Multi-wavelength properties of Spitzer selected starbursts at z ∼ 2

Context. Wide-field Spitzer surveys allow identification of thousands of potentially high-z submillimeter galaxies (SMGs) through their bright 24 mu m emission and their mid-IR colors. Aims. We want to determine the average properties of such z similar to 2 Spitzer-selected SMGs by combining millimeter, radio, and infrared photometry for a representative IR-flux (lambda(rest) similar to 8 mu m) limited sample of SMG candidates. Methods. A complete sample of 33 sources believed to be starbursts (5.8 mu m-peakers) was selected in the (0.5 deg(2)) J1046 + 56 field with selection criteria F(24 mu m) > 400 mu Jy, the presence of a redshifted stellar emission peak at 5.8 mu m, and r(Vega)' > 23. The field, part of the SWIRE Lockman Hole field, benefits from very deep VLA/GMRT 20 cm, 50 cm, and 90 cm radio data (all 33 sources are detected at 50 cm), and deep 160 mu m and 70 mu m Spitzer data. The 33 sources, with photometric redshifts similar to 1.5-2.5, were observed at 1.2mm with IRAM-30m/MAMBO to an rms similar to 0.7-0.8mJy in most cases. Their millimeter, radio, 7-band Spitzer, and near-IR properties were jointly analyzed. Results. The entire sample of 33 sources has an average 1.2mm flux density of 1.56 +/- 0.22 mJy and a median of 1.61mJy, so the majority of the sources can be considered SMGs. Four sources have confirmed 4 sigma detections, and nine were tentatively detected at the 3 sigma level. Because of its 24 mu m selection, our sample shows systematically lower F(1.2mm)/F(24 mu m) flux ratios than classical SMGs, probably because of enhanced PAH emission. A median FIR SED was built by stacking images at the positions of 21 sources in the region of deepest Spitzer coverage. Its parameters are T(dust) = 37 +/- 8K, L(FIR) = 2.5 x 10(12) L(circle dot), and SFR = 450 M(circle dot)yr(-1). The FIR-radio correlation provides another estimate of L(FIR) for each source, with an average value of 4.1 x 10(12) L(circle dot); however, this value may be overestimated because of some AGN contribution. Most of our targets are also luminous star-forming BzK galaxies which constitute a significant fraction of weak SMGs at 1.7 less than or similar to z less than or similar to 2.3. Conclusions. Spitzer 24 mu m-selected starbursts and AGN-dominated ULIRGs can be reliably distinguished using IRAC-24 mu m SEDs. Such 5.8 mu m-peakers with F(24 mu m) > 400 mu Jy have L(FIR) greater than or similar to 10(12) L(circle dot). They are thus z similar to 2 ULIRGs, and the majority may be considered SMGs. However, they have systematically lower 1.2mm/24 mu m flux density ratios than classical SMGs, warmer dust, comparable or lower IR/mm luminosities, and higher stellar masses. About 2000-3000 5.8 mu m-peakers may be easily identifiable within SWIRE catalogues over 49 deg(2).