Nature of Diffuse UV Background : A Study Using GALEX Observations
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[edit] ABSTRACT
We have analyzed five GALEX observed regions to study the nature of diffuse UV background and its components.
[edit] INTRODUCTION
It is well known that extended regions of faint nebulosities are present at high and moderate galactic lattitudes. While some of these nebulosities are in diffuse form others are in filamentary form. Sandage region consists of one such bright filamentary nebulosity near the galaxies M81 and M82. The cloud distance is approximately estimated as 100 pc (Sandage 1976)). According to Sandage (1976), the predicted scattered light from the galactic disc is in good agreement with the observations of the nebulosities in the field near M81 and M82 pointing out that it should be a reflection nebulae whose illuminating source is the integrated star light from the galactic plane.
Although it's been believed since long that the amount of diffuse UV radiation is well correlated with the amount of HI, especially in high lattitudes, we know several cases where this assumption is not true (ref). Our main objective of using the GALEX data is to explore these apparent contradictions and arrive at a better understanding of the nature and physical origin of diffuse UV background and its dependence on the amount of dust in the high lattitudes.
[edit] OBSERVATIONS
The Galaxy Evolution Explorer (GALEX) was launched into a low-Earth orbit (690 km) by a Pegasus XL launch vehicle on April 28, 2003 and it imaged different regions of the sky in moderately and high galactic lattitudes in two wavelength bands FUV(1528 \AA: 1344-1786\AA) and NUV (2271\AA: 1771-2831\AA). The field of view of the instrument is 1.25 deg in diameter with a spatial resolution of about 4" - 6". The GALEX imaged the SANDAGE region in January 2005-2006 as part of our own GALEX guest investigator program. The center of this observation was around an RA, DEC of 142.0, 70.4 (gl, gb => 142.3,38.2). The data are provided as FITS files from the Space Telescope MAST system.
[edit] DATA ANALYSIS AND RESULTS
The GALEX image consists of point sources and diffuse radiation which is the sum of dust scattered star light, airglow, zodiacal light (which is negligible in the FUV), integrated light of faint galaxies, dark counts, etc. We need to remove the point sources from the observed images in order to separate the diffuse background required for our analysis. The positions of stars from each of the GALEX field are extracted from three catalogs - TD-1 (Thompson et al. 1978), Tycho2 (Hog et al. 2000), and SK2 (Myers et al. 2002) - using the program SExtractor (Bertin & Arnouts 1996) and are tabulated in a catalog. We have blanked out these positions and rebinned the image to increase the signal-to-noise ratio of the diffuse background for our further studies.
