N. V. Sujatha

Dust Properties in the Far-Ultraviolet in Ophiuchus

We have derived the albedo (a) and phase function asymmetry factor (g) of interstellar dust grains at 1100 A using archival Voyager observations of diffuse radiation in Ophiuchus. We have found that the grains are highly forward-scattering, with g = 0.55 ± 0.25 and a = 0.40 ± 0.10. Even though most of the gas in this direction is in the Ophiuchus molecular cloud, the diffuse FUV radiation is almost entirely due to scattering in a relatively thin foreground cloud. This suggests that one cannot assume that the UV background is directly correlated with the total amount of gas in any direction.

Mapping the Diffuse Ultraviolet Sky with the Galaxy Evolution Explorer

We present a map of the diffuse ultraviolet cosmic background in two wavelength bands (FUV: 1530 {\AA}; NUV: 2310 {\AA}) over almost 75% of the sky using archival data from the GALEX mission. Most of the diffuse flux is due to dust-scattered starlight and follows a cosecant law with slopes of 545 photons cm-2 s-1 sr-1 {\AA}-1 and 433 photons cm-2 s-1 sr-1 {\AA}-1 in the FUV and NUV bands, respectively. There is a strong correlation with the 100 {\mu}m IRAS flux with an average UV/IR ratio of 300 photons cm-2 s-1 sr-1 {\AA}-1 (MJy sr-1)-1 in the FUV band and 220 photons cm-2 s-1 sr-1 {\AA}-1 (MJy sr-1)-1 in the NUV but with significant variations over the sky. In addition to the large scale distribution of the diffuse light, we note a number of individual features including bright spots around the hot stars Spica and Achernar.

GALEX OBSERVATIONS OF DIFFUSE UV RADIATION AT HIGH SPATIAL RESOLUTION FROM THE SANDAGE NEBULOSITY

We have observed a region of nebulosity first identified as starlight scattered by interstellar dust by Sandage using the Galaxy Evolution Explorer (GALEX) ultraviolet imaging telescope. Apart from airglow and zodiacal emission, we have found a diffuse UV background of between 500 and 800 photons cm–2 sr–1 s–1 A–1 in both the GALEX far-ultraviolet (FUV) (1350-1750 A) and NUV (1750-2850 A) bands. Of this emission, up to 250 photons cm–2 sr–1 s–1 A–1 is due to H2 fluorescent emission in the FUV band. The remainder is consistent with scattering from interstellar dust with forward scattering grains of albedo about 0.4. These are the highest spatial resolution observations of the diffuse UV background to date and show an intrinsic scatter beyond that expected from instrumental noise alone. Further modeling is required to understand the nature of this scatter and its implications for the ISM.

Measurement of dust optical properties in the coalsack nebula

We have used FUSE and Voyager observations of dust scattered starlight in the neighborhood of the Coalsack Nebula to derive the optical constants of the dust grains. The albedo is consistent with a value of 0.28 ± 0.04 and the phase function asymmetry factor with a value of 0.61 ± 0.07 throughout the spectral range from 900 – 1200 u in agreement with previous determinations as well as theoretical predictions. We have now observed two regions (Ophiuchus and Coalsack) with intense diffuse background radiation and in both cases have found that the emission is due to light from nearby hot stars scattered by a relatively thin foreground cloud, with negligible contribution from the background molecular cloud.

Survey of O vi absorption in the Large Magellanic Cloud

We present a survey of interstellar O VI absorption in the Large Magellanic Cloud (LMC) towards 70 lines of sight based onFarUltravioletSpectroscopicExplorer(FUSE) observations. The survey covers O VI absorption in a large number of objects in different environmental conditions of the LMC. Overall, a high abundance of O VI is present in active and inactive regions of the LMC with mean logN(O VI) = 14.23 atoms cm −2 . There is no correlation observed between O VI absorption and emissions from the hot gas (X-ray surface brightness) or 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 present observations in 10 superbubbles of the LMC of which we detect O VI absorption in five superbubbles for the first time and the superbubbles show an excess O VI absorption of about 40 per cent compared to non-superbubble lines of sight. 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, a good correlation between logN(O VI) and X-ray surface brightness for 30 Doradus region is present. We also find that O VI abundance decreases with increasing distance from the star cluster R136.

Diffuse Radiation from the Aquila Rift

We present an analysis of the diffuse ultraviolet (UV) background in a low latitude region near the Aquila Rift based on observations made by the Galaxy Evolution Explorer (GALEX). The UV background is at a level of about 2000 ph cm^-2 s^-1 sr^-1 \AA^-1 with no correlation with either the Galactic latitude or the 100 micron infrared (IR) emission. Rather, the UV emission falls off with distance from the bright B2 star HIP 88149, which is in the centre of the field. We have used a Monte Carlo model to derive an albedo of 0.6 - 0.7 in the UV with a phase function asymmetry factor (g) of 0.2 - 0.4. The value for the albedo is dependent on the dust distribution while g is determined by the extent of the halo.

Extragalactic survey using GALEX–Spitzer matching fields

We have used Spitzer observations of galaxies in the ELAIS-N1 region centered at 16h10m01s, +54◦30′36″ to extend the effective sensitivity of GALEX UV observations of the same region. Spitzer is more sensitive to external galaxies than GALEX and we have searched for UV sources at the positions of galaxies in the Spitzer catalog. This has allowed us to extend the sensitivity of the GALEX instrument to 27th magnitude. We have compared the derived number counts of galaxies in the two GALEX bands to standard models of galaxy evolution finding good agreement.The total contribution of these galaxies to the UV background is about 40 photons cm−2 sr−1 s−1 Å−1, in agreement with previous determinations.

Ultraviolet and Infrared Correlation Studies in Orion

Abstract We have studied the variation of diffuse ultraviolet (UV) radiation in the northern part of the Orion constellation using a set of eight areas of the GALEX All-Sky Imaging Survey in the far and near UV. Different components of diffuse UV radiation, like dust scattered emission and H2 fluorescence, were quantified and separated after removing the point sources and the foreground emission in each of the fields. Then the dependence of the individual UV components on the infrared 100 μm dust emission was studied. We did not find any positive correlation between the diffuse-UV and IR-100 micron intensities, probably due to the high optical depth of the region or the entire dust column not contributing to the diffuse UV radiation. However, in the far UV we noticed the presence of an excess emission in addition to the dust scattered radiation, which is clearly absent in the near UV. This excess emission, identified as the H2 fluorescence, is produced by the Trapezium stars in the surrounding molecular clouds. We also compare our results with those of previous studies in the region, based on Far Ultraviolet Spectroscopic Explorer (FUSE) observations.