A Radio-to-mm Census of Star-forming Galaxies in Protocluster 4C23.56 at Z = 2.5: Gas Mass and Its Fraction Revealed with ALMA

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}_\star > 4× {10}¹⁰ M _⊙) 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]× {10}¹¹× ({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 ({H}₂)∼ 5× {10}⁹ {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.