Optically faint massive Balmer break galaxies at z > 3 in the CANDELS/GOODS fields

Abstract

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^(3.4)_(2.8) mag ). 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)^(2.0) mag) 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 > 10^(3) 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.