Balmer Break Galaxy Candidates at z ∼ 6: A Potential View on the Star Formation Activity at z ⪆ 14

Ken Mawatari, Akio K. Inoue, Takuya Hashimoto, John Silverman, Masaru Kajisawa, Satoshi Yamanaka, Toru Yamada, Iary Davidzon, Peter Capak, Lihwai Lin, Bau Ching Hsieh, Yoshiaki Taniguchi, Masayuki Tanaka, Yoshiaki Ono, Yuichi Harikane, Yuma Sugahara, Seiji Fujimoto, Tohru Nagao

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27 Citations (Scopus)


We search for galaxies with a strong Balmer break (Balmer break galaxies; BBGs) at z ∼ 6 over a 0.41 deg2 effective area in the COSMOS field. Based on rich imaging data, including data obtained with the Atacama Large Millimeter/submillimeter Array (ALMA), three candidates are identified by their extremely red K-[3.6] colors, as well as by nondetection in the X-ray, optical, far-infrared, and radio bands. The nondetection in the deep ALMA observations suggests that they are not dusty galaxies but BBGs at z ∼ 6, although contamination from active galactic nuclei at z ∼ 0 cannot be completely ruled out for the moment. Our spectral energy distribution analyses reveal that the BBG candidates at z ∼ 6 have stellar masses of ≈5 × 1010 M o˙ dominated by old stellar populations with ages of ⪆ 700 Myr. Assuming that all three candidates are real BBGs at z ∼ 6, we estimate the stellar mass density to be 2.4-1.3 +2.3× 10-4,M Mpc-3. This is consistent with an extrapolation from the lower-redshift measurements. The onset of star formation in the three BBG candidates is expected to be several hundred million yr before the observed epoch of z ∼ 6. We estimate the star formation rate density (SFRD) contributed by progenitors of the BBGs to be 2.4-12 × 10-5 M o˙ yr-1 Mpc-3 at z > 14 (99.7% confidence range). Our result suggests a smooth evolution of the SFRD beyond z = 8.

Original languageEnglish
Article number137
JournalAstrophysical Journal
Issue number2
Publication statusPublished - 2020 Feb 1

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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