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Astron. Astrophys. 336, L9-L12 (1998)

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4. Discussion

The flux calibration can be made using either the COBE background measurements listed in Table 2 or the IRAS point source fluxes. Note that the intensity of interplanetary dust emission varies little at such high ecliptic latitude ([FORMULA]) as the solar elongation changes. The fields contain only one IRAS source (F10507+5723). This is the brightest source detected in our survey and identified with a Sb galaxy UGC 06009 (Thuan & Sauvage 1992). The IRAS fluxes are 533 mJy at 60 µm and 1218 mJy at 100 µm (IRAS FSC 1990). Its flux ratio f(100 µm)/f(60 µm) = 2.29 is fitted to a combination of the infrared cirrus and starburst spectra given by Pearson & Rowan-Robinson (1996), if 76% of the 100 µm flux comes from the cirrus component. This predicts f(175 µm) = 1133 mJy from f(175 µm)/f(100 µm) = 0.93 which is comparable to that in the inner arm of a Sc galaxy M101 (Hippelein et al. 1996). The COBE based scaling yields 347 mJy at 95 µm and 451 mJy at 175 µm, implying that the COBE based scaling may underestimate point source fluxes by 3.5 at 95 µm and 2.5 at 175 µm. The origin of these differences is unclear. In the following discussion, the ISO fluxes are scaled based on the IRAS measurements.

The maps in Fig. 3 contain numerous spots. Bright spots cannot be attributed to peaks of the infrared cirrus. The total HI column density [FORMULA] in our fields is approximately [FORMULA] cm-2 (Jahoda et al. 1990). The correlation between [FORMULA] and the infrared cirrus brightness indicates [FORMULA](95 µm) = 0.41 MJy sr-1 and [FORMULA](175 µm) = 0.81 MJy sr-1 (Table 2 in Boulanger et al. 1996). Using the models by Gautier et al. (1992) with [FORMULA] for the index of the spatial power spectrum of the infrared cirrus, [FORMULA](95 µm) [FORMULA] 0.14 mJy and [FORMULA](175 µm) [FORMULA] 1.7 mJy are obtained for one sigma cirrus confusion noise. The 3 [FORMULA] flux levels reached in our survey are tentatively estimated to be [FORMULA] mJy at 95 µm and [FORMULA] mJy at 175 µm. Hence, a probability of having a cirrus peak brighter than the detection limits is so low that most of spots are likely to be galaxies.

In the two fields LHEX and LHNW, covering 1.1 square degrees in total, there are 36 and 45 sources brighter than 150 mJy at 95 and 175 µm, respectively. Out of 45 175 µm sources, 36 sources are within the area which was observed at the both wavelength. 31 of the 36 sources have the 95 µm counterparts within [FORMULA] that were detected above the 3 [FORMULA] flux level. The completeness down to 150 mJy is estimated to be almost 100%, because of the low probability of cirrus confusion and the high SNR (signal to noise ratios) detection (typical SNR [FORMULA] 10). The cumulative counts down to 150 mJy are thus [FORMULA] sources sr-1 at 95 µm and [FORMULA] sources sr-1 at 175 µm. The number density of 175 µm sources brighter than 150 mJy are ten times higher than that expected from the no-evolution model by Guiderdoni et al. (1997, 1998). When the COBE based scaling applies, the 175 µm source density down to 60 mJy (i.e., 150/2.5 = 60) is three times higher than the no-evolution model.

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© European Southern Observatory (ESO) 1998

Online publication: July 7, 1998