CO J = 2-1 Line Emission in Cluster Galaxies at z ~ 1: Fueling Star Formation in Dense Environments

Abstract

We present observations of CO J = 2–1 line emission in infrared-luminous cluster galaxies at z ∼ 1 using the IRAM Plateau de Bure Interferometer. Our two primary targets are optically faint, dust-obscured galaxies (DOGs) found to lie within 2Mpc of the centers of two massive (>10^(14)M_☉) galaxy clusters. CO line emission is not detected in either DOG. We calculate 3σ upper limits to the CO J = 2–1 line luminosities, L_(CO) < 6.08×10^9 and <6.63×10^9 K km s^(−1) pc^2. Assuming a CO-to-H_2 conversion factor derived for ultraluminous infrared galaxies in the local universe, this translates to limits on the cold molecular gas mass of M_(H2) < 4.86×10^9 M_☉ and M_(H2) < 5.30×10^9 M_☉. Both DOGs exhibit mid-infrared continuum emission that follows a power law, suggesting that an active galactic nucleus (AGN) contributes to the dust heating. As such, estimates of the star formation efficiencies in these DOGs are uncertain. A third cluster member with an infrared luminosity, L_(IR) < 7.4×10^(11)L_☉, is serendipitously detected in CO J = 2–1 line emission in the field of one of the DOGs located roughly two virial radii away from the cluster center. The optical spectrum of this object suggests that it is likely an obscured AGN, and the measured CO line luminosity is L'_(CO)= (1.94 ± 0.35)× 10^(10) K km s^(−1) pc^2, which leads to an estimated cold molecular gas mass M_(H2)= (1.55 ± 0.28) × 10^(10)M_☉ . A significant reservoir of molecular gas in a z ∼ 1 galaxy located away from the cluster center demonstrates that the fuel can exist to drive an increase in star formation and AGN activity at the outskirts of high-redshift clusters.

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