Detections of CO Molecular Gas in 24 μm Bright ULIRGs at z ~ 2 in the Spitzer First Look Survey

Abstract

We present CO observations of nine ultra-luminous infrared galaxies (ULIRGs) at z ~ 2 with f_(ν)(24 μm) ≳ 1 mJy, previously confirmed with the mid-IR spectra in the Spitzer First Look Survey. All targets are required to have accurate redshifts from Keck/GEMINI near-IR spectra. Using the Plateau de Bure millimeter-wave Interferometer at the Institute for Radioastronomy at Millimeter Wavelengths, we detect CO J(3-2) (seven objects) or J(2-1) (one object) line emission from eight sources with integrated intensities I_c ~ 5σ-9σ. The CO-detected sources have a variety of mid-IR spectra, including strong polycyclic aromatic hydrocarbon, deep silicate absorption, and power-law continuum, implying that these molecular gas-rich objects at z ~ 2 could be either starbursts or dust-obscured active galactic nuclei (AGNs). The measured line luminosity L'_(CO) is (1.28-3.77) × 10^(10) K km/s pc^2. The averaged molecular gas mass is 1.7 × 10^(10) M_☉, assuming CO-to-H_2 conversion factor of 0.8 M (K km/s pc^2)^(–1). Three sources (33%)—MIPS506, MIPS16144, and MIPS8342—have double peak velocity profiles. The CO double peaks in MIPS506 and MIPS16144 show spatial separations of 45 kpc and 10.9 kpc, allowing the estimates of the dynamical masses of 3.2 × 10^(11) sin^(–2)(i) M_☉ and 5.4 × 10^(11) sin^(–2)(i) M, respectively. The implied gas fraction, M_(gas)/M_(dyn), is 3% and 4%, assuming an average inclination angle. Finally, the analysis of the Hubble Space Telescope/NIC2 images, mid-IR spectra, and IR spectral energy distribution revealed that most of our sources are mergers, containing dust-obscured AGNs dominating the luminosities at (3-6) μm. Together, these results provide some evidence suggesting submillimeter galaxies, bright 24 μm, z ~ 2 ULIRGs, and QSOs could represent three different stages of a single evolutionary sequence, however, a complete physical model would require much more data, especially high spatial resolution spectroscopy.

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