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

Minju M. Lee, Ichi Tanaka, Ryohei Kawabe, Kotaro Kohno, Tadayuki Kodama, Masaru Kajisawa, Min S. Yun, Kouichiro Nakanishi, Daisuke Iono, Yoichi Tamura, Bunyo Hatsukade, Hideki Umehata, Toshiki Saito, Takuma Izumi, Itziar Aretxaga, Ken Ichi Tadaki, Milagros Zeballos, Soh Ikarashi, Grant W. Wilson, David H. HughesR. J. Ivison

Research output: Contribution to journalArticlepeer-review

33 Citations (Scopus)


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 × 1010 Mo) 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 (Mgas) and gas fraction (fgas) are comparable to those of field galaxies, with Mgas = [0.3, 1.8] × 1011 × (αco/4.36 × A(Z))) M, where αco is the CO-to-H2 conversion factor and A(Z) is the additional correction factor for the metallicity dependence of αco, and fgas = 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 p(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.

Original languageEnglish
Article number55
JournalAstrophysical Journal
Issue number1
Publication statusPublished - 2017 Jun 10
Externally publishedYes


  • galaxies: ISM
  • galaxies: clusters: general
  • galaxies: evolution
  • galaxies: high-redshift
  • large-scale structure of universe
  • submillimeter: galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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