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

Abstract

We search for galaxies with a strong Balmer break (Balmer Break Galaxies; BBGs) at z ∼ 6 over a 0.41 deg 2 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 × 10 10 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 +2.3 −1.3 × 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 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.

A puzzling non-detection of [O III] and [C II] from a z ≈ 7.7 galaxy observed with ALMA

“…In order to get an understanding of the possible stellar populations that may lead to results that are consistent with the HST, VLT and ALMA observations, we performed SED fitting using the panhit code (Mawatari et al 2020). We used measurements from HST (F105W, F125W, F160W) and ground based infrared photometry (VLT/HAWK-I Ks) from the CANDELS catalog (Guo et al 2013) combined with the ALMA upper limits on the [O iii] 88 µm emission (assuming a line-width of 100 km s −1 ) and dust continuum emission in band 6 and band 8.…”

“…In order to account for IGM attenuation of the model spectra shortward of Lyα, panhit uses the model proposed in Inoue et al (2014b). For a more in-detail description of the panhit SED-fitting code, see Mawatari et al (2020) Formally, the best-fitting solution (χ 2 ν = 3.40) obtained from panhit gives a high age of 0.57 Gyr and a low metallicity of Z = 0.0004. The SFR of the model is 7.95 M yr −1 , with a stellar mass of M = 3.05 × 10 9 M and a dust-extinction of A V = 0.00 magnitudes.…”

The size and pervasiveness of Ly α–UV spatial offsets in star-forming galaxies at z ∼ 6

“…These studies, especially those over the last decade, have now become powerful enough to begin to explore the epoch known as "reionization" (EoR), during which the first stars, galaxies, and active galactic nuclei (AGN) were beginning to ionize what was a neutral in-tergalactic medium of hydrogen gas (e.g., Treu et al 2013;Tilvi et al 2014Tilvi et al , 2020Schmidt et al 2016;Hashimoto et al 2018aHashimoto et al ,b, 2019Mason et al 2018Mason et al , 2019Hoag et al 2019b;Tamura et al 2019;Fuller et al 2020). It is expected that the very first galaxies started to form when the universe was less than 1 Gyr old (see, e.g., Mawatari et al 2020;Strait et al 2020 and references therein), i.e., at z ∼ > 6, and that these first sources played a crucial role in the reionization that appears to have ended at z∼6 (Becker et al 2001;Fan et al 2006). Such a timescale is consistent with observations of the Cosmic Microwave Background from the Planck telescope, which use the Thomson optical scattering depth to infer the epoch of an instantaneous reionization at 7.0 ∼ < z ∼ < 8.5 (Planck Collaboration et al 2016.…”

A census of optically dark massive galaxies in the early Universe from magnification by lensing galaxy clusters

“…Future ALMA observations of a larger sample of optically dark galaxies in cluster fields will uncover even fainter objects. This is important to verify the presence of optically dark galaxies with very low specific SFRs, which may represent the rare population of quenched galaxies in the early Universe (e.g., Schreiber et al 2018;Mawatari et al 2020;Santini et al 2021).…”