2009A&A...508..117F

Wide-field Spitzer surveys allow identification of thousands of potentially high-z submillimeter galaxies (SMGs) through their bright 24µm emission and their mid-IR colors. We want to determine the average properties of such z∼2 Spitzer-selected SMGs by combining millimeter, radio, and infrared photometry for a representative IR-flux (λ_rest∼8µm) limited sample of SMG candidates.A complete sample of 33 sources believed to be starbursts (``5.8µm-peakers'') was selected in the (0.5deg<sup>2</sup>) J1046+56 field with selection criteria F<sub>24µm</sub>400µJy, the presence of a redshifted stellar emission peak at 5.8µm, and r'<sub>Vega</sub>23. The field, part of the SWIRE Lockman Hole field, benefits from very deep VLA/GMRT 20cm, 50cm, and 90cm radio data (all 33 sources are detected at 50cm), and deep 160µm and 70µm Spitzer data. The 33 sources, with photometric redshifts ∼1.5-2.5, were observed at 1.2mm with IRAM-30m/MAMBO to an rms ∼0.7-0.8mJy in most cases. Their millimeter, radio, 7-band Spitzer, and near-IR properties were jointly analyzed. The entire sample of 33 sources has an average 1.2mm flux density of 1.56±0.22mJy and a median of 1.61mJy, so the majority of the sources can be considered SMGs. Four sources have confirmed 4σ detections, and nine were tentatively detected at the 3σ level. Because of its 24µm selection, our sample shows systematically lower F<sub>1.2mm</sub>/F<sub>24µm</sub> 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<sub>dust</sub>=37±8K, L<sub>FIR</sub>=2.5x10<sup>12</sup>L<sub>☉</sub>, and SFR=450M<sub>☉</sub>/yr. The FIR-radio correlation provides another estimate of L<sub>FIR</sub> for each source, with an average value of 4.1x10<sup>12</sup>L<sub>☉</sub>; 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≲z≲ 2.3. Spitzer 24µm-selected starbursts and AGN-dominated ULIRGs can be reliably distinguished using IRAC-24µm SEDs. Such ``5.8µm-peakers'' with F_24µm>400µJy have L_FIR>10¹²L_☉. They are thus z∼2 ULIRGs, and the majority may be considered SMGs. However, they have systematically lower 1.2mm/24µm flux density ratios than classical SMGs, warmer dust, comparable or lower IR/mm luminosities, and higher stellar masses. About 2000-3000 ``5.8µm-peakers'' may be easily identifiable within SWIRE catalogues over 49deg².