2010A&A...522L...4L

Serendipitously we have discovered a rare, bright submillimeter galaxy (SMG) with a flux density of 30±2mJy at λ=1.2mm, using MAMBO2 at the IRAM 30-m millimeter telescope. Although no optical counterpart is known for MM18423+5938, we were able to measure the redshift z=3.92960±0.00013 from the detection of CO lines using the IRAM Eight MIxer Receiver (EMIR). In addition, by collecting all available photometric data in the far-infrared and radio to constrain its spectral energy distribution, we derive the FIR luminosity 4.8x10¹⁴/m*L_☉ and mass 6.0x10⁹/m*M_☉ for its dust, allowing for a magnification factor m caused by a probable gravitational lens. The corresponding star-formation rate is 8.3x10⁴/m*M_☉/yr. The detection of three lines of the CO rotational ladder, and a significant upper limit for a fourth CO line, allow us to estimate an H₂ mass of between 1.9x10¹¹/m*M_☉ and 1.1x10¹²/m*M_☉. The two lines CI(³P₁-³P₀) and CI(³P₂-³P₁) were clearly detected and yield a [CI]/[H₂] number abundance of between 1.4x10^–5 and 8.0x10^–5. Upper limits are presented for emission lines of HCN, HCO⁺, HNC, H₂O, and of other molecules. The moderate excitation of the CO lines is indicative of an extended starburst, and excludes the dominance of an AGN in heating this high-z SMG.