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A Study of Diffuse UV Emission from SPITZER First Look Survey Region Using GALEX DIS Observations

[edit] Introduction

GALEX observations provide the highest spatial resolution images of the diffuse UV background, with an effective spatial resolution of about 5\arcsec. We analyzed the diffuse UV radiation from the boundary of a high latitude molecular cloud complex, MBM 41-44, also known as Draco Nebula, using 11 GALEX DIS observations both in FUV (1350-1750\AA) and NUV (1750-2850\AA) bands. These targets are from an optically thin region with optical depth less than 1.0 which might be part of the nebula. The observational details are given in Table.1.

MBM 41-44 is an intermediate velocity cloud (vlsr =-21 km/s) at high galactic latitude subtends 5 degrees on the sky and exhibits a number of compact cores and a distinct filamentary structure trailing towards high-b and high-l from the high-density complex at (l ∼ 90, b ∼ 38). This object was discovered on the basis of a H I survey conducted by Goerigk et al. (1983). A detailed H I map of the Spitzer First Look Survey by the 100 micron Green Bank Telescope (Lockman & Condon 2005) detected a portion of the Draco nebula at a peak H I column density of 7.8 × 10¹⁹ cm^-2. This molecular cloud is at several hundred parsecs distant from the Sun (between 327 and 861 pc; Gladders et al. 1998), and is well away from the galactic plane.

Point sources were removed from each GALEX observation to extract the diffuse signal using the merged catalogue given in the GALEX archive. The signal to noise was increased by binning the image to 2' x 2' (80 x 80 pixels) which also enabled us to minimize the effect of any unidentified sources present in the field. The major components of diffuse UV radiation such as airglow, zodiacal light, dust scattered starlight and H2 fluorescence were extracted. The method of extraction is explained in Sujatha et al. (2009). We then studied the dependencies of these components to factors such as solar activity and Infrared 100 micron intensity, specific to each of these components.

[edit] Observed Region

[edit] Observational Details

The latitudes of 11 locations under study and that of the SANDAGE region (cloud MBM 30) are close to each other and are between 34 deg and 38.5 degree but the longitude of the 11 targets are around 88 deg and that of the SANDAGE region is about 142.3 deg.

RA: 16 50 30, DEC: 61 09 17

The number of total (fuv+nuv) sources, % area blanked out, total fuv, nuv count rate and % contribution of the sources and the bright points to the total count rate etc are tabulated in Table.1b. for each field. The source contribution is very high in the fields SIRTFFL_04, 06 & 09 because of the presence of a bright FUV source in the overlap region of these three observations whose galactic position is (89.28, 35.905). Similarly the source contribution is high in the field SIRTFFL_05 due to the presence of an A0 star (BD+60 1753, gl=89.359, gb=33.957) with B & V magnitude near 9.7.

[edit] Schlegel's Dust Map & IRAS 100 micron Map of the Region

Schlegel's dust map of the region with GALEX field of view (1.26^O) of 11 DIS targets under study are shown in Fig. 2a. The same on an IRAS 100 micron map is shown in Fig 2b. Stars brighter than 30,000 counts sec-1 (in blue diagonal hatch) & Stars fainter than 30,000 counts sec-1 (in green) in the region are also shown. The intensity of the diffuse galactic light in the image has been enhanced to maximize the contrast within the region plotted.

E(B-V) variation in the field is 0.01 - 0.05. Optical depth variation is between 0.44 to 1.0. The optical depth in the SANDAGE region varies between 0.8 and 3.4. The effective cross-section for the dust grains is almost the same in FUV and NUV at 1.4 x 10^-21 cm².

[edit] ISRF Variation in the Region

In our locations, on an average there is 22% and 26% increase in FUV and NUV ISRF compared to SANDAGE region.

[edit] Neutral Hydrogen Column Density Map of the Region

Fig.4. N(HI) in the field from GBT (NRAO).

Lockman & Condon (2005) observed the area of SPITZER FLS survey, where most part of our targets FOV encloses, in the 21 cm line of HI with the Robert C.Byrd Green Bank Telescope (GBT), which covered an area of 3^ox3^o centered at (J2000.0) RA=17h 18m, DEC=59d 30m (l=88.32, b=+34.89) with a FWHM beam of 9.2' in the 21 cm line at 1.420 GHz. Within this area HI spectra were measured every 3' in both coordinates with an integration time of 3 sec/pixel and corrected for stray-radiation contamination. These data produced accurate values of the neutral hydrogen column density N(HI) throughout the field and is shown in Fig.4. We have made use of these values in our study. The dust ridge seen in the bottom right corner of Fig. 4 is part of our target SIRTFFL_03 & SIRTFFL_10. It is a dense, nearby (d < 86 pc), low velocity HI cloud (LVC 88+36-2) which is located within the Local Hot Bubble.

Fig.NHI. Different components of HI map in the SFL field.

Fig.NHI-IR. LVC N(HI) (left) & total N(HI) (right) are plotted against IRIS 100 micron emission for the SFL field.

[edit] Distance of Various clouds

Cloud	Distance (d)	Method	Reference
LVC	60 20 pc	NaI absorption line study	Wennmacher et al. (1992)
LVC closest part	~55-60 pc		Lallement et al. (2003)
LVC densest part	<85 pc		Lallement et al. (2003)
IVC	300pc < d < 800pc	star counts	Mebold et al. (1985)
IVC	800 to 2500 pc	photometric study	Goerigk & Mebold (1986)
IVC	d > 180 pc	NaI absorption lines toward nearby bright stars	Lilienthal et al (1991)
IVC	463+192-136 pc < d < 618+243-174 pc	NaI absorption line study	Gladders et al (1998)
IVC	average d = 105 pc		Danner (1998)
IVC	800 to 1300 pc	reddening study	Penprase et al. (2000)
HVC	4-10 kpc		van Woerden et al. (1999)
HVC	10 2.5 kpc		Wakker et al (2007), Thom et al. (2008)

[edit] Scatter in the Data

[edit] Sources of Scatter

For a photon counting instrument such as GALEX, the instrumental scatter will be either due to photon noise or to errors in the flat fielding (calibration) of the instrument.

Although, I had originally thought that different visits could address the issue of flat fields, there is a roll between different visits which means that different pixels are being observed. Therefore all we can hope to probe is photon statistics.

In both cases, we assumed a linear relationship between the two sets of data but allowed a baseline shift to allow for different airglow/zodiacal light contributions.

[edit] Separation into different visits

We broke the FUV & NUV visits of each observation into two sets and compared the values with each other. The results are tabulated in the title link. The errors are more or less compatible with photon statistics alone.

[edit] Study of Overlap Regions

There is quite a bit of scatter in the data which shows up as a poor correlation when we plot individual regions of overlap. The reason for this is that there is not much of a variation in the values for most of the regions. However, if we plot all the regions together, there is an excellent correlation (Fig 3c &3d) with correlation coefficient of 0.88/0.75 in FUV/NUV. If we avoid those observations (5&7, 4&9) which has high photon noise then the correlation will further increase to 0.94/0.87 in FUV/NUV bands (Fig 3a & 3b).

Fig.3a. FUV-FUV correlation in the overlap regions.

Fig.3b. NUV-NUV correlation in the overlap regions.

If we assume that the chisq values are 1 for this region and adjust the error bars accordingly, as we did with the individual visits, we can find the errors empirically. These are typically about the same as for individual visits; that is, they are consistent with photon noise alone. However, the NUV data are somewhat higher. We don't yet have the data to check this properly but, for now, it will suffice if we assume an error of about 40 photon units.

[edit] Summary

Based on these data, the FUV data are more or less consistent with photon errors only while the NUV is somewhat higher (Fig.5). It is possible that either there is a flat-fielding issue with the NUV data or, more likely, that the baseline is about 40 photon units, irrespective of the photon flux. The long exposure times for the NUV data bring the level of the photon noise to less than 10 photon units. The error is larger in the regions of overlap as opposed to the separation by visits. This may be because there is much less overlap (see Fig.5a) in the different observations. One thing to try is to divide the visits into 2 rings - the central part and a ring around that and see what the scatter is there.

Fig.5. Intrinsic scatter versus predicted level.

Fig.5a. Percentage area overlaped versus the scatter level.

In general, the observed scatter is consistent with photon noise alone but in the overlap region, between different observations, the scatter is somewhat higher than the photon noise due to the many fewer points in the overlap regions and their location near the edge of the detector. Please note that all our comparisons are in sky coordinates because there are arbitrary roll angle differences between different visits which do not allow a comparison between physical detector pixels.

[edit] Summary of Statistics

[edit] Photon Statistics

Assumption: Total signal is 1500 photon units
FUV:        Corresponds to 1.4e7 counts/sr/s
Detector:   5300 counts/s over the detector.
Time:       5000 s
Total counts in the 1 arcminute pixel: 6000 counts
Photon statistics: 77 counts
In photon units: 20 photon units.

[edit] Components of Observed Diffuse Radiation

[edit] Foreground Emission

[edit] Airglow

Airglow in the GALEX bands is due to the resonantly scattered geocoronal oxygen lines at 1304 (assume 10% leakage to the GALEX FUV band), 1356 and 2471 \AA from the exosphere of the Earth. The typical values of these lines observed by HST from 600 km altitude and the possible values through GALEX bands, derived from this, are tabulated in Table 2. According to this, we can expect not less than 220/100 photon units in the GALEX FUV/NUV bands from these lines. Note that these values will vary depending on the position of the telescope with respect to the day-night terminator, solar activity and the position of the target relative to the Earth limb.

We extracted this major component of diffuse radiation (Airglow) from our observed fields using the total count rate from the spacecraft state file of each visit. While extracting the airglow, in our observed fields, we found that the airglow constitutes of two parts - time variable part and uniform level added to the astrophysical signal at local midnight.

Fig.6a. Total FUV count rate as a function of local time.

Fig.6b. Total NUV count rate as a function of local time.

In Figure 6 we have plotted local time of each observation versus the time variable component of airglow, derived after subtracting the astrophysical signal and ZL, in the NUV. Since the shape of the curve is same for all the observations, we did a curve fit by minimizing the chi-square and derived empirical equations (given below (equ. 1)). The average area under this curve is the variable component of airglow and we found that its value is approximately 77.5/67.2 photon units in the fuv/nuv bands.

FUV_AG(t) = 24.5*t² +11.6*t ------ (1a)

NUV_AG(t) = 16.1*t² + 5.94*t ------ (1b)

Since the 10.7 cm (2.8GHz) radio emission from the Sun has been used as an indicator of Solar activity for various kinds of Solar terrestrial effects (Chatterjee et al 1995). The solar activity data are from Space Weather Canada Daily Measurements[1]. The minimum TEC levels & Solar flux (SF) are correlating well with some shift in the non-zero intercept values which is due to the difference in the astrophysical signal and the zodiacal light in the observed regions. However the slope of the lines corresponding to different observations are almost same, which clearly indicates a linear relationship between the two.

Fig.8a. FUV AG_C level versus solar flux at the Earth.

Fig.8b. NUV AG_C level versus solar flux at the Earth.

We subtracted the offset values (tabulated in Table 5), assuming zero solar flux corresponds to zero airglow, from the minimum TEC level and extracted the uniform airglow component which is intrinsically embedded in this TEC level of each visit. We have plotted these values as a function of solar flux in Figure 8 and found very good correlation with coefficients 0.91/0.89 in the FUV/NUV bands. By minimizing the chisq value of the linear fit, we derived empirical equations (see equs. 2 & 3) to predict the FUV/NUV levels of uniform airglow component at any time.

FUV_TEC_MIN = 3.4*SF + I_S + I_DGL ------ (2)

NUV_TEC_MIN = 3.7*SF + I_ZL + I_S + I_DGL ---- (3)

If we combine the two parts of the airglow then the equation for the total airglow becomes:

FUV_AG = 3.4*SF + 24.5*t² + 11.6*t ------ (4)

NUV_AG = 3.7*SF + 16.1*t² + 5.94*t ------ (5)

Using equation 4 & 5, we estimated the average airglow contribution in each of the observations (Table 6) and found that this varies between 300 and 370 photons cm^-2 s^-1 sr^-1 A^-1 (hereafter 'photon units' or 'pu') both in the FUV & NUV bands which is higher than the expected values from the HST measurements. The possible error bar (due to the scatter) calculated from the predicted levels of airglow is 41 and 28 photon units in FUV and NUV bands respectively.

[edit] Zodiacal Light

Zodiacal light will contribute to the NUV signal. The best estimate of the zodiacal light comes from Leinert et al. (1998) and we have incorporated this into a zodiacal light calculator. The value will vary per visit and is tabulated in Table 6. The value varies between 342 and 407 photon units.

[edit] Total Foreground Emission

There are several measures of the foreground emission (by which we mean airglow and zodiacal light).

1. Using the airglow equation and the zodiacal light calculations. This results in a relative uncertainty of about 50 photon units.
2. Minimizing the scatter between FUV-FUV & NUV-NUV data in the overlap regions. This is likely to be the most accurate method because it makes no assumptions about the source of the signal.
3. Minimizing the scatter between FUV and NUV. This makes the assumption that FUV and NUV are linear and minimizes the chisq. This method may not give accurate values if the FUV/NUV ratio varies across the field.
4. Minimizing the scatter between FUV and IR100 and NUV and IR100. This assumes that the UV is correlated with the IR. The numbers are entirely consistent with the above so we'll use one. Note that they should be the same because minimizing FUV/IR and NUV/IR is equivalent to minimizing FUV/NUV. This also may not give accurate values if the ratio varies.

Fig.6a. Overlap versus calculated.

There is good agreement between the calculated offsets and the overlap offsets. We have estimated about 30 photon unit uncertainty in the calculated offsets, which is probably a decent estimate.

The total foreground emission (Table 8), ranges from 20% to 50% of the total emission with an uncertainty of about 30 photons cm⁻² sr⁻¹ s⁻¹ A˚⁻¹ , estimated using the spatial overlap between different observations. It should be emphasized that the foreground emission affects only the level of the offset and will not affect the spatial variability of the diffuse radiation field.

[edit] Background Emission

Apart from the uniform foreground emissions discussed above, we found a level of 300-1200 photon units in FUV and 400-800 photon units in NUV band as the total background emission. The zero level we used to normalize each image is the value obtained from the overlap regions. Each of the sky background images are here: PII_sky_background_images.

[edit] Correlation Studies of the Background Emission

[edit] FUV and NUV correlations

Figure 9 shows the overall correlation between the FUV and NUV emission from the observed field (3^o x 3^o). The correlation here is 67 %. We can select smaller areas such as the obvious ridge which yields a correlation of 0.8, much better than the 0.55 in the rest of the region. None of the other clouds make much of a difference, nor does it matter if we choose the entire region. The coefficients of the linear fit to the data are tabulated in Table 9. The FUV versus the NUV is shown in Fig. 9 with the equivalent Sandage data added. There is essentially no difference between the two. Note that I have made no effort to subtract the putative molecular hydrogen from either field.

Fig.9. NUV - FUV Correlation in SIRTFFL and Sandage Regions.

[edit] IR and UV correlations

There is a strong correlation between the 100 micron and diffuse UV data, unlike in the Sandage region (Fig. 11). The ratios follow a very nice slope as would be expected for saturated data. Only the ridge stands out in the FUV/IR ratio but not so much in the NUV/IR ratio.

Fig.11a. IR 100 - FUV Correlation.

Fig.11b. IR 100 - NUV Correlation.

[edit] UV/IR Ratio Fit Using Exponential Function

Fig.11c. IR 100 - UV/IR ratio Correlation.

A different way of looking at this is seen in Fig. 11c where we have plotted the ratio between the UV bands and the IR. There is a clear trend from the low optical depth Draco region to the high optical depth (in the UV) Region I (but still low in the IR) with an empirical formula of

UV/IR = 415 * exp(−0.22×IR).

It is interesting that the UV/IR ratio in our GALEX data follows a continuous curve very similar to that found by Murthy et al. (2001) in Orion using data from the Midcourse Space Experiment (MSX) even though the UV and the IR fluxes in Orion were each greater by a factor of about 200, reflecting the intense radiation field there. However, quite different values are cited in the literature for other regions with ratios ranging from near -50 to almost 260 photons cm⁻² sr⁻¹ s⁻¹ A˚⁻¹ (MJy sr⁻¹ )⁻¹ (see Table uv/ir slope below) with little dependence on the IR (see Sasseen et al. (1995); Sasseen & Deharveng (1996)). It is likely that these relations are only apparent when observed at a high enough spatial resolution; the MSX data were at a resolution of 20′′ and these data are at a resolution of 2′ , while the other observations are at resolutions of 0.5◦or worse. Because both the IR and the UV vary on smaller scales, the measured UV/IR ratio may not be a reliable estimator of the true ratio. In fact, Sasseen & Deharveng (1996) found a total UV/IR ratio of 255 photons cm⁻² sr⁻¹ s⁻¹ A˚⁻¹ (MJy sr⁻¹ )⁻¹ when they calculated the slope using all their data, higher than any of the individual data sets.

[edit] FUV-NUV RATIO

As in the Sandage region, there is a linear dependence of the FUV/NUV ratio with the FUV but not with NUV or with IR (Fig. 10a, 10b). We had identified this as molecular hydrogen earlier but there are other sources which could contribute including CIV.

If we leave out the ridge, the correlation of the FUV with the LVC column densities is only 0.26. The correlation of the LVC with the ridge is 0.84. There is no correlation with either the IVC or the HVC clouds. Lockman & Condon (2005) found that the reddening per HI atom was greater in the ridge than outside, suggesting the presence of H2.

Fig.10a. FUV vs FUV/NUV Ratio (Total).

Fig.10b. IR 100 vs FUV/NUV Ratio (Total).

Fig.10b. LVC vs FUV/NUV Ratio (Total).

Fig.10b. Excess radiation in this field.

If we now look at the ratio with respect to the FUV flux we find a nice linear fit, as mentioned above. The correlation with the ridge is 0.91 and the slope is 0.0011. If we look at the non-ridge region, the correlation is 0.73 and the slope is 0.00102, almost no difference although the correlation is much better, perhaps only because there is more scatter outside the ridge.

We can now separate the FUV emission into two parts: the dust emission and an excess over the dust component which contributes to the increasing ratio and is hence correlated with the FUV. These are plotted in Fig. 11 below. The excess radiation is 0.74*(FUV-309) with a correlation coefficient of 0.92. The remaining emission is the same as the NUV and so the correlation is 0.65 and FUV(dust) = FUV*0.26 + 209. The Sandage region is overplotted but with the new scale factor of 0.7 instead of 0.8.

[edit] Modeling the Emission

[edit] Fitting with HI

We would expect the NUV emission to be only dust so our first attempt is to assume a linear relationship between the LVC and NUV. This gives a correlation coefficient of 0.61 with a slope of 132 and a y intercept of 292. Nominally, this would imply an extragalactic contribution of 292 photon units. The quality of the model is not good enough to actually distinguish this, however. We definitely do not see shadows from the clouds but would only expect about 30 photon unit difference anyway. The offset from the IR is about 260 photon units.

[edit] Level of Excess Emission

By definition, the excess emission in the FUV is defined by setting the FUV/NUV ratio to 0.7, a little less than the minimum in the field. We have assumed that this is due to the Werner band(900-1300A) and Lyman band(1400-1700A) emission of molecular hydrogen but it could also be due to line emission such as CIV (seen in the SPEAR data).

The excess emission is strongly correlated with the total FUV, whatever this means. Perhaps most significantly, it is correlated with the ridge where it is almost certainly due to molecular hydrogen. The correlation with the total FUV doesn't change here so it must be the same the whole way through - that is, the entire bit is due to molecular hydrogen. Other lines should be anti-correlated with HI but nothing like that is seen. Diffuse FUV, NUV & IRIS maps of the region is shown in the following figure.

Fig.fd. Diffuse FUV map of the field.

Fig.nd. Diffuse NUV map of the field.

Fig.excess Excess emission in field.

Fig.iris. IRIS 100 micron map of the field.

[edit] FIR Emissivity & Molecular Hydrogen Formation Rate In the SFL Region

Fig.IRNH. Ratio IR100/N(HI) vs. IR100 emission over the SFL field.

Fig.H2E. Predicted levels of H2 emission assuming R as 1 x 10^-17 /sec vs. the observed level of H2 emission.

Good correlation (82%: see Table. Correlation) between IR 100 and LVC+IVC N(HI) indicates negligible contribution (IR-HVC correlation -0.2) of HVC to IR 100 micron emission. Hence we estimated the FIR emissivity per HI atom using the LVC+IVC cloud and plotted against the IR 100 micron emission in Fig. IRNH. From the plot it is clear that the emissivity is almost constant about 8.89 x 10^-21 MJy/sr cm², very close to the mean value (8.96 x 10^-21 MJy/sr cm²) derived for the Galactic cirrus clouds (Moritz et al 1998), for the region except for the ridge. This might be due to the presence of considerable amount of molecular hydrogen in addition to the neutral hydrogen in the ridge. By fixing the FIR emissivity at 8.89 x 10^-21 MJy/sr cm², we estimated the H2 column density for the region and found that it varies upto 5.5 x 10¹⁹ cm^-2.

We estimated the H2 emission using the model of Martin et al (1991)(equ(2)), assuming the H2 formation rate, R as 1 x 10^-17 s^-1. The estimated level is plotted against the excess emission observed in Fig.H2E. The overall correlation obtained here is 77 % with a chisq of 2.12 (without ridge the correlation is 61%). However, within the ridge the correlation is 88 % with a chisq of 2.46 for a uniform R value. Further, if we assume the formation rate is not a constant and is less in the less dense region and more in the densest regions than the assumed value of 1 x 10^-17, the correlation between the predicted and observed level will further increase to 96% with a chisq of 0.54, which is much better than the former case (see Fig. FRC).

The theoretical estimation of the formation rate constant (R) is in the range (1-3) x 10^-17 s^-1 and is depends on the grain and gas temperatures. However when we tried to estimate the R value by assuming the excess radiation as H2 emission and found that it varies between 0.5 to 1.4 in the ridge (see Fig.FRC (left)).

N(H2) = [IR100/8.89 x 10-21 - N(HI)]/2.0  --- equ(1)

IH2F (model) = R<n(HI)>(1-exp(-sigma*N(H))/(4*PI*<k>*Sigma*BW) --- equ(2)

where R = 1 x 10^-17 cm^-3 s^-1, n(H_I) is LVC N(H_I)/3.09 x 10⁺¹⁸, N(H) = LVC N(H_I), <k> = 0.11, Sigma = 1.37 x 10^-21 and BW = 450 A

Number density in the region, nH = IR100/8.96 x 10-21/3.09E+18  --- equ(3)

Predicted R = I_H2F(observed)/I_H2F(model) in unit of 10^-17 cm-3 s-1 --- equ(4)

Fig.FRC. Estimated formation rate of H2 molecules are plotted against the excess emission in the ridge (left). The predicted level of H2 emission, assuming a lower R in the less dense region and a higher R in the more dense region, is plotted against the observed H2 emission (right). The predicted level for a uniform R is over plotted as squares.

[edit] Space Density Estimation from H2 Model

Fig.nH. Predicted space density is plotted against the distance from the cloud center. The best fit curves and the corresponding chisq are given in the plot.

We estimated the space density for the region using the observed formation rate, R, and is plotted against the distance from the cloud center in Fig. nH. We also noticed an exponential decrease (i.e., n(H)=141*exp(-0.59*R)) of cloud density from the center towards the edges of the cloud. The best fit curve and corresponding chi-square are given in the graph. The thickness of the cloud assumed as 1 pc for the calculation.

Park et al (2009) reported that Si II* emission is more bright outside the molecular cloud which indicates its origin from hot & warm ionized media located beyond the molecular cloud whereas CIV line is more intense inside the molecular cloud. The lack of strong correlation between the modeled H2 emission and the observed excess emission outside the ridge area (correlation is 61%) might be due to the contamination of above mentioned line emissions with the H2 emission in the region.

[edit] Summary

1. FUV-NUV Correlation:
   1. Overall correlation for the region is moderate due to the presence of H2F.
   2. Individual correlation for the field is not good except for SIRTFFL_03 & SIRTFFL_10 where the dust ridge present. This could be due to the presence of FUV excess in the form of H2F.
2. IR 100-UV Correlation:
   1. Correlation is moderate due to the presence of both neutral and molecular hydrogen in the region
   2. FUV excess is visible
   3. The IR-UV slope is not in agreement with that of Haikala et al (1995).
3. Reasonable IR100- total N(HI) correlation (70%)
4. The predicted H2 column density level found to be varying between 2.71 x 10¹⁸ to 2.13 x 10²⁰ cm^-2 over the SFL field.
5. High IR100- N(H2) correlation (88%) in the region agrees with Boulanger & Perault (1988).
6. UV & N(HI) correlation is 51%
7. According to previous studies the dust ridge present in the region is purely low velocity HI cloud. But H2 emission is strong here.
8. Existence of 3 layers of clouds (LVC, IVC & HVC) in the region: LVC is about 60 pc, MBM 41-44, which is an IVC is located between 300 pc and 1 kpc and HVC is between 4-10 kpc.
9. IVC is well separated from the foreground and background gas (Herbstmeier et al 1993).
10. As per the E(B-V) vs Hipparcos distance of stars present within 3 deg field toward SFL region, the E(B-V) varies between 0 to 0.35 whereas the Schlegel's dust map shows the maximum E(B-V) in this region is only 0.17. Why this difference?

[edit] Modeling of the Dust Scattered Emission

The total background emission observed is the dust scattered component in NUV band whereas in the FUV, we derived the component after subtracting the H2F contribution from the total background. Then we run our four parameter (a,g,d, C) single scattering model (Sujatha et al 2005) to predict the NUV dust scattered emission assuming the major dust emission in the region is from the LVC cloud (since more than 80% correlation with IR) and hence to derive the optical parameters of dust grains, LVC distance and the possible extragalactic contribution.

This model make use of stellar details from the Hipparcos catalog to predict the radiation field at the location dust from which the dust scattered light originates. The LVC distance varied between 40 pc to 90 pc in step of 5 pc and the constant component, C, varied between 0 to 250 photon units in step of 5 photon units in the model. We found that the LVC cloud distance is 55 pc and the constant component, C is 84 photon units from the chi-square minimization. But it should be noted that the chi-square difference is only less than 0.001 if we use the LVC distance as 60 pc as per the literature. The 99.9 % confidence levels obtained for various parameters are shown in Fig.ctr.

When we included the IVC cloud (which has ~12% contribution) in the model by assuming its distance as 300 pc, we find that the limits of each parameters are shifted as seen in Fig.ctr1.

Fig.ctr. Plot of a 99.9 % confidence a, g contour for the NUV data with only LVC cloud. Corresponding EGL is 30 -- 150 photon units.

Fig.ctr1. Plot of a 99.9 % confidence a, g contour for the NUV data with LVC and IVC clouds. Corresponding EGL is 70 -- 190 photon units.

[edit] Ridge Modeling

The ridge is a low velocity cloud at a distance of about 60 pc. If we separate this region and model it the 3 sigma levels of various parameters are as follows.

Observed FUV and NUV diffuse light for the region with the foreground light subtracted are plotted as "+" against the IR 100 emission in Fig. fdmodel and Fig. ndmodel. The best fit model prediction of the same are over plotted as "x" in each figure. Both the FUV & NUV model fits include a dust-scattered component corresponding to a => 0.25/0.27; g => 0.7/0.0 and a flat component of 84 pu representing the uncertainty in the foreground emission estimated. The FUV model fit also includes the prediction of H2 emission using the model of Martin et al 1991 for a formation rate of 1 x 10^-17 /sec in Fig.fdmodel and is different in in Fig.fdmodel1. Reduced chi-square for the fits are 3.6/1.1 (with 7369 degrees of freedom). The best fit models in FUV & NUV indicates that using the same optical constants it is not possible to fit both the regions simultaneously well, which suggest that the dust properties could be slightly different in these regions.

Fig.fdmodel. Observed FUV diffuse emission with foreground subtracted (+) is plotted against the IR 100 emission. Best-fit model (a=0.32,g=0.7,flat component=84 PU) predictions of dust-scattered star light plus modeled H2 emission for R = 1 x 10^-17 /s is over plotted.

Fig.fdmodel1. Observed FUV background diffuse emission (+) is plotted against the IR 100 emission. Best-fit model (a=0.25,g=0.7,flat component=84 PU) predictions of dust-scattered star light plus modeled H2 emission with different R in different region is over plotted.

Fig.ndmodel. Observed NUV diffuse emission with foreground subtracted (+) is plotted against the IR 100 emission. Best-fit model (a=0.27,g=0,flat component=84 PU) prediction of dust-scattered star light is overplotted.

[edit] UV/IR100 Slope History

Table uv/ir slope - UV/IR100 Slope History

Region	Observation	Wavelength (A)	UV/IR100 Slope (CU MJy/sr)	Reference
NGC 6752	FAUST	1600	199 35	Sasseen & Deharveng (1996)
Centaurus	FAUST	1600	36 37	Sasseen & Deharveng (1996)
M83	FAUST	1600	134 30	Sasseen & Deharveng (1996)
M87	FAUST	1600	36 26	Sasseen & Deharveng (1996)
NGP	FAUST	1600	33 16	Sasseen & Deharveng (1996)
p1	FAUST	1600	17 52	Sasseen & Deharveng (1996)
p2	FAUST	1600	49 14	Sasseen & Deharveng (1996)
Hydra 21a	FAUST	1600	87 42	Sasseen & Deharveng (1996)
Hydra 21b	FAUST	1600	-49 28	Sasseen & Deharveng (1996)
Hydra 21c	FAUST	1600	148 14	Sasseen & Deharveng (1996)
Hydra 20a	FAUST	1600	66 53	Sasseen & Deharveng (1996)
Hydra 20b	FAUST	1600	25 51	Sasseen & Deharveng (1996)
Hydra 20c	FAUST	1600	95 46	Sasseen & Deharveng (1996)
Total	FAUST	1600	255 5	Sasseen & Deharveng (1996)
Total	FAUST	1600	23326	Sasseen et al (1995)
Northern Hemisphere	D2B-AURA	1600	244	Perault et al (1991)
Southern Hemisphere	D2B-AURA	1600	214	Perault et al (1991)
-65 < b < 65 deg	low intensity DE-1 data of Fix et al (1989)	1500	203	Wright (1992)
UVX targets	UVX data	1600	164	Sasseen et al
l(40 -- 110 deg); b ~ 50 deg	Rocket data (1984)	1600	6525	Jakobsen et al (1987)
UVX targets	UVX data	1600	213 -- 474	Hurwitz et al (1991)
G251.2+73.3 near NGP	FAUST	1600	128 3	Haikala et al (1995)
One quarter of the sky	NUVIEWS	1740	47	Schiminovich et al (2001)
SFL Ridge	GALEX	1525	496	Sujatha et al (This work)
SFL Ridge	GALEX	2320	214	Sujatha et al (This work)
SFL without Ridge	GALEX	1525	224	Sujatha et al (This work)
SFL without Ridge	GALEX	2320	136	Sujatha et al (This work)
SFL field Total	GALEX	1525	300	Sujatha et al (This work)
SFL field Total	GALEX	2320	147	Sujatha et al (This work)

Sasseen & Deharveng (1996) found that the UV/IR100 slope increase as a function of UV, IR fluxes and when the galactic latitude decreases. Also that the slope is dependent on the size of the region being compared.

[edit] Possible line emissions in the GALEX FUV/NUV bands

[edit] Extragalactic Contribution

[edit] Results And Discussions

We analyzed a set of 11 GALEX observations in the vicinity of MBM 41-44 (Draco Nebula) where the SPITZER has made its FLS data. These targets are from an optically thin region where most of the diffuse emission expected from the starlight scattered from interstellar dust.

The scatter in the FUV data is almost consistent with the photon noise alone whereas the scatter in the NUV data is ~ 3 times higher than from the photon noise. We observe that the airglow, one of the major component of the foreground emission, is strongly dependent on the solar activity with a correlation coefficient of 0.91/0.89 in the FUV/NUV bands. The average total foreground emissions estimated in the fields, by minimizing the scatter between FUV & IR100 and NUV & IR100 assuming that the UV is correlated with the IR, varies between 300 and 370 photon units in the FUV band and varies between 650 and 800 photon units in the NUV band with a possible error bar of about 50 photon units.

The FUV emission from the region is moderately correlated with the NUV emission indicates the dominance of dust scattered starlight in the region. However, an increase in the ratio between the two bands (FUV/NUV) with the FUV & IR emissions suggesting a part of the total background emission in the FUV band is coming from a source whose contribution in the NUV band is negligible. Since these targets are in the vicinity of MBM 41-44 cloud complex, this additional contribution in the FUV band is most probably the emission from the Werner bands of molecular hydrogen. An interesting point here is that the CO line emission is not detected in our field (Wakker et al. 1997), however Martin et al. (1990) found that clouds showing no CO line emission do contain molecular hydrogen. Our targets are typical examples for this case. According to Blitz et al (1990) the lower IR 100 micron limit for the detectability of CO line emission is 4 MJy/sr. The IR-UV and UV-UV correlation in the field is found to be only moderate in the filed due to the presence of molecular cloud and its contribution.

The FUV excess is also clearly evident in the IR-FUV plot and we estimated its level by assuming the minimum observed ratio between the bands as 0.8, which is the same that we noticed for the SANDAGE region. We found that this level varies between 0 to 450 photon units spatially over the field which is highly correlated with the IR 100 micron Intensity as expected due to the lack of correlation between the neutral and molecular hydrogen in the field (Fig.10).

We could not correlate the IR-UV slope of our region with that of the observations of G251.2+73.3 by Haikala et al (1995), even though both cases are from optically thin isolated clouds, indicating that the IR-UV slope can not be a unique number and will vary depends on the type of the medium and anisotropies of the UV radiation field in the area.

Lockman & Condon (2005) observed this area in the 21 cm line of HI with GBT and produced an accurate values of the neutral Hydrogen column density over the field which helped us to predict the dust scattered starlight and hence to derive the parameters responsible for it. The previous studies shows that the neutral Hydrogen column density depends on the FUV emission as

I_FUV (0.3–2.5) * N(HI) + 300

,where N(HI) is of the units of 10¹⁸/cm² and the range of slope is due to the anisotropy of the radiation field and difference in the dust properties of the region (Bowyer, 1991). The equation derived for our medium is

I_UV 0.68 * N(HI) + 260

and is in good agreement with Bowyer (1991) for medium latitudes where the stellar radiation field is weaker. Schiminovich et al. (2001) found the UV-N(HI) slope of 0.42 in units of 10^-18 PU/cm2 with his NUVIEWS data in FUV at 1740 A. When we converted this to the FUV/IR slope, assuming the mean FIR emissivity, we got the range 47 photon units/MJy/sr. Here our conclusion is that the slope between the background emission and its related parameters are not uniform over the sky and is dependent strongly on the type of medium.

Even after the absence of CO line emission, we could predict the level of H2 column density by fixing the Far-IR emissivity (IR 100/N(H) ratio) at 8.89 x 10^-21 MJy/sr cm² - the minimum observed ratio in the region- and assuming the H2 formation rate constant as 1 x 10-17 s-1 , and found that the column density varies upto 5.5 x 1019 cm^-2 in the SFL region. This is the first time prediction & estimation of molecular hydrogen & its emission in this field. The Far-IR emissivity (about 8.89 x 10^-21 MJy/sr cm²) in our region found to be close to the mean value found for the galactic cirrus clouds. We noticed a good correlation between IR 100 micron emission & level of N(H2) (r=0.88).

Apart from the foreground and H2 fluorescent emission, the dust scattered radiation in the field found to be varying between 300 to 700 photon units both in the FUV & NUV channels of GALEX originating from a low velocity cloud (LVC 88+36-2) located at 55 pc, which is part of the Local Hot Bubble. We have determined optical constants of 0.24 ≤ a ≤ 0.32 and 0 ≤ g ≤ 0.45 in the NUV band for the dust here, largely consistent with previous observational and theoretical determinations (Gordon 2004). Regardless of the actual value of the optical constants, we find that the ratio between the FUV dust scattered light and the NUV is 0.7 over a wide range of optical depths through both this region and Sandage.

At present, we completed the analysis of two sets of GALEX DIS observations to study the diffuse emission over the sky, in which the observations near MBM 30 (SANDAGE target) is from an optically thick region and those near MBM 41-44 are from an optically thin region. When we combined the dust scattered component from these two regions together, we found that this emission linearly correlates with the amount of dust in optically thin regions and is saturated in optically thick regions (Fig 12c).

[edit] PII References

(1) Leinert et al. 1998, A&AS, 127, 1-99.

(2) Schlegel, D. J., Finkbeiner, D. P., & Davis, M. 1998, ApJ, 500, 525.

(3) Sujatha et al. 2005, ApJ, 633, 257.

(4) Sujatha et al. 2009, ApJ, 692, 1333.

(5) Mebold, U.; Cernicharo, J.; Velden, L.; Reif, K.; Crezelius, C.; Goerigk, W., A&A, vol. 151, no. 2, Oct. 1985, p. 427-434.

(6) Wakker, B. P. 2006, ApJS, 163, 282-305.

(7) Chatterjee, T. N.; Das, T. K., 1995, MNRAS, 274, 858, Relation between solar UV flux and 10.7-cm radio emission

(8) Herbstmeier et al 1993

(9) Reach et al 1998

(10) R. Lallement, B. Y. Welsh, J. L. Vergely, F. Crifo and D. Sfeir, 2003, A&A,411, 447-464

(11) F. J. Lockman & J. J. Condon, The Astronomical Journal, 129:1968–1977, 2005

[edit] GALEX_PII_Paper

[edit] Action items for Sujatha

1. Publication quality diffuse FUV and NUV maps. - done

OLD FIGURES