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Signatures of diffuse interstellar gas in the Galaxy Evolution Explorer all sky survey

The all sky survey run by the Galaxy Evolution Explorer (GALEX AIS) mapped about 85\% of the Galaxy at ultraviolet (UV) wavelengths and detected the diffuse UV background produced by the scattering of the radiation from OBA stars by interstellar dust grains. Against this background, diffuse weak structures are detected as well as the UV counterparts to nebulae and molecular clouds. To make full profit of the survey, unsupervised and semi-supervised procedures need to be implemented. The main objective of this work is to implement and analyze the results of the method developed by us for the blind detection of ISM features in the GALEX AIS . Most ISM features are detected at very low signal levels (dark filaments, globules) against the already faint UV background. We have defined an index, the UV background fluctuations index (or UBF index), to identify areas of the sky where these fluctuations are detected. The algorithm is applied to the images obtained in the FUV 1344 -- 1786 Angstroms) band since this is less polluted by stellar sources, facilitating the automated detection. The UBF index is shown to be sensitive to the main star forming regions within the Gould's Belt, as well as to some prominent loops like Loop I or the Eridanus and Monogem areas. The catalogue with the UBF index values is made available on-line to the community.


Introduction
In 2003, the GALaxy Evolution eXplorer (GALEX) was launched into low Earth orbit. The mission lasted ten years and was equipped with wide field cameras that provided simultaneous far ultraviolet or FUV (1344 -1786 Å) and near ultraviolet or NUV (1771 -2831 Å) imaging using photon-counting, MCPtype, detectors (Martin et al. 2005). GALEX run the first all sky survey at UV wavelengths providing highly valuable data for interstellar medium (ISM) studies.
The UV radiation produced by the massive OBA stars in the Galaxy is scattered and absorbed by the dust in the ISM providing important clues on the grains albedo and composition (Saslaw & Gastaud 1969, Draine, 2003 especially, at the low size end of the size distribution. An additional contribution to the UV ISM radiation comes from the molecular hydrogen in the envelopes of the molecular clouds that absorbs efficiently the Lyman-α photons from the UV background and reprocess them into H 2 fluorescent emission in the electronic Lyman band; these transitions are significantly stronger than the infrared (magnetic quadrupole) transitions, often used in ISM studies. Moreover, the envelopes of ionized nebulae are detected in the GALEX bands; the resonance transitions from neutral (H I, C I, O I), singly ionized (Mg II, C II, O II, Fe II, Al II) or multiply ionized gas (C IV, Si IV, N V) are observed within the range covered by GALEX. Some examples of GALEX images of interstellar objects are shown in Figures 1-3. The Rosette nebula is an excellent example of a reflection nebula. Rosette is illuminated by a cluster of B8-9 stars and dense (dark) filaments are observed against the bright background produced by the radiation scattered by the icy dust grains in the nebula (see Figure 1); GALEX sensitivity to faint absorption nebula is shown in Figure 2 for Barnard 29. The exposure time is about 200 s in the FUV band, significantly less than required to obtain an image of the dark clump with similar SNR in the optical range.
The Vela loop is shown in Figure 3. The loop is prominent both in the NUV (Fe II, C II) and in the FUV (C IV, Si IV, He II) bands and the braiding of the filaments is neatly observed even in the ∼ 90 s exposure time images of the GALEX AIS.
All these features are characterized by being observed at intensity levels similar to the UV background level. The dark globules in Figure 2 would have passed unnoticed to a simple inspection of the GALEX tiles.
During its lifetime GALEX run an All Sky Survey (or AIS) that included 28,707 images (or tiles) in the FUV band and 34,285 images in the NUV band (Bianchi et al. 2014), with a total sky coverage of 26,000 square degrees. The wealth of information contained in GALEX/AIS images is huge and requires of unsupervised procedures to search for weak interstellar structures. This work deals with this challenge; we describe, implement and analyze the results of a method developed by us, for the blind detection of nebular structures, condensations and filaments in the GALEX survey. In Section 2, we describe the method and derive the index to search for diffuse features in the GALEX AIS. In Section 3, the results are presented and com-

Description of the method
For each field imaged by GALEX, the mission provides detailed information including raw data, calibration files and the end data products (see, Morrisey, 2007), all accessible through the Mikulski Archive of Space Telescopes 1 (MAST). Among the whole set of data products provided for each field or tile, there are three relevant to this work namely, Upon the mission completion, GALEX/AIS data products were used to study the galactic UV background at the Sun's location in the Galaxy. Murthy et al. (2010) built a map of the local UV background at NUV and FUV wavelengths using the sky background data derived by the mission (the -skybg files). An all-sky map of the UV background was provided after normalization by the exposure time and mosaicking the full set of -skybg images into an Aitoff projection. Later work by Handem et al. (2013) applied a more sophisticated treatment that involved the removal of the point-like UV sources from the intensity images (-int ) by implementing mask files including the effect of dust grains scattering of the UV radiation from point-like sources. For our work, we use the end, data products from the mission, the calibrated images (-intbgsub images) distributed through the MAST.
The first challenge for the blind detection of ISM features is their low signal; ISM filaments are observed as small fluctuations over the background that it is already very weak. For instance, the FUV background is < 1 × 10 −2 counts s −1 pixel −1 in the Taurus molecular complex (b gal −20 o ) hence, less than one photon is detected per pixel in the ∼ 100 s of exposure time, characteristic of GALEX AIS (Morrisey et al. 2007). To reach statistically significant levels, the images have been re-binned to resolution elements (or resels) of 30×30 pix 2 (or 45×45 arcsec 2 ); in this manner, the count rate is raised to tens of counts s −1 per resel even at high galactic latitudes; note that this degraded angular resolution is still significantly better than the resolution of the IRAS survey at 100 microns (Beichman et al., 1998).
After rebinning, the images have been inverted 2 to enhance the background. The statistical estimator of the background fluctuations is computed over these rebinned and inverted images, M, with 64 × 64 elements, M(i, j), as follows:.
1. All elements of matrix M are arranged into a vector, V, of 64×64 = 4096 elements such that element (i, j) in the matrix corresponds to element k in the vector with k = ( j−1) * 64+i. 2. Subsets, V l , with l ∈ [1, 10], are defined from V so, V l = [V((l−1) * 400), V(l * 400−1)], each containing 400 elements. 1 archive.stsci.edu 2 Each pixel has been assigned a new value that is the inverse of the original; each pixel (i, j) of the inverse image, M, is assigned a value m i, j = 1/gr i, j with gr i, j , the value of the pixel in the original GALEX rebinned image, GR 3. For each subset l, the arithmetic mean, the maximum, and the minimum are computed: Mean(l), Maximum(l), and Minimum(l), respectively; an outlier detection algorithm has been used. 4. For each tile, the Background Scale Variability Rating (BSVR) is computed. The BSVR is defined as, with l running from 1 to 10, the number of subsets.
The process is illustrated in Figure 4. The original GALEX image, as provided by MAST, is shown in the leftmost panel.
Only the central square (1920×1920 pix 2 ), marked in white in the image, is used for the calculation of the BSVR. This area is rebinned into resels of 30 × 30 pix 2 , as shown in the central panel. Then, the value of each pixel is plotted in the right panel; pixels are read from top to bottom and from left to right. Thus, the first 1,000 pixels correspond to the leftmost area of the image where no significant fluctuations (filaments) are detected while the last 1,000 pixels correspond to the rightmost area where the dark filaments concentrate. This results in an increasing dispersion from the top-left corner (pixel number 1) to the bottom-right corner (pixel 4,000).
Note that as M is made of the inverse (rebinned) values, the contribution to the BSVR of the bright points/areas in the image (mainly point-like sources) is negligible, provided that the number of those is small and does not affect significantly the mean. In this case, Mean(k) >> Minimum(k) resulting in, The number of point-like sources in the FUV band is a tenth of those detected in the NUV band. Henceforth, the BSVR index is determined from the FUV images in the survey to optimize its performance.
The resulting BSVR all-sky map is plotted in Aitoff projection in Figure 5. Two main features are clearly discernible: [1] the BSVR depends on the galactic latitude and [2] there are some clear stripes (maximum circles) where the BSVR is anomalously low.

Correction from the dependence on galactic latitude: the UV Background Fluctuations Index (UBFI)
In Figure 6, the histogram of observed BSVR values per galactic latitude is represented. The butterfly-like pattern indicates that there are two neatly defined areas in the sky. Over most of the sky, the BSVR increases with galactic latitude however, the area with BSVR < 0.6 does not follow the same trend. The general dependence of the BSVR on galactic latitude is well fitted by a power law of sin |b|: The UBFI histogram of occurrences is shown in Figure 7. It displays a maximum at UBFI 1 that can be well fitted to a normal distribution with dispersion σ = 0.272±0.001 and shows the goodness of the fitting in Eq. 3. Some excess is observed in the wings of the distribution produced by the outliers, the weak nebular structures we aim at detecting through the UBF index.
There is also, a secondary maxima at UBFI < 0.5 that comes from the areas with BSVR ≤ 0.6 in Figures 5 and 6. In these areas, i.e. these index values trace areas of the sky where the UV background is unusually uniform. The pattern is most clearly recognizable in equatorial coordinates, both Earth's Equator and some polar orbits are readily recognizable (see Figure 8).
The main conclusion to be drawn is that: the UV background is characterized by a natural level of fluctuation that is artificially damped in certain areas of the sky. This damping results most likely from the GALEX data processing.

Results: UV background fluctuations derived from the GALEX FUV AIS
The UBF index all-sky map is built from the 83.081 GALEX AIS FUV tiles and represented in galactic coordinates (Aitoff projection) in Figure 9  Additional areas with strong fluctuations are at high galactic latitudes, some seem to be associated with prominent ISM shells or loops such as the Eridanus loop or Loop I that are especially noticeable in the soft X-ray background map of the Galaxy built by the ROSAT mission (Snowden et al. 1995).
The catalogue with the UBF index computed for the 83.081 GALEX AIS FUV tiles is available on line at the Joint Center for Ultraviolet Astronomy (jcuva.ucm.es) and through the services of the Centre de Donneés Stellaires (CDS). For each GALEX AIS tile, the galactic and equatorial (ICRS) coordinates of the center of the field, as well as the UBF index are provided (see Table I, for an excerpt). Note that there typically 2 or 3 entries per location in the sky since the AIS mapped all the sky several times. As shown in the table, in most cases the UBFI is the same for all measurements of the same field. However, note that occasionally we have detected discrepancies (see the first three entries in Table 1). These values are left as calculated in the catalogue for reference.

Interstellar clouds in the far UV
According with the histogram in Figure 7, the fluctuations introduced by interstellar structures are expected to be at UBFI > 1.3 that corresponds to 2.5σ above the average (see also Figure 9a). In general, good matches are found between the far UV structures and the infrared structures in the nearby molecular clouds. A good example is the Taurus-Auriga region. In figure 7, the far UV absorption is compared with the infrared emission for some dark globules and filaments in the area. On the left panels, far UV maps are displayed in logarithmic scale; they are relativeextinction maps computed as, where C i, j is the count rate in resel (i, j) and C 0 k is the average count rate in the tile. On the right panel, IRAS (100 microns) images of the same features are plotted. Key contours from the left (FUV) panel are overimposed in the right panel for guidance. Note that the FUV extinction, A FUV , scales with the column density of dust grains 3 , N d , as, A FUV ∝ N d and the thermal emission, F 100µ , is also expected to be proportional to N d thus, A FUV ∝ F 100µ roughly. In practice, the details of the grains composition, albedo and size distribution will introduce deviations over this simple trend providing a unique tool for dust grains characterization.
There is not however, a similar good match with the diffuse structures detected by the Galactic Arecibo L-Band Feed Array H I (GALFA H I) survey. GALFA HI maps with unprecedented sensitivity and spatial resolution of 3.5 arc min, the distribution of neutral Hydrogen in the 21 cm line (Begun et al. 2010). 96 H I clouds were reported to have LSR velocities smaller than 90 km s −1 and, in principle, could be close enough to be detectable within the GALEX AIS. However, though some some good matches have been found, as shown in Figure 11 most of the clouds have not been detected by GALEX, probably because of the limited depth of the GALEX AIS survey.

Conclusions
In this work, we have mined the FUV images in the GALEX all sky survey to detect weak, extended structures in the ISM. We have derived an index, the UBF index, that can be reliably used to search for fluctuations of the FUV background in the sky. The U BFI > 1.3 in the main nearby star forming complexes and some prominent loops in the ISM. Fig. 1  The original GALEX images have been mosaiced together (coordinates as in Figure 1).      Table 1 of the catalogue.