9 Conclusion and summary

The analysis of the FIRBACKISO deep survey sources at 170 $\mu$m is presented. After a process of data reduction and calibration of extended emission (Lagache & Dole 2001), we performed extensive simulations to validate our source extraction process, and studied the sources of noise and accuracy in photometry and astrometry. The confusion $\sigma _{\rm c}$ equals 45 mJy.

We compiled the ISO FIRBACKSource Catalog ( $S_{170} > 4 \sigma_{\rm c}$) and the Complementary ISO FIRBACKSource Catalog ( $3 \sigma_{\rm c} < S_{170} < 4 \sigma_{\rm c}$, for follow-up purposes) containing 196 sources. It is important to note that the extended source calibration is in excellent agreement with DIRBE and the point source calibration is in agreement with IRAS. The differential source counts show a steep slope of $3.3 \pm 0.6$between 180 and 500 mJy, and a significant excess of faint sources with respect to low or moderate evolution expectations.

The steep slope of the source counts has important consequences on the sensitivity limits of the deep surveys conducted in the far infrared: the confusion noise is large, as it will be for future observatories, and will impact dramatically on the future IR deep surveys.

Table 7. FIRBACK Catalog in FSM: coordinates are in hours ( $\alpha _{2000}$) or degrees ( $\delta _{2000}$), minutes, seconds, the flux S and the flux uncertainty $\delta S$ at $170~\mu$m are in mJy.

Table 8. FIRBACK Catalog in FN1: coordinates are in hours ( $\alpha _{2000}$) or degrees ( $\delta _{2000}$), minutes, seconds, the flux S and the flux uncertainty $\delta S$ at $170~\mu$m are in mJy.

Table 9. FIRBACK Catalog in FN1 (continued).

Table 10. FIRBACK Catalog in FN2: coordinates are in hours ( $\alpha _{2000}$) or degrees ( $\delta _{2000}$), minutes, seconds, the flux S and the flux uncertainty $\delta S$ at $170~\mu$m are in mJy.

Table 11. FIRBACK Complementary Catalog in FSM: coordinates are in hours ( $\alpha _{2000}$) or degrees ( $\delta _{2000}$), minutes, seconds, the flux S and the flux uncertainty $\delta S$ at $170~\mu$m are in mJy.

Table 12: FIRBACK Complementary Catalog in FN1: coordinates are in hours ( $\alpha _{2000}$) or degrees ( $\delta _{2000}$), minutes, seconds, the flux S and the flux uncertainty $\delta S$ at $170~\mu$m are in mJy. Table 13: FIRBACK Complementary Catalog in FN2: coordinates are in hours ( $\alpha _{2000}$) or degrees ( $\delta _{2000}$), minutes, seconds, the flux S and the flux uncertainty $\delta S$ at $170~\mu$m are in mJy.
One important intention of the FIRBACKsurvey was to probe the nature of the extragalactic far-infrared sources. According to most of the models, the steep slope of the source counts is due to a strongly evolving population of LIRGs. Our model shows that the effect of the K-correction alone is insufficient to explain the observations. To definitively investigate this question, one has to identify the sources and understand their nature. Discussions of the nature of the FIRBACKsources is beyond of the scope of this paper and will be discussed elsewhere. The multiwavelength follow-up performed at 1.4 GHz, 1.3 mm, 850 and 450 $\mu$m, as well as other ISO and optical / NIR data, seems to show that most of the sources (typically 50%) are local (z < 0.3), and about 10% at high redshift (z > 1). Massive star formation seems also to be dominant. Nevertheless, identifying FIRBACKsources is not easy because of the uncertainty in the positions at 170 $\mu$m.

The summary of the FIRBACKsurvey is as follows:

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observation of about 4 sq. deg in 3 high galactic latitude fields: FSM, FN1 & FN2;

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ISOPHOT AOT P22 raster map mode with the C_200 array and the C_160 filter at 170 $\mu$m;

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128 or 256 seconds of integration per sky pixel;

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extraction of instrumental effects: long and short term transients, photometric correction;

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calibration of extended emission: excellent agreement between PHT and DIRBE;

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calibration of point sources compatible with IRAS;

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instrumental noise: 3 mJy $1 \sigma$;

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confusion noise: 45 mJy $1 \sigma$; $4 \sigma _{\rm c}$ sensitivity: 180 mJy;

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ISO FIRBACKSource Catalog: 106 sources between 180 mJy and 2.4 Jy;

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Complementary ISO FIRBACKSource Catalog: 90 sources between 135 and 180 mJy;

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Flux uncertainty error: 25% at $3 \sigma_{\rm c}$, 20% at $5 \sigma_{\rm c}$, and reduced to 10% at higher flux levels;

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positional error: 100 arcsecond diameter circle (99% of the sources);

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source density for S₁₇₀ > 225 mJy: $16 \pm 4$ sources per square degree;

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source density for S₁₇₀ > 180 mJy: $27 \pm 5$ sources per square degree;

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slope of the differential source counts: $3.3 \pm 0.6$ between 180 and 500 mJy;

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4 to 7% of the Cosmic Infrared Background at 170 $\mu$m is resolved into sources brighter than 135 mJy;

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Prediction that the CIB will be resolved at flux levels in the range 1 to 10 mJy at 170 $\mu$m;

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Catalogs, images, and plots available on line at: http://wwwfirback.ias.u-psud.fr.

Acknowledgements

HD, GL & JLP appreciate discussions with Rene Laureijs and Carlos Gabriel at Vilspa and Ulrich Klaas at Heidelberg. We also are greatful to Alain Abergel, Alain Coulais & Marc-Antoine Miville-Deschênes at IAS, for stimulating and helpful discussions through the analysis of the FIRBACKdata. Thanks also go to Martin Kessler and his team who did a great job in planning ISO observations so efficiently. MH acknowledges support of his participation on ISO through NASA grants and contracts. DE acknowledges support from NASA grant NAG5-8218.