydrogen.
We have obtained a total of 48 observations of 38 independent locations from the FUSE archive.These observations are shown in an optical R-band map of the LMC in fig(1). Most of these are as a result of MDRS pointing within the LMC but a few were calibration observations (S405/S505). Our data analysis has been fully described by Murthy & Sanhow (2004).We break up the FUSE focal plane into bands & extract the values from those bands. The presence of stars is revealed by the FWHW of slit profile which is much less for a point source than for a diffuse source.We have excluded all such observations. The bands are listed in table below:
Table
Bands Used for Background Extraction.
Number......detector........ columns.............Wavelenths (A)
1..........LiF 1A ...........1100-600.......... 987.08-1020.77
2..........LiF 1A .......... 7500-15,000.........1034.84-1081.37
3..........LiF 1B ..........2000-7000...........1100.28-1133.69
4....... ..LiF 1B ..........7000-14,000.........1133.69-1180.07
5..........LiF 2A .......... 2000-7000...........1175.32-1141.97
6..........LiF 2A ...........9000-14,000.........1128.57-1095.03
For our purpose of studying FUV diffuse emission we have taken the data through the LiF LWRS aperture at effective wavelength 1003A and the amount of emission for each location is given in the table (x). In LMC the intensity of FUV diffuse emission varies from 1500 photons cm-2 s-1 sr-1 A-1 to 75,000 photons cm-2 s-1 sr-1 A-1 (reffered as CU) The other data set we have used is the IR fluxes at 12 ,25 ,60 & 100 micron in MJy Sr-1 & we have measured these from the corresponding Infrared Astronomical satelite (IRAS) image.Total hydrogen column density is calculated using a foreground reddening of E(B-V) from Sclegel et al (1998) & gas to dust ratio of 2.22* 10^22 H cm-2 for LMC from draine (2003). The neutral hydrogen column density have been taken from Dickey & Lockman (1991). In order to study the correlation between the FUV and IR intensities, we checked our observation in each locations for nearby bright stars. If there is any nearby bright star, it will certainly affect our observation. To verify this we took circles of LWRS aperture size around each location on the Ultraviolet Imaging Telescope (UIT) images (Parker et al. 1998) and identified nearby UV sources for seven locations out of the 48 observations: Three are in N11 region, two are in supernovae field, one is in 30 Doradus region & one is in some other region of LMC. While measuring the IR intensities from the IRAS images, we got two points with very high IR intensities in all the four IR wavelengths (29 & 30th row from table:2). These two points are well inside the 30 Doradus region which is an immense star forming region in LMC having an interacting complex of massive stars, molecular clouds and diffuse gases. Gas and dust clouds in 30 Doradus have been sculpted into elongated shapes by powerful winds and ultraviolet radiation from these hot clusters of stars.The high values of IR in those regions is due to more molecular clouds in those regions which is obvious from the table (xx) that these two locations have higher values of total hydrogen column density.
Table:2
[edit] Result
Large Magellanic Cloud is a nice laboratory for astronomers to explore interesting results due to introduction of air based satellites. Large number of studies going on to depict amaging picture of various objects of it, which may be different from Milky Way.The diffuse interstellar medium is one of such object of LMC, different from our galaxy in several ways like in metalicity, gas to dust ratio etc. We have got some 48 diffuse sources from FUSE spacecraft to investigate the diffused light and dust in LMC. We have overplotted circles on an optical map of the LMC (figure:2) to show the intensities of all locations while the radius of the circle is proportional to the FUV diffuse emission. Cole et al (1999) found that scattered light from N11 complex strongly contributes to the diffuse galactic light of LMC .We can support their claim as we have some 15 diffuse sources in N11 region. There is no FUSE points near NGC 1818 as there is no dust to scatter star light.
FUV diffuse emission is mainly due to light from hot O,B stars scattered off of dust grains in ISM of galaxies. Several works have attempted to study its relation with Infrared wavelength for different celestial regions. The interest to study this connection is because the dust in ISM which scatters in UV does absorb a part of the star light and re-emit in IR wavelength. A good relation between the two wavelength can give a vivid pictures of dust & other physical parameters associated with it. For our purpose, we have ploted the FUV diffuse emission in CU against IR fluxes in MJy Sr-1 (fig: 3) at 12, 25, 60 & 100 microns for LMC. In this plot we have eliminated those two data points (29 & 30) beacause these two points were very discrepant with respect to the average trend. We calculated the slope of the correlations by polyfit method and the correlation coefficients for all of them which is given along with the plots. We found that all have good correlations showing that FUV emission is increasing with increase in IR fluxes. That means FUV emission is proportional to the amount of dusts as traced by the IR fluxes.
There are many studies of the UV/100micron correlation on different regions. Haikala et al. (1995) found a ratio of 128 CU/(MJy Sr-1) for a Galactic cirrus cloud G251.2+73.3 near the north galactic pole using the FAUST data, Schiminovich et al. (2001) found a latitute dependent ratio of 60 (b > 30) to 100 (b > 15) CU/(MJy Sr-1) using NUVIEWS instrument and Murthy et al. (2001) found the ratio varying between 30 and 300 CU/(MJy Sr-1) in Midcourse Space Experiment (MSX) observations around M42 in Orion. We have plotted our FUSE data against the IR 100 micron flux along with Haikala et al. (1995) & Schiminovich et al. (2001) in figure:(4) in which our FUV/100 micron ratio is 172.75 CU/(MJy Sr-1), a bit higher among them. It may be due to large number of OB associations in LMC which sends light that is getting scattered by dust.
Several works have been studied the correlation between diffuse UV background intensity and line of sight neutral hydrogen column density. It was found that a good correlation exits between the two at high and intermediate galactic latitutes. (Paresce et al,1980, Joubert et al,1983, Jakobsen et al, 1984). Existence of this correlation gives evidence of galactic component to the diffuse UV background due to scattering of starlight by high latitute interstellar dust mixed with neutral hydrogen.(Jakobsen 1982). LMC is one of our extragalactic glimpse to investigate the variousproperties related our galaxy. Cole et al (1999) tried to study the correlation between the H1 and diffuse UV of the western side of the LMC at a wavelength of 2150 A using Wide-Field Imaging Survey Polarimeter (WISP) and they found some correlation for North part of the WISP image of LMC where stellar density is low but not for the sourthen part. However for our FUSE data which covers larger part of LMC the diffuse FUV is not correlating with HI (figure :5) but it correlates with total hydrogen column density (figure:6) .
It is revealed that amount of molecular clouds are distributed through out the LMC and dust grains are well associated with the molecular clouds and the diffuse radiation we observe is the light from hot OB stars scattered off of dust. For Av>1, FUV radiation field inside the clouds can differ by orders of magnitude depending upon the grain properties & growth (J.R.Goicoechia,2007). This correlation is interesting beacause it is known that for optically thick regions FUV diffuse radiation gets saturated when column density is greater than 10^21/cm2 . That means the molecular clouds are existing in layer like structures in LMC with OB stars in between which gives the correlation eventhough the optical depth is very high. Figure: (6) shows the correlation between IR 100 micron intensity and total hydrogen column density with correlation coefficient r = 0.94 . One should not be surprised to find this since FUV diffuse emission correlates with both IR 100micron intensity & total hydrogen column density. It is obvious from the plot (6) that interstellar dusts and molecular clouds are uniformly distributed. As molecular clouds are partly optically thick to the external radiation field , the contribution of the ISRF to the infrared emission depends on the geometry and optical depth of the clouds.
We have done all the analysis for the data obtained throgh LiF 1A1 band at effective wavelenth 1003A. We have also same data through other bands and to prove these data to be adequate, we studied the correlations of FUV diffuse emission obtained through different bands . We got very good correlations among them as it is expected . The correlation between LiF 1A1 and LiF 2A1 is shown in the figure (7) with correlation cofficients.
[edit] CONCLUSION
[edit] References
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