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

<jats:title>Abstract</jats:title> <jats:p>We search for galaxies with a strong Balmer break (Balmer break galaxies; BBGs) at <jats:italic>z</jats:italic> ∼ 6 over a 0.41 deg<jats:sup>2</jats:sup> 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 <jats:italic>K</jats:italic>–[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 <jats:italic>z</jats:italic> ∼ 6, although contamination from active galactic nuclei at <jats:italic>z</jats:italic> ∼ 0 cannot be completely ruled out for the moment. Our spectral energy distribution analyses reveal that the BBG candidates at <jats:italic>z</jats:italic> ∼ 6 have stellar masses of ≈5 × 10<jats:sup>10</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> dominated by old stellar populations with ages of ≳ 700 Myr. Assuming that all three candidates are real BBGs at <jats:italic>z</jats:italic> ∼ 6, we estimate the stellar mass density to be <jats:inline-formula> <jats:tex-math> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjab6596ieqn1.gif" xlink:type="simple"/> </jats:inline-formula> Mpc<jats:sup>−3</jats:sup>. 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 <jats:italic>z</jats:italic> ∼ 6. We estimate the star formation rate density (SFRD) contributed by progenitors of the BBGs to be 2.4–12 × 10<jats:sup>−5</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> yr<jats:sup>−1</jats:sup> Mpc<jats:sup>−3</jats:sup> at <jats:italic>z</jats:italic> > 14 (99.7% confidence range). Our result suggests a smooth evolution of the SFRD beyond <jats:italic>z</jats:italic> = 8.</jats:p>