TWO BRIGHT SUBMILLIMETER GALAXIES IN Az= 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 field (GOODS-N). The identification of the millimeter emission lines as CO 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 850µm = 20.3mJy, z CO = 4.055 ± 0.001) and its companion (GN20.2; S 850µm = 9.9mJy, 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 AGN inside its primary counterpart revealed in the radio. Continuum emission of 0.3mJy at 3.3mm (0.65mm 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 infrared 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 SFRs than 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 proto-cluster structure with total mass ∼ 10 14 M ⊙ . Using photometry at mid-IR (24µm), submm (850µm) and radio (20cm) wavelengths, we show that reliable photometric redshifts (∆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 24µm , coupled to optical, near-IR and mid-IR photometry, can be used to select z > 3.5 starbursts, regardless of their submm/mm emission.

Dusty star-forming galaxies at high redshift

“…GN20 (Pope et al, 2005), whose redshift was serendipitously discovered by Daddi et al (2009), is a bright SMG in a proto-cluster of multiple DSFGs. GN20 is the brightest SMG in the GOODS-N field with a 850 µm flux density of 20.3 mJy, suggesting a SFR at the level of 1800 M yr −1 (Fig.…”

“…Sources with z > ∼ 2 are primarily identified via Lyα emission and sources at z < ∼ 1.3 are identified via OII, OIII or Hα. An updated version of the Chapman et al redshift distribution is shown as a dashed-blue line, including DEIMOS-observed radio SMGs from Banerji et al (2011) specifically targeted to fill the redshift desert gap of LRIS, and a handful of high-z 850µm-selected SMGs Daddi et al, 2009;Walter et al, 2012). Also over-plotted is the Lewis et al (2005) and Chapman et al (2003b) phenomenological model 850µm SMG redshift distribution without radio flux selection (dark green dot dashed line).…”

“…Serendipitously, Daddi et al (2009) discovered CO emission lines from the system, and placed the redshift of the SMGs at z ∼ 4 (the physical characterization of GN20 itself was discussed in § 6.3). The brightest galaxies in the area are GN20 with S 850 = 20.3 mJy at z = 4.055 (potentially the brightest unlensed SMG known to date), and GN20.2a and b with a total flux density of S 850 = 9.9 mJy and at z = 4.051.…”

Herschel Observed Stripe 82 Quasars and Their Host Galaxies: Connections Between Agn Activity and Host Galaxy Star Formation

“…This so-called star-forming main sequence (MS) is thought to reflect a large duty cycle of star formation in galaxies and exist over a large range of redshifts from z = 0 up to z = 7 (Brinchmann et al 2004;Daddi et al 2007Daddi et al , 2009Elbaz et al 2007;Noeske et al 2007;González et al 2011). Starburst galaxies are considered to be "off-sequence."…”

The intense starburst HDF 850.1 in a galaxy overdensity at z ≈ 5.2 in the Hubble Deep Field

“…Its redshift of z=5.183 enforces the presence of a high redshift tail (z>4) of sub-millimeter bright star-forming (non-AGN/quasar) galaxies (currently there are only about half a dozen systems known) [22][23][24][25][26] . Only a small fraction of sub-millimeter bright sources is expected to be at very high redshift 27 ---it is thus ironic that the first blank-field source belongs to this subgroup.…”