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

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⊙ 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 ${2.4}_{-1.3}^{+2.3}\times {10}^{4}\,{M}_{\odot }$ 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⊙ yr‑1 Mpc‑3 at z > 14 (99.7% confidence range). Our result suggests a smooth evolution of the SFRD beyond z = 8.