2017ApJ...842...55L

We investigate gas contents of star-forming galaxies associated with protocluster 4C23.56 at z = 2.49 by using the redshifted CO (3-2) and 1.1 mm dust continuum with the Atacama Large Millimeter/submillimeter Array. The observations unveil seven CO detections out of 22 targeted Hα emitters (HAEs) and four out of 19 in 1.1 mm dust continuum. They have high stellar mass (M_* 4×10¹⁰M_☉) and exhibit a specific star-formation rate typical of main-sequence star-forming galaxies at z∼2.5. Different gas-mass estimators from CO (3-2) and 1.1 mm yield consistent values for simultaneous detections. The gas mass (M_gas) and gas fraction (f_gas) are comparable to those of field galaxies, with M_gas=[0.3,1.8]×10¹¹×(α_CO/(4.36×A(Z))) M_☉, where α_CO is the CO-to-H₂ conversion factor and A(Z) is the additional correction factor for the metallicity dependence of α_CO, and <f_gas>=0.53±0.07 from CO (3-2). Our measurements place a constraint on the cosmic gas density of high-z protoclusters, indicating that the protocluster is characterized by a gas density higher than that of the general fields by an order of magnitude. We found ρ(H₂)∼5×10⁹ M_☉ Mpc^–3 with the CO(3-2) detections. The five ALMA CO detections occur in the region of highest galaxy surface density, where the density positively correlates with global star-forming efficiency (SFE) and stellar mass. Such correlations possibly indicate a critical role of the environment on early galaxy evolution at high-z protoclusters, though future observations are necessary for confirmation.