TWO BRIGHT SUBMILLIMETER GALAXIES IN A z= 4.05 PROTOCLUSTER IN GOODS-NORTH, AND ACCURATE RADIO-INFRARED PHOTOMETRIC REDSHIFTS

Abstract

We present the serendipitous discovery of molecular gas CO emission lines with the IRAM Plateau de Bure interferometer coincident with two luminous submillimeter galaxies (SMGs) in the Great Observatories Origins Deep Survey North (GOODS-N) field. The identification of the millimeter emission lines as CO[4–3] at z = 4.05 is based on the optical and near-IR photometric redshifts, radio-infrared photometric redshifts, and Keck+DEIMOS optical spectroscopy. These two galaxies include the brightest submillimeter source in the field (GN20; S 850 μm = 20.3 mJy, z CO = 4.055 ± 0.001) and its companion (GN20.2; S 850 μm = 9.9 mJy, z CO = 4.051 ± 0.003). These are among the most distant submillimeter-selected galaxies reliably identified through CO emission and also some of the most luminous known. GN20.2 has a possible additional counterpart and a luminous active galactic nucleus inside its primary counterpart revealed in the radio. Continuum emission of 0.3 mJy at 3.3 mm (0.65 mm in the rest frame) is detected at 5σ for GN20, the first dust continuum detection in an SMG at such long wavelength, unveiling a spectral energy distribution that is similar to local ultra luminous IR galaxies. In terms of CO to bolometric luminosities, stellar mass, and star formation rates (SFRs), these newly discovered z > 4 SMGs are similar to z ∼ 2–3 SMGs studied to date. These z ∼ 4 SMGs have much higher specific star formation rates than those of typical B -band dropout Lyman break galaxies at the same redshift. The stellar mass–SFR correlation for normal galaxies does not seem to evolve much further, between z ∼ 2 and z ∼ 4. A significant z = 4.05 spectroscopic redshift spike is observed in GOODS-N, and a strong spatial overdensity of B -band dropouts and IRAC selected z > 3.5 galaxies appears to be centered on the GN20 and GN20.2 galaxies. This suggests a protocluster structure with total mass ∼10 14 M ☉ . Using photometry at mid-IR (24 μm), submillimeter (850 μm), and radio (20 cm) wavelengths, we show that reliable photometric redshifts (Δ z /(1 + z ) ∼ 0.1) can be derived for SMGs over 1 ≲ z ≲ 4. This new photometric redshift technique has been used to provide a first estimate of the space density of 3.5 < z < 6 hyper-luminous starburst galaxies, and to show that they both contribute substantially to the SFR density at early epochs and that they can account for the presence of old galaxies at z ∼ 2–3. Many of these high-redshift starbursts will be within reach of Herschel . We find that the criterion S 1.4 GHz ≳ S 24 μm , coupled to optical, near-IR and mid-IR photometry, can be used to select z > 3.5 starbursts, regardless of their submillimeter/millimeter emission.