Submillimeter number counts at 250 μm, 350 μm and 500 μm in BLAST data

Context. The instrument BLAST (Balloon-borne Large-Aperture Submillimeter Telescope) performed the first deep and wide extragalactic survey at 250, 350 and 500 mu m. The extragalactic number counts at these wavelengths are important constraints for modeling the evolution of infrared galaxies. Aims. We estimate the extragalactic number counts in the BLAST data, which allow a comparison with the results of the P(D) analysis of Patanchon et al. (2009). Methods. We use three methods to identify the submillimeter sources. 1) Blind extraction using an algorithm when the observed field is confusion-limited and another one when the observed field is instrumental-noise-limited. The photometry is computed with a new simple and quick point spread function (PSF) fitting routine (FASTPHOT). We use Monte-Carlo simulations (addition of artificial sources) to characterize the efficiency of this extraction, and correct the flux boosting and the Eddington bias. 2) Extraction using a prior. We use the Spitzer 24 mu m galaxies as a prior to probe slightly fainter submillimeter flux densities. 3) A stacking analysis of the Spitzer 24 mu m galaxies in the BLAST data to probe the peak of the differential submillimeter counts. Results. With the blind extraction, we reach 97 mJy, 83 mJy and 76 mJy at 250 mu m, 350 mu m and 500 mu m respectively with a 95% completeness. With the prior extraction, we reach 76 mJy, 63 mJy, 49 mJy at 250 mu m, 350 mu m and 500 mu m respectively. With the stacking analysis, we reach 6.2 mJy, 5.2 mJy and 3.5 mJy at 250 mu m, 350 mu m and 500 mu m respectively. The differential submillimeter number counts are derived, and start showing a turnover at flux densities decreasing with increasing wavelength. Conclusions. There is a very good agreement with the P(D) analysis of Patanchon et al. (2009). At bright fluxes (> 100 mJy), the Lagache et al. (2004) and Le Borgne et al. (2009) models slightly overestimate the observed counts, but the data agree very well near the peak of the differential number counts. Models predict that the galaxy populations probed at the peak are likely z similar to 1.8 ultra-luminous infrared galaxies.