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 non-detection in X-ray, optical, far-infrared (FIR), and radio bands. The non-detection in the deep ALMA observations suggests that they are not dusty galaxies but BBGs at z ∼ 6, although contamination from Active Galactic Nuclei (AGNs) at z ∼ 0 cannot be completely ruled out for the moment. Our spectral energy distribution (SED) 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 the three candidates are real BBGs at z ∼ 6, we estimate the stellar mass density (SMD) to be 2.4⁺²₋₁^._.³₃ × 10⁴ 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 years 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.