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

Abstract

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 H$\alpha$ emitters (HAEs) and four out of 19 in 1.1 mm dust continuum. They have high stellar mass ($M_{\star }>4\times 10^{10}$ $M_{\odot }$) and exhibit a specific star-formation rate typical of main-sequence star forming galaxies at $z\sim 2.5$. Different gas mass estimators from CO(3-2) and 1.1 mm yield consistent values for simultaneous detections. The gas mass ($M_{\mathrm{gas}}$) and gas fraction ($f_{\mathrm{gas}}$) are comparable to those of field galaxies, with $M_{\mathrm{gas}}=[0.3,1.8]\times 10^{11}\times ({\alpha }_{\mathrm{CO}}/(4.36\times A(Z)$)) M${}_{\odot }$, where ${\alpha }_{\mathrm{CO}}$ is the CO-to-H${}_2$ conversion factor and $A(Z)$ the additional correction factor for the metallicity dependence of ${\alpha }_{\mathrm{CO}}$, and $⟨f_{\mathrm{gas}}⟩=0.53\pm 0.07$ from CO(3-2). Our measurements place a constraint on the cosmic gas density of high-$z$ protoclusters, indicating the protocluster is characterized by a gas density higher than that of the general fields by an order of magnitude. We found $\rho (H_2)\sim 5\times 10^9{M}_{\odot }\mathrm{Mp}{\mathrm{c}}^{-3}$ 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 imply a potentially critical role of environment on early galaxy evolution at high-z protoclusters, although future observations are necessary for confirmation.

33 pages, 16 figures, 2 tables ; Accepted for publication in The Astrophysical Journal

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