2021ApJ...908..163M

We present Atacama Large Millimeter/submillimeter Array Band 5 observations of a galaxy at z = 1.91, GDS24569, in search of molecular gas in its vicinity via the [C I] ³P₁-³P₀ line. GDS24569 is a massive (logM_*/M_☉=11), passively evolving galaxy, characterized by compact morphology with an effective radius of ∼0.5 kpc. We apply two blind detection algorithms to the spectral data cubes and find no promising detection in or around GDS24569 out to a projected distance of ∼320 kpc, while a narrow tentative line (4.1σ) is identified at +1200 km s^–1 by one of the algorithms. From the nondetection of [C I], we place a 3σ upper limit on molecular hydrogen mass, ∼ 7.1 x 10⁹ M_☉, which converts to an extremely low gas-to-stellar mass fraction of <= 5%. We conduct a spectral energy distribution modeling by including optical to far-infrared data and find a considerably high ( ∼ 0.1%) dust-to-stellar mass ratio, i.e., ∼10-100x higher than those of local early-type galaxies. In combination with a previous result of an insufficient number of surrounding satellite galaxies, it is suggested that GDS24569 is unlikely to experience significant size evolution via satellite mergers. We discuss possible physical mechanisms that quenched GDS24569.