2 Data - the ISSA maps

2.1 The IRAS Sky Survey Atlas

The IRAS mission was originally designed to measure point sources but it also provided spectacular images of the Galactic dust diffuse emission. These images, constituted of $500\times500$ pixels with a pixel size of 1.5' (i.e. the size of the field is $12.5^\circ \times 12.5^\circ$), are gathered in the IRAS Sky Survey Atlas (ISSA). Each ISSA map is the result of the combination of up to three individual maps (named HCON for Hours CONfirmation). The HCON images were constructed from different observations of the same region separated by several months. The angular resolution of the 60 and 100 $\mu$m band images are respectively $1.5'\times4.7'$ and $3.0'\times5.0'$. To study the power spectrum of the CIB emission at 60 and 100 $\mu$m, we have worked on 12 ISSA maps with particularly low cirrus emission.

2.2 Calibration of the ISSA maps

Special care has been taken to have a consistent diffuse emission calibration through the Atlas. But, as the IRAS mission was designed to provide absolute photometry only for point sources, the ISSA images give only relative photometry and cannot be used to determine the absolute surface brightness for diffuse emission. Based on a comparison with the DIRBE data, it has been shown (Wheelock et al. 1993; Schlegel et al. 1998) that, at large scale (> a few degrees), the amplitude of the fluctuations are overestimated in the ISSA maps by a factor 1.15 at 60 $\mu$m and 1.39 at 100 $\mu$m. We have thus applied these factors to our maps prior to the analysis.

There is also an uncertainty on the zero level of the ISSA maps, for which the zodiacal emission has been subtracted (Wheelock et al. 1993). This uncertainty is dominated at 60 and 100 $\mu$m by an imperfect knowledge of the detector offsets and of the zodiacal emission. As we are looking at fluctuations of the signal, a global additive offset has no impact on the power spectrum. On the other hand, an imperfect zodiacal emission correction applied to the ISSA maps will have an impact on the large scale structure of the maps and then on the power spectrum (for low k values). To restore appropriately the large scale structure of the selected fields we have compared each ISSA map with the DIRBE data (for which the zodiacal emission correction was better done). The ISSA maps, multiplied by the appropriate gain value (0.87 at 60 $\mu$m and 0.72 at 100 $\mu$m) were convolved by the DIRBE beam and then subtracted from the DIRBE data. This offset map is then added to the ISSA map. Note that we do not use the Schlegel et al. (1998) IRAS rescaled maps, constructed in a similar manner, for which it is impossible to recover the associated instrumental noise.

2.3 Fields with low cirrus emission

In this paper we present a power spectrum analysis of twelve high latitude fields. These fields were selected on the basis of their low cirrus emission (mean brightness $\sim$1 MJy/sr at 100 $\mu$m) but also on their redundancy; we have selected only fields for which each sky position has been observed at least twice in order to be able to estimate the contribution of the noise to the power spectrum. The typical stripping of the ISSA maps is very efficiently removed from the signal power spectrum by the noise estimate procedure using the difference between observations of the different HCON as will be shown in Sect. 3.3 and Fig. 2.

The 60 and 100 $\mu$m ISSA maps of the twelve selected fields, gain and offset corrected, are shown in Figs. A.2 to A.9 and their central coordinates are gathered in Table 1. Five fields are located in the southern hemisphere, most of them in the neighborhood of the Marano field (Marano et al. 1988), and seven are spread over the northern hemisphere (ISSA map number 376 contains the Lockman Hole).

 

 

Table 1: Central pixel coordinates (ecliptic and Galactic) of the 12 ISSA maps selected for our analysis.

ISSA	$\alpha_{2000}$	$\delta_{2000}$	l	b
47	4h 1m 24.8s	-49$^\circ$51'41.5''	258.32$^\circ$	-47.41$^\circ$
66	23h 2m 53.8s	-49$^\circ$43'50''	338.21$^\circ$	-59.30$^\circ$
69	1h 46m 9.0s	-39$^\circ$45'1''	264.42$^\circ$	-73.02$^\circ$
71	3h 29m 49.5s	-39$^\circ$49'46''	244.45$^\circ$	-54.96$^\circ$
97	1h 34m 18.6s	-29$^\circ$44'39''	230.96$^\circ$	-80.22$^\circ$
322	13h 4m 23.3s	29$^\circ$43'55''	76.14$^\circ$	86.14$^\circ$
323	13h 50m 16.1s	29$^\circ$45'9''	47.86$^\circ$	76.81$^\circ$
348	9h 35m 7.0s	39$^\circ$46'35''	182.59$^\circ$	47.71$^\circ$
349	10h 26m 56.1s	39$^\circ$44'41''	180.70$^\circ$	57.59$^\circ$
356	16h 29m 42.1s	39$^\circ$53'31''	63.41$^\circ$	43.50$^\circ$
375	10h 3m 12.9s	49$^\circ$45'28''	166.14$^\circ$	50.80$^\circ$
376	11h 2m 53.8s	49$^\circ$43'50''	158.21$^\circ$	59.30$^\circ$