Detection of a Substantial Molecular Gas Reservoir in a Brightest Cluster Galaxy at z = 1.7

Abstract

We report the detection of CO(2–1) emission coincident with the brightest cluster galaxy (BCG) of the high-redshift galaxy cluster SpARCS1049+56, with the Redshift Search Receiver (RSR) on the Large Millimeter Telescope (LMT). We confirm a spectroscopic redshift for the gas of z  = 1.7091 ± 0.0004, which is consistent with the systemic redshift of the cluster galaxies of z  = 1.709. The line is well fit by a single-component Gaussian with an RSR-resolution-corrected FWHM of 569 ± 63 km s −1 . We see no evidence for multiple velocity components in the gas, as might be expected from the multiple image components seen in near-infrared imaging with the Hubble Space Telescope . We measure the integrated flux of the line to be 3.6 ± 0.3 Jy km s −1 , and using  = 0.8 M ⊙ (K km s −1 pc 2 ) −1 , we estimate a total molecular gas mass of 1.1 ± 0.1 × 10 11 M ⊙ and a M H2 / M ⋆  ∼ 0.4. This is the largest gas reservoir detected in a BCG above z  > 1 to date. Given the infrared-estimated star formation rate of 860 ± 130 M ⊙ yr −1 , this corresponds to a gas depletion timescale of ∼0.1 Gyr. We discuss several possible mechanisms for depositing such a large gas reservoir to the cluster center—e.g., a cooling flow, a major galaxy–galaxy merger, or the stripping of gas from several galaxies—but conclude that these LMT data are not sufficient to differentiate between them.