2019ApJ...876..135A

We present a sample of 33 Balmer break galaxies (BBGs) selected as Hubble Space Telescope/F160W dropouts in the deepest CANDELS/GOODS fields (H >= 27.3 mag) but relatively bright in Spitzer/IRAC ([3.6], [4.5] < 24.5 mag), implying red colors (median and quartiles: <H-[3.6]>=3.1_2.8^3.4mag). Half of these BBGs are newly identified sources. Our BBGs are massive (<log(M/M_☉)>=10.8_10.4^11.0), high-redshift (<z>=4.8_4.4^5.1), dusty (<A(V)>=2.0_1.5^2.0mag) galaxies. The spectral energy distributions (SEDs) of half of our sample indicate that they are star-forming galaxies with typical specific star formation rates (SFRs) of 0.5-1.0 Gyr^–1, qualifying them as main-sequence (MS) galaxies at 3 < z < 6. One-third of these SEDs indicate the presence of prominent emission lines (Hβ + [O III], Hα + [N II]) boosting the IRAC fluxes and red colors. Approximately 20% of the BBGs are very dusty (A(V) ∼ 2.5 mag) starbursts with strong mid-to-far-infrared detections and extreme SFRs (SFR > 10³ M_☉ yr^–1) that place them above the MS. The rest, 30%, are post-starbursts or quiescent galaxies located >2σ below the MS with mass-weighted ages older than 700 Myr. Only two of the 33 galaxies are X-ray-detected active galactic nuclei (AGNs) with optical/near-infrared SEDs dominated by stellar emission, but the presence of obscured AGNs in the rest of the sources cannot be discarded. Our sample accounts for 8% of the total number density of log(M/M_☉)> 10 galaxies at z > 3, but it is a significant contributor (30%) to the general population of red log(M/M_☉)> 11 galaxies at 4 < z < 6. Finally, our results point out that one of every 30 massive log(M/M_☉)> 11 galaxies in the local universe was assembled in the first 1.5 Gyr after the big bang, a fraction that is not reproduced by state-of-the-art galaxy formation simulations.